New observations support the idea that hot, diffuse threads of gas called cosmic filaments connect clusters of galaxies across the cosmos. That is the conclusion of Konstantinos Migkas at Leiden University and colleagues who say that their study strengthens the idea that much of the normal matter in the universe resides in these structures.

About 5% of the universe’s mass–energy content appears to be baryonic matter – the familiar nuclei and particles that make up atoms and molecules. The rest is believed to be dark energy and dark matter, which are both hypothetical entities. Although they know what baryonic matter is, astronomers have a poor understanding of where much of it is distributed in the universe.

Combining the Standard Model of cosmology with the rigid constraints enforced by observations of cosmic microwave background radiation tells us that structures including stars, black holes, and gas clouds account for around 60% of baryonic matter in the universe. This leaves 40% of baryonic matter unaccounted for.

Previously, cosmologists have argued that this discrepancy could point to a fundamental error in the Standard Model. Recently, however, a growing body of evidence suggests that this matter could be found in vast yet elusive structures, hidden deep within intergalactic space.

On a WHIM

“Large-scale structure simulations of the universe tell us this material should reside within long strings of gas called ‘cosmic filaments’, which connect clusters of galaxies,” Migkas explains. “These missing baryons should be found in the so-called ‘warm-hot intergalactic medium’ (WHIM).”

Despite being extremely sparse, models also predict that the WHIM should be extremely hot – primarily heated by shock waves produced as matter collapses into the large-scale cosmic web, as well as by phenomena including active galactic nuclei and mergers between galaxy clusters. As a result, these cosmic filaments should be emitting a faint yet detectable X-ray signal.

On top of this, the Standard Model places tight theoretical constraints on several physical properties of the WHIM – including its density, temperature, and composition. If X-rays are indeed being emitted by cosmic filaments, these properties should be encoded in their energies, intensities, and frequency spectra – providing astronomers with a clear target in their search for the elusive structures.

These X-ray signals have so far evaded detection because they are extremely faint compared to powerful X-ray signals such as those coming from supermassive black holes

To overcome this, researchers combined data from two of the world’s most advanced X-ray observatories. One is the Suzaku satellite, which was jointly operated by JAXA and NASA and was very good at detecting very faint signals. The other is the ESA’s XMM-Newton, which is very good at observing powerful X-ray signals.

Eliminating black holes

“Combining the two instruments, we carefully and appropriately eliminated the contaminating signal of the black holes throughout our filament,” Migkas explains. “This enabled us to isolate the signal of WHIM and measure its density and temperature for the very first time with such accuracy.”

For an observational target, Migkas’ team searched for cosmic filaments in the Shapley supercluster. This vast structure around 650 million light-years from the Milky Way contains one of the highest concentrations of galaxies in the known universe.

With the combined abilities of Suzaku and XMM-Newton, the researchers detected an X-ray signal indicating the presence of a filament – consistent with predictions of the Standard Model. As they expected, this intergalactic material was extremely hot and sparse: boasting temperatures close to 10 million Kelvin, while containing just around 10 electrons per cubic metre.

“We also found that on average, the filament is around 40 times denser than the average density of the universe – which is pretty empty in general – and around 1000 times less dense than the cores of the four-galaxy cluster it connects,” Migkas describes. Despite having gone undetected so far, this filament also carries a total mass around 10 times that of the Milky Way – making it a vast reservoir of previously hidden matter.

“For the very first time, our work confirms the validity of the predictions of the Standard Model of cosmology regarding the properties of a big part of the missing baryons,” Migkas concludes.

The research is described in Astronomy and Astrophysics.

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Quantum technology is rapidly growing with job demand tripling in the US along with venture capital bringing in billions of dollars into the field. That is according to the inaugural Massachusetts Institute of Technology (MIT) Quantum Index Report 2025, which finds, however, that large-scale commercial applications for quantum computing still remain “far off”.

Carried out by the Initiative on the Digital Economy (IDE) at MIT, the report is a result of data collection from academia, industry and policy sources. It sets out to track, measure and visualize trends across several areas such as education, funding, research and development.

One aim of the report is to reduce the complexity of quantum technology and to make the field more accessible and inclusive for entrepreneurs, investors, designers, teachers and decision makers. This in turn, the report says, can help to shape how the technology is developed, commercialized and governed.

The inaugural edition focusses on quantum computing and networks, due to their higher potential impact compared to quantum sensing and simulation. The report says that $1.6bn has been raised by quantum-computing firms in 2024 compared with $621m by quantum-software companies.

The report also finds that jobs in the quantum sector have increased with demand tripling in the US since 2018. This has led to a higher number of education initiatives, with Germany having the most Master’s degrees that include “quantum” in the name.

A ‘community-led project’

The report says that corporations and universities dominate innovation efforts, claiming up to 91% of quantum computing patents. When it comes to academic research, the report finds that while China produces the most papers in quantum computing, US research tends to have a greater impact and influence.

The report also indexes and analyzes published data on over 200 quantum processing units (QPUs) from 17 countries to provide insight into how the performance of different types of quantum computers can be verified. The report finds that despite QPUs making impressive progress in performance, they remain far from meeting the requirements for running large-scale commercial applications such as chemical simulations or cryptanalysis.

Principal investigator Jonathan Ruane from MIT Sloan calls the report a “community-led project” and encourages people to contribute additional data. He says that while a report will be published annually, data on its website will be updated “as often as input is given”.

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Grid operators around the world are under intense pressure to expand and modernize their power networks. The International Energy Authority predicts that demand for electricity will rise by 30% in this decade alone, fuelled by global economic growth and the ongoing drive towards net zero. At the same time, electrical transmission systems must be adapted to handle the intermittent nature of renewable energy sources, as well as the extreme and unpredictable weather conditions that are being triggered by climate change.

High-voltage capacitors play a crucial role in these power networks, balancing the electrical load and managing the flow of current around the grid. For more than 40 years, the standard dielectric for storing energy in these capacitors has been a thin film of a polymer material called biaxially oriented polypropylene (BOPP). However, as network operators upgrade their analogue-based infrastructure with digital technologies such as solid-state transformers and high-frequency switches, BOPP struggles to provide the thermal resilience and reliability that are needed to ensure the stability, scalability and security of the grid.

“We’re trying to bring innovation to an area that hasn’t seen it for a very long time,” says Dr Mike Ponting, Chief Scientific Officer of Peak Nano, a US firm specializing in advanced polymer materials. “Grid operators have been using polypropylene materials for a generation, with no improvement in capability or performance. It’s time to realize we can do better.”

Peak Nano has created a new capacitor film technology that address the needs of the digital power grid, as well as other demanding energy storage applications such as managing the power supply to data centres, charging solutions for electric cars, and next-generation fusion energy technology. The company’s Peak NanoPlex™ materials are fabricated from multiple thin layers of different polymer materials, and can be engineered to deliver enhanced performance for both electrical and optical applications. The capacitor films typically contain polymer layers anywhere between 32 and 156 nm thick, while the optical materials are fabricated with as many as 4000 layers in films thinner than 300 µm.

“When they are combined together in an ordered, layered structure, the long polymer molecules behave and interact with each other in different ways,” explains Ponting. “By putting the right materials together, and controlling the precise arrangement of the molecules within the layers, we can engineer the film properties to achieve the performance characteristics needed for each application.”

In the case of capacitor films, this process enhances BOPP’s properties by interleaving it with another polymer. Such layered films can be optimized to store four times the energy as conventional BOPP while achieving extremely fast charge/discharge rates. Alternatively, they can be engineered to deliver longer lifetimes at operating temperatures some 50–60°C higher than existing materials. Such improved thermal resilience is useful for applications that experience more heat, such as mining and aerospace, and is also becoming an important priority for grid operators as they introduce new transmission technologies that generate more heat.

“We talked to the users of the components to find out what they needed, and then adjusted our formulations to meet those needs,” says Ponting. “Some people wanted smaller capacitors that store a lot of energy and can be cycled really fast, while others wanted an upgraded version of BOPP that is more reliable at higher temperatures.”

The multilayered materials now being produced by Peak Nano emerged from research Ponting was involved in while he was a graduate student at Case Western Reserve University in the 2000s, where Ponting was a graduate student. Plastics containing just a few layers had originally been developed for everyday applications like gift wrap and food packaging, but scientists were starting to explore the novel optical and electronic properties that emerge when the thickness of the polymer layers is reduced to the nanoscale regime.

Small samples of these polymer nanocomposites produced in the lab demonstrated their superior performance, and Peak Nano was formed in 2016 to commercialize the technology and scale up the fabrication process. “There was a lot of iteration and improvement to produce large quantities of the material while still maintaining the precision and repeatability of the nanostructured layers,” says Ponting, who has been developing these multilayered polymer materials and the required processing technology for more than 20 years. “The film properties we want to achieve require the polymer molecules to be well ordered, and it took us a long time to get it right.”

As part of this development process, Peak Nano worked with capacitor manufacturers to create a plug-and-play replacement technology for BOPP that can be used on the same manufacturing systems and capacitor designs as BOPP today. By integrating its specialist layering technology into these existing systems, Peak Nano has been able to leverage established supply chains for materials and equipment rather than needing to develop a bespoke manufacturing process. “That has helped to keep costs down, which means that our layered material is only slightly more expensive than BOPP,” says Ponting.

Ponting also points out that long term, NanoPlex is a more cost-effective option. With improved reliability and resilience, NanoPlex can double or even quadruple the lifetime of a component. “The capacitors don’t need to be replaced as often, which reduces the need for downtime and offsets the slightly higher cost,” he says.

For component manufacturers, meanwhile, the multilayered films can be used in exactly the same way as conventional materials. “Our material can be wound into capacitors using the same process as for polypropylene,” says Ponting. “Our customers don’t need to change their process; they just need to design for higher performance.”

Initial interest in the improved capabilities of NanoPlex came from the defence sector, with Peak Nano benefiting from investment and collaborative research with the US Defense Advanced Research Projects Agency (DARPA) and the Naval Research Laboratory. Optical films produced by the company have been used to fabricate lenses with a graduated refractive index, reducing the size and weight of head-mounted visual equipment while also sharpening the view. Dielectric films with a high breakdown voltage are also a common requirement within the defence community.

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A computational paradigm that can accurately simulate interactions between powerful laser beams and quantum fluctuations in a vacuum has been unveiled by researchers in the UK and Portugal. Led by Lily Zhang at the University of Oxford, the team hopes that their solver could lead to important new insights into the quantum nature of the vacuum.

Quantum electrodynamics (QED) provides a detailed picture of how light and matter interact, and has withstood decades of experimental scrutiny. So far, however, evidence for one of the theory’s key predictions about the nature of the vacuum has remained elusive.

Far from being empty, the vacuum contains a sea of virtual particles that are associated with quantum fluctuations. These particle–antiparticle pairs spontaneously pop into existence before annihilating almost instantly.

QED predicts that virtual particles create nonlinearities within the vacuum that can interact with powerful laser pulses. Underlying this effect is photon–photon scattering, something that particle physicists have tried to observe for several decades in accelerator experiments.

Powerful lasers

“So far, there has been no successful direct tests of photon–photon scattering,” Zhang explains. “However, the global emergence of multi-petawatt lasers has rekindled interest in testing the vacuum using just light itself.” For these experiments to succeed, robust analytical tools which can model the quantum vacuum’s responses to such immensely powerful lasers will be crucial.

So far, researchers have used computational tools that can only model simplified laser setups using 2D models. Zhang’s team addressed these limitations using a numerical technique called the Yee scheme. This is used to solve Maxwell’s equations of electromagnetism and is already widely used in plasma simulation. The method works by separately calculating electric and magnetic fields at staggered times and positions, ensuring greater stability and accuracy.

“The key challenge here is the nonlinear terms, which depend on the electromagnetic fields themselves,” Zhang explains. “We addressed this by combining the Yee scheme with an iterative loop that updates the nonlinear response at each time step until the solution converges.” Once integrated with a state-of-the-art plasma simulation code, the team was left with a fully 3D solver, capable of simulating arbitrary laser interactions within a vacuum.

To test their platform’s performance, they benchmarked it against existing theoretical predictions of vacuum birefringence, which is an effect triggered when a vacuum is distorted by and intense laser pulse that “pumps” the vacuum. In the context of photon–photon scattering, QED predicts that that these distortions will cause the light in a weaker probe beam to split into two separate rays, each with a different polarization and refractive index.

In addition, the team extended their solver to modelling four-wave mixing, which is a more complex effect whereby three input light beams interact in a vacuum to generate a fourth beam.

Tracking asymmetries

“The real-time simulation capability allowed us to track the evolving properties of this output signal, including its size, intensity, and duration, and link these to physical conditions at earlier stages of the interaction,” Zhang explains. “For instance, asymmetries in the signal beam were traced back to asymmetries in the interaction region, which is clearly observable from the simulation data.”

Just as the team hoped, their simulations of birefringence and four-wave mixing both closely matched their theoretical predictions – clearly showcasing their platform’s advanced capabilities.

For now, Zhang and colleagues hope that their solver could vastly reduce the computing resources required to develop robust 3D simulations of laser–vacuum interactions, making them far more efficient and accessible in turn. With its high flexibility in simulating arbitrary laser configurations, the team is now confident that their platform could soon be used to studying a diverse array of quantum vacuum effects – including birefringence, four-wave mixing, and many others.

“Looking ahead, we’re using the solver to explore new regimes, including novel beam profiles and exotic field interactions,” Zhang says. “Its structure also allows for easy extension to other nonlinear theories, such as Born–Infeld electrodynamics and axion-like particle fields. Ultimately, our goal is to create a versatile simulation platform for probing fundamental physics in the quantum vacuum.”

The research is described in Communications Physics.

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A new experiment has offered the clearest view yet of how gluons behave inside atomic nuclei. Conducted at the Thomas Jefferson National Accelerator Facility in the US, the study focused on a rare process called photoproduction. This involves high-energy photons interacting with protons confined in nuclei to produce J/psi mesons. The research sheds light on how gluons are distributed in nuclear matter and is a crucial step toward understanding the nature of protons within nuclei.

While gluons are responsible for generating most of the visible mass in the universe, their role inside nuclei remains poorly understood. These massless particles mediate the strong nuclear force, which binds quarks as well as protons and neutrons in nuclei. Gluons carry no electric charge and cannot be directly detected.

The theory that describes gluons is called quantum chromodynamics (QCD) and it is notoriously complex and difficult to test – especially in the dense, strongly interacting environment of a nucleus. That makes precision experiments essential for revealing how matter is held together at the deepest level.

Probing gluons with light

The Jefferson Lab experiment focused on photoproduction, a process in which a high-energy photon strikes a particle and creates something new, in this case, a J/psi meson.

The J/psi comprises a charm quark and its antiquark and is especially useful for studying gluons. Charm quarks are much heavier than those found in ordinary matter and are not present in protons or neutrons. Therefore, they must be created entirely during the interaction, making the J/psi a particularly clean and sensitive probe of gluon behaviour inside nuclei.

Earlier studies had observed this process using free protons. This new experiment extends the approach to protons confined in nuclei to see how that environment affects gluon behaviour. The modification of quarks inside nuclei has been known since the 1980s and is called the EMC effect. However, much less is known about how gluons behave under the same conditions.

“Protons and neutrons do behave differently when they are bound inside nuclei than they do on their own,” says Jackson Pybus, now a postdoctoral fellow at Los Alamos National Laboratory and one of the experiment’s collaborators. “The nuclear physics community is still trying to work out the mechanisms behind the EMC effect. Until now, the distribution of high-momentum gluons in nuclei has remained an unexplored area.”

Pybus and colleagues used Jefferson Lab’s Experimental Hall D, which delivers an intense beam of high-energy photons. This setup had previously been used to study simpler systems, but this was the first time it was applied to heavier nuclei.

“This study looked for events where a photon strikes a proton inside the nucleus to knock it out while producing a J/psi,” Pybus explains. “By measuring the knocked-out proton, the produced J/psi, and the energy of the photon, we can reconstruct the reaction and learn how the gluons were behaving inside the nucleus.” This was done using the GlueX spectrometer.

Unexpected signals

Significantly, the experiment was accessing the “threshold” region – where the photon has just enough energy to produce a J/psi meson. Near-threshold interactions are particularly valuable because they are highly sensitive to the gluon structure of the target. Creating a heavy charm-anticharm pair requires a large energy transfer so interactions in this region reveal how gluons behave when little momentum is available. This is a regime where theoretical uncertainties in QCD are especially large.

Even more striking were the observations below this threshold. In so-called “sub-threshold” photoproduction, the incoming photon does not carry enough energy to produce the J/psi on its own, so it must draw additional energy from the internal motion of protons or from the nuclear medium itself. This is a well-understood mechanism in principle, but the rate at which it occurred in the experiment came as a surprise.

“Our study was the first to measure J/psi photoproduction from nuclei in the threshold region,” Pybus said. “The data indicate that the J/psi is produced more commonly than expected from protons that are moving with large momentum inside the nucleus, suggesting that these fast-moving protons could experience significant distortion to their internal gluons.”

The sub-threshold results were even harder to explain. “The number of subthreshold J/psi exceeded expectations,” Pybus added. “That raises questions about how the photon is able to pick up so much energy from the nucleus.”

Towards a deeper theory

The results suggest that gluons may be modified inside nuclei in ways that are not described by existing models – suggesting a new frontier in nuclear physics.

“This study has given us the first look at this sort of rare phenomenon that can teach us about the gluon inside the nucleus – just enough data to point to unexpected behaviours,” said Pybus. “Now that we know this measurement is possible, and that there are signs of interesting and unexplored phenomena, we’d like to perform a dedicated measurement focused on pinning down the sort of exotic effects we’re just now glimpsing.”

Follow-up experiments, including those planned at the future Electron-Ion Collider, are expected to build on these results. For now, this first glimpse at gluons in nuclei reveals that even decades after QCD’s development, the inner workings of nuclear matter remain only partially illuminated.

The research is described in Physical Review Letters.

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Modern-day communications rely on both fibre-optic cables and wireless radiofrequency (RF) microwave communications. Reaching higher data transmission capabilities is going to require technologies that can efficiently process and convert both optical and microwave signals in a small and energy-efficient package that’s compatible with existing communication networks.

Microwave photonics (MWP) is one of the frontrunning technologies, as it can perform signal processing tasks within the optical domain. Current MWP approaches, however, are typically power intensive and often require many off-chip devices to achieve the desired device capabilities and functionalities – so are not very scalable. Researchers from Belgium and France have now managed to overcome some of these limitations, reporting their findings in Nature Communications.

“We wanted to demonstrate that photonic chips can be as versatile as electronic chips, and one of the fields where the two overlap is that of microwave photonics,” one of the paper’s lead authors, Wim Bogaerts from Ghent University, tells Physics World.

A photonic engine

The researchers have created a photonic engine that processes microwave and optical signals and can convert the signals between the two domains. It is a silicon chip that can generate and detect optical and analogue electrical signals. The chip uses a combination of tuneable lasers (created by using an optical amplifier with on-chip filter circuits), electro-optic modulators and photodetectors, low-loss waveguides and passive components, and a programmable optical filter – which enables the chip to filter signals in both domains.

“We managed to integrate all key functionalities for manipulation of microwave signals and optical signals together on a single silicon chip and use that chip as a programmable engine in different experimental demonstrations,” says Bogaerts.

This setup allowed the researchers to operate the chip as a black-box microwave photonics processor, where the user can process high-frequency RF signals, without being exposed to the internal optical operations (they are hidden).

Optical signals from an external optical fibre are coupled to the chip using a grating coupler and high-speed RF signals are fed into the chip using electro-optic modulators. The RF signal is imprinted into an optical carrier wavelength – which is generated by the on-chip laser – and the signal is then processed on the chip using an optical filter bank. The signal then gets converted back into an RF signal using photodetectors.

All of the signals travelling into and out of the chip can be confined to the RF domain, so the chip doesn’t require any external optical components, unlike many other MWP devices. Moreover, the signals are locally programmed and tuned using thermo-optic phase shifters, enabling users to select any combination of microwave and optical inputs and outputs across the chip.

Extensive applications

The researchers used the photonic engine to create multiple systems that showcase its different optical and RF signal processing capabilities and demonstrate a potential pathway towards smaller MWP systems for high-speed wireless communication networks and microwave sensing applications.

As well as being used for simple light-tuning applications, the chip can also perform optical-to-electrical signal conversion, electrical-to-optical signal conversion, microwave frequency doubling, and microwave/optical filtering and equalization. These functions allow it to be used as a transmitter, receiver, optical/microwave filter, frequency converter or a tuneable opto-electronic oscillator.

When asked about the future of the chip, Bogaerts states that “we plan combine this functionality with more general purpose photonic circuits to enable even more functions and applications to help product developers roll out new photonic products as easily as new electronics products”.

Some other potential applications for the chip that have been touted – but not physically tested in this study – include RF instantaneous frequency measuring, radio-over-fibre links, RF phase tuning, optical and RF switching, optical sensing and signal temporal computing. With so many possibilities, this small-scale and low-power chip could become increasingly important as technologies such as communications advance further.

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It was a crisp, chilly morning in Bombay (Mumbai) on 4 December 2024 as delegates made their way to the Indian Institute of Technology, Bombay (IITB) for the opening day of the Women in Optics and Photonics in India-Asia (WOPI) conference. The cool, air-conditioned auditorium was soon packed with almost 300 people – mostly female postgraduate students and researchers – with notepads in hand and lanyards around their necks, eager to learn from the talks ahead.

The three-day event was organized by the IITB and sponsored by institutions, companies and publishers, including IOP Publishing, which publishes Physics World. Aimed at highlighting female voices that often go unheard, the meeting brought together scientists and experts from all over the world.

Yet not everything went according to plan. Although the room was full of curious students, each time the floor opened for questions, nervous glances were exchanged. A few moments of silence lingered as though everyone was waiting for a “better” question to come along. The silence rung of missed opportunities.

This kind of reticence is not new. From classrooms and labs to meetings, we see hesitance by students, and especially female students, everywhere. Growing up, I was told to look up the basics first and only ask so-called “high-level” questions. Good advice in theory but not if you’re already unsure of your place in the room. Add to that a dismissive answer such as “You should have known this by now” and the urge to speak up is nipped right in the bud.

Perhaps the hesitation is understandable. We live in a world where every answer is an Internet search away. Why ask a “silly” question when everyone else probably already knows better – or at least looks like they do? Yet asking questions is a fundamental part of learning. Science doesn’t move forward because people stay quiet until they have the perfect question, it moves because someone dares to ask – even when they aren’t sure.

Another contributing factor to such silence is imposter syndrome –  the feeling that you don’t deserve to be there and aren’t good at your work. Multiple studies have shown that women consistently score higher on measures of imposter syndrome than men. Women in technical fields such as engineering, science and maths also report lower self-efficacy than men, regardless of actual performance or ability – so not only do we question whether we belong but we also underestimate ourselves.

All of this means women are less likely to ask questions or speak up when uncertain. But for a scientist, uncertainty is the norm. We spend most of our time sitting with the unknown and with it the need to ask questions and chip away at it. Yet the very process of scientific inquiry can feel to women like a trap.

A new way

So what can we do differently? Events like WOPI create time and space not just for presenting research and innovation, but for mentorship, for insight into the real-world machinery of science. It’s not just about “What are you working on?” but also “Where are you heading, and who’s with you?”

WOPI 2024 modelled a new approach to inclusivity by running panel sessions that included the families of successful leaders to showcase the kind of support that is necessary to “make it”. Women were invited who fearlessly shared their stories of pivoted careers and/or failures, while acknowledging the challenges that they encountered along the way. It reminded us that you don’t just grow by knowing but by asking, exploring and doing.

But we need to do more. Encouraging young female scientists today means accounting for the weight of cultural expectations, social atmosphere and gender-based tendencies. Events and conferences need to go beyond formal settings to highlight not just the science but the scaffolding that holds it all together – networking, informal mentorship, vulnerability and visibility. This must be integrated into the very fabric of all scientific events, not just those for women.

The real pulse of the WOPI conference was in the moments after the talks – students lining up to talk to speakers. Nervous, curious and determined to ask their questions anyway. That moment stayed with me. It was steadfast curiosity that had survived immense self-doubt and fear of speaking in front of an audience.

Hopefully at the next WOPI meeting to be held in India in 2026, students will be much more confident in asking questions. So, to every student: don’t wait for the perfect moment. Ask the question that’s been sitting at the edge of your mind, making you wonder. After all, that is where discovery starts.

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Proton energies achievable in laser accelerators could be tripled by using specially designed micronozzle targets, according to computer simulations done by physicists in Japan and India. In their design, the electric field generated in the micronozzle would be funnelled towards the outgoing protons, allowing the acceleration to proceed for much longer. The researchers believe that the research could be useful in nuclear fusion, hadron therapy and materials science.

Conventional accelerators use oscillating electric fields to drive charged particles to relativistic speeds. The Large Hadron Collider at CERN, for example, uses radio-frequency oscillations to achieve proton energies of nearly 7 TeV.

These accelerators tend to be very large, which limits where they can be built. Laser acceleration, which involves using high-energy laser pulses to accelerate charged particle, offers a way to create much more compact accelerators.

Crucial to inertial confinement

Laser acceleration is crucial to inertial confinement fusion, and high energy proton beams produced by laser accelerators are used in scientific laboratories for a variety of scientific applications including laboratory astrophysics.

The standard techniques for laser acceleration involve firing a laser pulse at a proton target surrounded by metal foil. Solid hydrogen only exists near absolute zero, so the proton target can be a hydrogen-rich compound such as a hydride or a polymer. The femtosecond laser pulse concentrates a huge amount of energy into a tiny area and this instantly turns the target into a plasma. The light’s oscillating electromagnetic field drives electrons through the plasma, leaving behind the much heavier ions and creating a huge electric field that can accelerate protons.

In the new work, physicist Masakatsu Murakami and colleagues at the University of Osaka in Japan, together with researchers at the Indian Institute of Technology Hyderabad, used computer modelling to examine the effect of changing the shape of the metal surrounding the target from a simple planar foil to a two-headed nozzle, with the target placed at the narrowest point. During the first stage of the acceleration process, the wide head of the nozzle behaves like a lens, concentrating the electric field from a wide area to produce an enhanced flow of hot electrons towards the centre. This electric current on the nozzle enhances ablation of protons from the hydrogen rod, kicking them forward into the vacuum.

“Just like a rocket nozzle”

Subsequently, the electrons keep moving through the “skirt” of the nozzle, creating a powerful electric field that, owing to the nozzle’s shape, remains focused on the accelerating proton pulse as it travels away into the vacuum. “With the single hydrogen rod and the single foil, the protons are accelerated only during the laser illumination,” explains Murakami. “However, interestingly with the micronozzle target, the acceleration keeps going even after the laser pulse illumination…Most of the plasma expands in a small volume together with the protons – just like a rocket nozzle,” he says. Whereas the standard proton energies achievable with a laser accelerator today are around 400 MeV, the researchers estimate that their micronozzle design could allow energies into the gigaelectronvolt regime without changing anything else.

Murakami has been studying nuclear fusion for 40 years and believes that “this method will be used for fast ignition of laser fusion”. However, he says, its potential uses go far beyond this. Proton beam therapy generally uses protons with energies of 200–300 MeV to treat cancer by delivering a high dose of radiation to the tumour and a much lower dose to surrounding healthy tissue. “Even higher energy is required to target cancers that are located in deeper parts of the body,” he says. The technique could also be useful for materials science techniques such as proton radiography or for simulation of the physics of astrophysical objects such as neutron stars. “I’m planning to do proof of principle experiments in the near future,” says Murakami. 

Accelerator physicist Nicholas Dover of Imperial College London describes the work as “very interesting,” adding, “This target that they propose is a very complex thing to make. It would be a big project for a target fabrication lab to generate something like this – it’s not something we just cook up in our lab. Having these numerical optimizations is really helpful for us.” He notes, however, that one reason accelerator physicists often use planar targets (essentially pieces of kitchen foil) is the need to replace them in every shot. In scientific applications, this may not matter, he says. Applications in fields like medicine, however, would probably require the development of mass production facilities to fabricate the targets economically.

The research is described in Scientific Reports.

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A bumper crop of measurements of the expansion rate of the universe have stretched the Hubble tension as taut as it has ever been, with scientists grappling with trying to find a solution.

Over 500 researchers have come together in the “CosmoVerse” consortium to produce a new white paper that delves into the various cosmological tensions between theory and observation. These include the Hubble tension, which is the bewildering discrepancy in the expansion rate of the universe, referred to as the Hubble constant (H₀).

Predictive measurements made by applying the standard model of cosmology to the cosmic microwave background (CMB) give H₀ as 67.4 km/s/Mpc. In other words, every volume of space a million parsecs across (one parsec is 3.26 light years) should be expanding by 67.4 kilometres every second.

Yet that’s not what Hubble’s law – which tells us the expansion rate based on a given object’s velocity away from us and its distance – says, as demonstrated by the CosmoVerse White Paper.

“The paper’s been getting a lot of attention in our field,” Joe Jensen of Utah Valley University tells Physics World. “You can easily see that the vast majority of measurements fall around 73 km/s/Mpc, with varying uncertainties.”

There’s no known reason why local measurements of H₀ (based on supernovae observations) should differ from the CMB measurement. This discrepancy leads to two possibilities. Either there are unknown systematic uncertainties in measurements that skew the results, or cosmology’s standard model is wrong and new physics is needed.

A lot at stake

The highest rung on the cosmic distance ladder is a type Ia supernova – a white dwarf explosion. They have a standardizable brightness that makes them perfect for judging how far away they are, based on their luminosity curve. These measurements are calibrated by lower rungs on the ladder, such as Cepheid variable stars or the peak brightness of red giant stars (referred to as the “tip of the red giant branch”, or TRGB).

If the tension is real, then different calibrators should still give the same result. One of the few outliers is found in a new paper published in The Astrophysical Journal by the Chicago–Carnegie Hubble Program (CCHP) led by the University of Chicago’s Wendy Freedman.

CCHP’s latest paper uses the TRGB to arrive at a best value of 70.39 km/s/Mpc when combining measurements from the James Webb Space Telescope (JWST) – which is able to better resolve red giant stars in other galaxies – with Hubble Space Telescope data.

The CCHP team argue that this result is in line with the CMB measurements and removes the tension. However, their conclusion has met opposition.

“Their result is sort of in the middle of the Hubble tension, so I’m surprised that they would say they rule it out,” Dan Scolnic, an astrophysicist at Duke University in the United States, tells Physics World.

At a meeting of the American Astronomical Society in January 2025, Scolnic declared that the Hubble tension was now a crisis. CCHP’s results do not dissuade him from this conclusion.

“For some reason they don’t include a number of supernovae in their sample that they could have,” says Scolnic. “Siyang Li [of Johns Hopkins University] led a paper [on which Scolnic is a co-author] that showed that if one uses their TRGB measurements, and the complete sample of supernovae, one goes back to higher H₀.”

Freedman did not respond to Physics World‘s request for an interview.

Different approaches

Jensen has also led a team that recently conducted measurements of H₀ using TRGB stars, but in a different way by looking for surface brightness fluctuations (SBF).

“SBF is a statistical method that measures the brightnesses of red giant stars even when they cannot be measured individually,” says Jensen.

Individual stars in galaxies cannot be resolved at great distance – their light blends together, and the more distant the galaxy, the smoother this blend is. We describe this blended light as the galaxy’s surface brightness, and fluctuations are statistical in nature and result from the discrete nature of stars.

In old elliptical galaxies, the surface brightness is dominated by red giant stars, which are evolved Sun-like stars. Measuring the SBF therefore provides a value for the TRGB, from which a distance can be determined.

Using JWST images to measure the SBF of 14 elliptical galaxies, then using those to calibrate the distances to 60 more distant ellipticals, and then using that calibration to determine H₀, Jensen’s team arrived at a value of 73.8 km/s/Mpc.

“The reason that we don’t get the same answer [as CCHP] is that we are not using the same JWST calibrators, and we don’t use type Ia to measure H₀,” says Jensen.

This contradicts CCHP’s main assertion, which is that there must be unknown systematic uncertainties in either the type Ia supernovae or the Cepheids. Jensen’s team use neither, yet still find a tension.

Perhaps the most convincing evidence for the tension comes from the TDCOSMO (time-delay cosmography) team, who utilize gravitationally lensed quasars to measure H₀.

Quasars fluctuate in brightness over a matter of days. When light from a quasar takes paths of varying lengths around a lensing object, it produces multiple images that have time lags relative to one another. The expansion of space can extend this time delay, providing a completely independent measure of H₀.

In 2019 the H0LiCOW project used six gravitational lenses to arrive at a value of 73.3 km/s/Mpc. This result came with some scepticism. So they formed the new TDCOSMO consortium and “went on a six-year journey to see if their original measurement was okay,” says Scolnic.

TDCOSMO’s final conclusion is 72.1 km/Mpc/s, strongly supporting the tension. However, in all these measurements there’s wriggle room from various known measuring uncertainties.

“It’s important to remember that the uncertainties put us in only mild disagreement,” says Jensen. “I expect that we will soon know if the disagreement can be explained by the mundane choices of calibration galaxies and processing techniques.”

If it cannot, then the inescapable conclusion is that there’s something wrong with our understanding of the universe. Figuring that out could be the next great quest in cosmology.

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Independent measurements of the charge radius of the helium-3 nucleus using two different  methods have yielded significantly different results – prompting a re-evaluation of underlying theory to reconcile them. The international CREMA Collaboration used muonic helium-3 ions to determine the radius, whereas a team in the Netherlands used a quantum-degenerate gas of helium-3 atoms.

The charge radius is a statistical measure of how far the electric charge of a particle extends into space. Both groups were mystified by the discrepancy in the values – which hints at physics beyond the Standard Model of particle physics. However, new theoretical calculations inspired by the results may have already resolved the discrepancy.

Both groups studied the difference between the charge radii of the helium-3 and helium-4 nuclei. CREMA used muonic helium ions, in which the remaining electrons replaced by muons. Muons are much more massive than electrons, so they spend more time near the nucleus – and are therefore more sensitive to the charge radius.

Shorter wavelengths

Muonic atoms have spectra at much shorter wavelengths than normal atoms. This affects values such as the Lamb shift. This is the energy difference in the 2S_1/2 and 2P_1/2 atomic states, which are split by interactions with virtual photons and vacuum polarization. This is most intense near the nucleus. More importantly, a muon in an S orbital becomes more sensitive to the finite size of the nucleus.

In 2010, CREMA used the charge radius of muonic hydrogen to conclude that the charge radius of the proton is significantly smaller than the current accepted value. The same procedure was then used with muonic helium-4 ions. Now, CREMA has used pulsed laser spectroscopy of muonic helium-3 ions to extract several key parameters including the Lamb shift and used them to calculate the charge radius of muonic helium-3 nuclei. They then calculated the difference with the charge radius in helium-4. The value they obtained was 15 times more accurate than any previously reported.

Meanwhile, at the Free University of Amsterdam in the Netherlands, researchers were taking a different approach, using conventional helium-3 atoms. This has significant challenges, because the effect of the nucleus on electrons is much smaller. However, it also means that an electron affects the nucleus it measures less than does a muon, which mitigates a source of theoretical uncertainty.

The Amsterdam team utilized the fact that the 2S triplet state in helium is extremely long-lived. ”If you manage to get the atom up there, it’s like a new ground state, and that means you can do laser cooling on it and it allows very efficient detection of the atoms,” explains Kjeld Eikema, one of the team’s leaders after its initial leader Wim Vassen died in 2019. In 2018, the Amsterdam group created an ultracold Bose–Einstein condensate (BEC) of helium-4 atoms in the 2S triplet state in an optical dipole trap before using laser spectroscopy to measure the ultra-narrow transition between the 2S triplet state and the higher 2S singlet state.

Degenerate Fermi gas

In the new work, the researchers turned to helium-3, which does not form a BEC but instead forms a degenerate Fermi gas. Interpreting the spectra of this required new discoveries itself. “Current theoretical models are insufficiently accurate to determine the charge radii from measurements on two-electron atoms,” Eikema explains. However, “the nice thing is that if you measure the transition directly in one isotope and then look at the difference with the other isotope, then most complications from the two electrons are common mode and drop out,” he says. This can be used to the determine the difference in the charge radii.

The researchers obtained a value that was even more precise than CREMA’s and larger by 3.6σ. The groups could find no obvious explanation for the discrepancy. “The scope of the physics involved in doing and interpreting these experiments is quite massive,” says Eikema; “a comparison is so interesting, because you can say ‘Well, is all this physics correct then? Are electrons and muons the same aside from their mass? Did we do the quantum electrodynamics correct for both normal atoms and muonic atoms? Did we do the nuclear polarization correctly?’” The results of both teams are described in Science (CREMA, Amsterdam).

While these papers were undergoing peer review, the work attracted the attention of two groups of theoretical physicists – one led by Xiao-Qiu Qi f the Wuhan Institute of Physics and Mathematics in China, and the other by Krzysztof Pachucki of the University of Warsaw in Poland. Both revised the calculation of the hyperfine structure of helium-3, finding that incorporating previously neglected higher orders into the calculation produced an unexpectedly large shift.

“Suddenly, by plugging this new value into our experiment – ping! – our determination comes within 1.2σ of theirs,” says Eikema; “which is a triumph for all the physics involved, and it shows how, by showing there’s a difference, other people think, ‘Maybe we should go and check our calculations,’ and it has improved the calculation of the hyperfine effect.” In this manner the ever improving experiments and theory calculations continue to seek the limits of the Standard Model.

Xiao-Qiu Qi and colleagues describe their calculations in Physical Review Research, while Pachucki’s team have published in Physical Review A.

Eikema adds “Personally I would have adjusted the value in our paper according to these new calculations, but Science preferred to keep the paper as it was at the time of submission and peer review, with an added final paragraph to explain the latest developments.”

Theoretical physicist Marko Horbatsch at Canada’s York University is impressed by the experimental results and bemused by the presentation. “I would say that their final answer is a great success,” he concludes. “There is validity in having the CREMA and Eikema work published side-by-side in a high-impact journal. It’s just that the fact that they agree should not be confined to a final sentence at the end of the paper.”

The post Conflicting measurements of helium’s charge radius may be reconciled by new calculations appeared first on Physics World.

Artificial intelligence (AI) has the potential to generate a sea change in the practice of radiology, much like the introduction of radiology information system (RIS) and picture archiving and communication system (PACS) technology did in the late 1990s and 2000s. However, AI-driven software must be accurate, safe and trustworthy, factors that may not be easy to assess.

Machine learning software is trained on databases of radiology images. But these images might lack the data or procedures needed to prevent algorithmic bias. Such algorithmic bias can cause clinical errors and performance disparities that affect a subset of the analyses that the AI performs, unintentionally disadvantaging certain groups of patients.

A multinational team of radiology informaticists, biomedical engineers and computer scientists has identified potential pitfalls in the evaluation and measurement of algorithmic bias in AI radiology models. Describing their findings in Radiology, the researchers also suggest best practices and future directions to mitigate bias in three key areas: medical image datasets; demographic definitions; and statistical evaluations of bias.

Medical imaging datasets

The medical image datasets used for training and evaluation of AI algorithms are reflective of the population from which they are acquired. It is natural that a dataset acquired in a country in Asia will not be representative of the population in a Nordic country, for example. But if there’s no information available about the image acquisition location, how might this potential source of bias be determined?

Lead author Paul Yi, of St. Jude Children’s Research Hospital in Memphis, TN, and coauthors advise that many existing medical imaging databases lack a comprehensive set of demographic characteristics, such as age, sex, gender, race and ethnicity. Additional potential confounding factors include the scanner brand and model, the radiology protocols used for image acquisition, radiographic views acquired, the hospital location and disease prevalence. In addition to incorporating these data, the authors recommend that raw image data are collected and shared without institution-specific post-processing.

The team advise that generative AI, a set of machine learning techniques that generate new data, provides the potential to create synthetic imaging datasets with more balanced representation of both demographic and confounding variables. This technology is still in development, but might provide a solution to overcome pitfalls related to measurement of AI biases in imperfect datasets.

Defining demographics

Radiology researchers lack consensus with respect to how demographic variables should be defined. Observing that demographic categories such as gender and race are self-identified characteristics informed by many factors, including society and lived experiences, the authors advise that concepts of race and ethnicity do not necessarily translate outside of a specific society and that biracial individuals reflect additional complexity and ambiguity.

They emphasize that ensuring accurate measurements of race- and/or ethnicity-based biases in AI models is important to enable accurate comparison of bias evaluations. This not only has clinical implications, but is also essential to prevent health policies being established in error from erroneous AI-derived findings, which could potentially perpetuate pre-existing inequities.

Statistical evaluations of bias

The researchers define bias in the context of demographic fairness and how it reflects differences in metrics between demographic groups. However, establishing consensus on the definition of bias is complex, because bias can have different clinical and technical meanings. They point out that in statistics, bias refers to a discrepancy between the expected value of an estimated parameter and its true value.

As such, the radiology speciality needs to establish a standard notion of bias, as well as tackle the incompatibility of fairness metrics, the tools that measure whether a machine learning model treats certain demographic groups differently. Currently there is no universal fairness metric that can be applied to all cases and problems, and the authors do not think there ever will be one.

The different operating points of predictive AI models may result in different performance that could lead to potentially different demographic biases. These need to be documented, and thresholds should be included in research and by commercial AI software vendors.

Key recommendations

The authors suggest some key courses of action to mitigate demographic biases in AI in radiology:

• Improve reporting of demographics by establishing a consensus panel to define and update reporting standards.
• Improve dataset reporting of non-demographic factors, such as imaging scanner vendor and model.
• Develop a standard lexicon of terminology for concepts of fairness and AI bias concepts in radiology.
• Develop standardized statistical analysis frameworks for evaluating demographic bias of AI algorithms based on clinical contexts
• Require greater demographic detail to evaluate algorithmic fairness in scientific manuscripts relating to AI models.

Yi and co-lead collaborator Jeremias Sulam, of Hopkins BME, Whiting School of Engineering, tell Physics World that their assessment of pitfalls and recommendations to mitigate demographic biases reflect years of multidisciplinary discussion. “While both the clinical and computer science literature had been discussing algorithmic bias with great enthusiasm, we learned quickly that the statistical notions of algorithmic bias and fairness were often quite different between the two fields,” says Yi.

“We noticed that progress to minimize demographic biases in AI models is often hindered by a lack of effective communication between the computer science and statistics communities and the clinical world, radiology in particular,” adds Sulam.

A collective effort to address the challenges posed by bias and fairness is important, notes Melissa Davis of Yale School of Medicine, in an accompanying editorial in Radiology. “By fostering collaboration between clinicians, researchers, regulators and industry stakeholders, the healthcare community can develop robust frameworks that prioritize patient safety and equitable outcomes,” she writes.

The post AI algorithms in radiology: how to identify and prevent inadvertent bias appeared first on Physics World.

Laser World of Photonics, the leading trade show for the laser and photonics industry, takes place in Munich from 24 to 27 June. Attracting visitors and exhibitors from around the world, the event features 11 exhibition areas covering the entire spectrum of photonic technologies – including illumination and energy, biophotonics, data transmission, integrated photonics, laser systems, optoelectronics, sensors and much more.

Running parallel and co-located with Laser World of Photonics is World of Quantum, the world’s largest trade fair for quantum technologies. Showcasing all aspects of quantum technologies – from quantum sensors and quantum computers to quantum communications and cryptography – the event provides a platform to present innovative quantum-based products and discuss potential applications.

Finally, the World of Photonics Congress (running from 22 to 27 June) features seven specialist conferences, over 3000 lectures and around 6700 experts from scientific and industrial research.

The event is expecting to attract around 40,000 visitors from 70 countries, with the trade shows incorporating 1300 exhibitors from 40 countries. Here are some of the companies and product innovations to look out for on the show floor.

HOLOEYE unveils compact 4K resolution spatial light modulator

HOLOEYE Photonics AG, a leading provider of spatial light modulator (SLM) devices, announces the release of the GAEA-C spatial light modulator, a compact version of the company’s high-resolution SLM series. The GAEA-C will be officially launched at Laser World of Photonics, showcasing its advanced capabilities and cost-effective design.

The GAEA-C is a phase-only SLM with a 4K resolution of 4094 x 2400 pixels, with an exceptionally small pixel pitch of 3.74 µm. This compact model is equipped with a newly developed driver solution that not only reduces costs but also enhances phase stability, making it ideal for a variety of applications requiring precise light modulation.

The GAEA-C SLM features a reflective liquid crystal on silicon (LCOS) display (phase only). Other parameters include a fill factor of 90%, an input frame rate of 30 Hz and a maximum spatial resolution of 133.5 lp/mm.

The GAEA-C is available in three versions, each optimized for a different wavelength range: a VIS version (420–650 nm), a NIR version (650–1100 nm) and a version tailored for the telecommunications waveband around 1550 nm. This versatility ensures that the GAEA-C can meet the diverse needs of industries ranging from telecoms to scientific research.

HOLOEYE continues to lead the market with its innovative SLM solutions, providing unparalleled resolution and performance. The introduction of the GAEA-C underscores HOLOEYE’s commitment to delivering cutting-edge technology that meets the evolving demands of its customers.

• For more information about the GAEA-C and other SLM products, visit HOLOEYE at booth #225 in Hall A2.

Avantes launches NIR Enhanced spectrometers

At this year’s Laser World of Photonics, Avantes unveils its newest generation of spectrometers: the NEXOS NIR Enhanced and VARIUS NIR Enhanced. Both instruments mark a significant leap in near-infrared (NIR) spectroscopy, offering up to 2x improved sensitivity and unprecedented data quality for integration into both research and industry applications.

Compact, robust and highly modular, the NEXOS NIR Enhanced spectrometer redefines performance in a small form factor. It features enhanced NIR quantum efficiency in the 700–1100 nm range, with up to 2x increased sensitivity, fast data transfer and improved signal-to-noise ratio. The USB-powered spectrometer is designed with a minimal footprint of just 105 x 80 x 20 mm and built using AvaMation production for top-tier reproducibility and scalability. It also offers seamless integration with third-party software platforms.

The NEXOS NIR Enhanced is ideal for food sorting, Raman applications and VCSEL/laser system integration, providing research-grade performance in a compact housing. See the NEXOS NIR Enhanced product page for further information.

Designed for flexibility and demanding industrial environments, the VARIUS NIR Enhanced spectrometer introduces a patented optical bench for supreme accuracy, with replaceable slits for versatile configurations. The spectrometer offers a dual interface – USB 3.0 and Gigabit Ethernet – plus superior stray light suppression, high dynamic range and enhanced NIR sensitivity in the 700–1100 nm region.

With its rugged form factor (183 x 130 x 45.2 mm) and semi-automated production process, the VARIUS NIR is optimized for real-time applications, ensuring fast data throughput and exceptional reliability across industries. For further information, see the VARIUS NIR Enhanced product page.

Avantes invites visitors to experience both systems live at Laser World of Photonics 2025. Meet the team for hands-on demonstrations, product insights and expert consultations. Avantes offers free feasibility studies and tailored advice to help you identify the optimal solution for your spectroscopy challenges.

• For more information, visit www.avantes.com or meet Avantes at booth #218 in Hall A3.

HydraHarp 500: a new era in time-correlated single-photon counting

Laser World of Photonics sees PicoQuant introduce its newest generation of event timer and time-correlated single-photon counting (TCSPC) unit – the HydraHarp 500. Setting a new standard in speed, precision and flexibility, the TCSPC unit is freely scalable with up to 16 independent channels and a common sync channel, which can also serve as an additional detection channel if no sync is required.

At the core of the HydraHarp 500 is its outstanding timing precision and accuracy, enabling precise photon timing measurements at exceptionally high data rates, even in demanding applications.

In addition to the scalable channel configuration, the HydraHarp 500 offers flexible trigger options to support a wide range of detectors, from single-photon avalanche diodes to superconducting nanowire single-photon detectors. Seamless integration is ensured through versatile interfaces such as USB 3.0 or an external FPGA interface for data transfer, while White Rabbit synchronization allows precise cross-device coordination for distributed setups.

The HydraHarp 500 is engineered for high-throughput applications, making it ideal for rapid, large-volume data acquisition. It offers 16+1 fully independent channels for true simultaneous multi-channel data recording and efficient data transfer via USB or the dedicated FPGA interface. Additionally, the HydraHarp 500 boasts industry-leading, extremely low dead-time per channel and no dead-time across channels, ensuring comprehensive datasets for precise statistical analysis.

The HydraHarp 500 is fully compatible with UniHarp, a sleek, powerful and intuitive graphical user interface. UniHarp revolutionizes the interaction with PicoQuant’s TCSPC and time tagging electronics, offering seamless access to advanced measurement modes like time trace, histogram, unfold, raw and correlation (including FCS and g²).

Step into the future of photonics and quantum research with the HydraHarp 500. Whether it’s achieving precise photon correlation measurements, ensuring reproducible results or integrating advanced setups, the HydraHarp 500 redefines what’s possible – offering precision, flexibility and efficiency combined with reliability and seamless integration to achieve breakthrough results.

For more information, visit www.picoquant.com or contact us at info@picoquant.com.

• Meet PicoQuant at booth #216 in Hall B2.

SmarAct showcases integrated, high-precision technologies

With a strong focus on turnkey, application-specific solutions, SmarAct offers nanometre-precise motion systems, measurement equipment and scalable micro-assembly platforms for photonics, quantum technologies, semiconductor manufacturing and materials research – whether in research laboratories or high-throughput production environments.

At Laser World of Photonics, SmarAct presents a new modular multi-axis positioning system for quantum computing applications and photonic integrated circuit (PIC) testing. The compact system is made entirely from titanium and features a central XY stage with integrated rotation, flanked by two XYZ modules – one equipped with a tip-tilt goniometer.

For cryogenic applications, the system can be equipped with cold plates and copper braids to provide a highly stable temperature environment, even at millikelvin levels. Thanks to its modularity, the platform can be reconfigured for tasks such as low-temperature scanning or NV centre characterization. When combined with SmarAct’s interferometric sensors, the system delivers unmatched accuracy and long-term stability under extreme conditions.

Also debuting is the SGF series of flexure-based goniometers – compact, zero-backlash rotation stages developed in collaboration with the University of Twente. Constructed entirely from non-ferromagnetic materials, the goniometers are ideal for quantum optics, electron and ion beam systems. Their precision has been validated in a research paper presented at EUSPEN 2023.

Targeting the evolving semiconductor and photonics markets, SmarAct’s optical assembly platforms enable nanometre-accurate alignment and integration of optical components. At their core is a modular high-performance toolkit for application-specific configurations, with the new SmarAct robot control software serving as the digital backbone. Key components include SMARPOD parallel kinematic platforms, long-travel SMARSHIFT electromagnetic linear stages and ultraprecise microgrippers – all seamlessly integrated to perform complex optical alignment tasks with maximum efficiency.

Highlights at Laser World of Photonics include a gantry-based assembly system developed for the active alignment of beam splitters and ferrules, and a compact, fully automated fibre array assembly system designed for multicore and polarization-maintaining fibres. Also on display are modular probing systems for fast, accurate and reliable alignment of fibres and optical elements – providing the positioning precision required for chip- and wafer-level testing of PICs prior to packaging. Finally, the microassembly platform P50 from SmarAct Automation offers a turnkey solution for automating critical micro-assembly tasks such as handling, alignment and joining of tiny components.

Whether you’re working on photonic chip packaging, quantum instrumentation, miniaturized medical systems or advanced semiconductor metrology, SmarAct invites researchers, engineers and decision-makers to experience next-generation positioning, automation and metrology solutions live in Munich.

• Visit SmarAct at booth #107 in Hall B2.

 

The post Laser World of Photonics showcases cutting-edge optical innovation appeared first on Physics World.

A new solid-state laser can make a vast number of precise optical measurements each second, while sweeping across a broad range of optical wavelengths. Created by a team led by Qiang Lin at the University of Rochester in the US, the device can be fully integrated onto a single chip.

Optical metrology is a highly versatile technique that uses light to gather information about the physical properties of target objects. It involves illuminating a sample and measuring the results with great precision – using techniques such as interferometry and spectroscopy. In the 1960s, the introduction of lasers and the coherent light they emit boosted the technique to an unprecedented level of precision. This paved the way for advances ranging from optical clocks, to the detection of gravitational waves.

Yet despite the indispensable role they have played so far, lasers have also created a difficult challenge. To ensure the best possible precision, experimentalists much achieve very tight control over the wavelength, phase, polarization and other properties of the laser light. This is very difficult to do within the tiny solid-state laser diodes that are very useful in metrology.

Currently, the light from laser diodes is improved externally using optical modules. This added infrastructure is inherently bulky and it remains difficult to integrate the entire setup onto chip-scale components – which limits the development of small, fast lasers for metrology.

Two innovations

Lin and colleagues addressed this challenge by designing a new laser with two key components. One is a laser cavity that comprises a thin film of lithium niobate. Thanks to the Pockels effect, this material’s refractive index can vary depending on the strength of an applied electric field. This provides control over the wavelength of the light amplified by the cavity.

The other component is a distributed Bragg reflector (DBR), which is a structure containing periodic grooves that create alternating regions of refractive index. With the right spacing of these grooves, a DBR can strongly reflect light at a single, narrow linewidth, while scattering all other wavelengths. In previous studies, lasers were created by etching a DBR directly onto a lithium niobate film – but due to the material’s optical properties, this resulted in a broad linewidth.

“Instead, we developed an ‘extended DBR’ structure, where the Bragg grating is defined in a silica cladding,” explains team member Mingxiao Li at the University of California Santa Barbara. “This allowed for flexible control over the grating strength, via the thickness and etch depth of the cladding. It also leverages silica’s superior etchability to achieve low scattering strength, which is essential for narrow linewidth operation.”

Using a system of integrated electrodes, Lin’s team can adjust the strength of the electric field they applied to the lithium niobate film. This allows them to rapidly tune the wavelengths amplified by the cavity via the Pockels effect. In addition, they used a specially designed waveguide to control the phase of light passing into the cavity. This design enabled them to tune their laser over a broad range of wavelengths, without needing external correction modules to achieve narrow linewidths.

Narrowband performance

Altogether, the laser demonstrated an outstanding performance on a single chip – producing a clean, single wavelength with very little noise. Most importantly, the light had a linewidth of just 167 Hz – the smallest range achieved to date for a single-chip lithium niobate laser. This exceptional performance enabled the laser to rapidly sweep across a bandwidth of over 10 GHz – equivalent to scanning quintillions of points per second.

“These capabilities translated directly into successful applications,” Li describes. “The laser served as the core light source in a high-speed LIDAR system, measuring the velocity of a target 0.4 m away with better than 2 cm distance resolution. The system supports a velocity measurement as high as Earth’s orbital velocity – around 7.91 km/s – at 1 m.” Furthermore, Lin’s team were able to lock their laser’s frequency with a reference gas cell, integrated directly onto the same chip.

By eliminating the need for bulky control modules, the team’s design could now pave the way for the full miniaturization of optical metrology – with immediate benefits for technologies including optical clocks, quantum computers, self-driving vehicles, and many others.

“Beyond these, the laser’s core advantages – exceptional coherence, multifunctional control, and scalable fabrication – position it as a versatile platform for transformative advances in high-speed communications, ultra-precise frequency generation, and microwave photonics,” Lin says.

The new laser is described in Light: Science & Applications.

The post Tiny laser delivers high-quality, narrowband light for metrology appeared first on Physics World.

The Shape of Wonder: How Scientists Think, Work and Live
By Alan Lightman and Martin Rees

In their delightful new book, cosmologist Martin Rees and physicist and science writer Alan Lightman seek to provide “an honest picture of scientists as people and how they work and think”. The Shape of Wonder does this by exploring the nature of science, examining the role of critical thinking, and looking at how scientific theories are created and revised as new evidence emerges. It also includes profiles of individual scientists, ranging from historical Nobel-prize winners such as physicist Werner Heisenberg and biologist Barbara McClintock, to rising stars like CERN theorist Dorota Grabowska. Matin Durrani

• 2025 Pantheon Books

Our Accidental Universe: Stories of Discovery from Asteroids to Aliens
By Chris Lintott

TV presenter and physics professor Chris Lintott brings all his charm and wit to his new book Our Accidental Universe. He looks at astronomy through the lens of the human errors and accidents that lead to new knowledge. It’s a loose theme that allows him to skip from the search for alien life to pulsars and the Hubble Space Telescope. Lintott has visited many of the facilities he discusses, and spoken to many people working in these areas, adding a personal touch to his stated aim of elucidating how science really gets done. Kate Gardner

• 2024 Penguin

Science is Lit: Awesome Electricity and Mad Magnets
By Big Manny (Emanuel Wallace)

Want to feed your child’s curiosity about how things work (and don’t mind creating a mini lab in your house)? Take a look at Awesome Electricity and Mad Magnets, the second in the Science is Lit series by Emanuel Wallace – aka TikTok star “Big Manny”. Wallace introduces four key concepts of physics – force, sound, light and electricity – in an enthusiastic and fun way that’s accessible for 8–12 year olds. With instructions for experiments kids can do at home, and a clear explanation of the scientific process, your child can really experience what it’s like to be a scientist. Sarah Tesh

• 2025 Puffin

Space Posters & Paintings: Art About NASA
By Bill Schwartz

Astronomy is the most visually gifted of all the sciences, with endless stunning photographs of our cosmos. But perhaps what sets NASA apart from other space agencies is its art programme, which has existed since 1962. In Space Posters and Paintings: Art about NASA, documentary filmmaker Bill Schwartz has curated a striking collection of nostalgic artworks that paint the history of NASA and its various missions across the solar system and beyond. Particularly captivating are pioneering artist Robert McCall’s paintings of the Gemini and Apollo missions. This large-format coffee book is a perfect purchase for any astronomy buff. Tushna Commissariat

• 2024 ACC Art Books

The post Delving into the scientific mind, astronomy’s happy accidents, lit science experiments at home, the art of NASA: micro reviews of recent books appeared first on Physics World.

The administration of US president Donald Trump has proposed drastic cuts to science that would have severe consequence for physics and astronomy if passed by the US Congress. The proposal could involve the cancellation of one of the twin US-based gravitational-wave detectors as well as the axing of a proposed next-generation ground-based telescope and a suite of planned NASA mission. Scientific societies, groups of scientists and individuals have expressed their shock over the scale of the reductions.

In the budget request, which represents the start of the budgeting procedure for the year from 1 October, the National Science Foundation (NSF) would see its funding plummet from $9bn to just  $3.9bn – imperilling several significant projects. While the NSF had hoped to support both next-generation ground-based tele­scopes planned by the agency – the Giant Magellan Tele­scope (GMT) and the Thirty Meter Telescope (TMT) – the new budget would only allow one to be supported.

On 12 June the GMT, which is already 40% completed thanks to private funds, received NSF approval confirming that the observatory will advance into its “major facilities final design phase”, one of the final steps before becoming eligible for federal construction funding. The TMT, meanwhile, which is set to be built in Hawaii, has been hit with delays following protests over adding more telescopes to Mauna Kea. In a statement from the TMT International Observatory, it said it was “disappointed that the NSF’s current budget proposal does not include TMT”.

It is also possible that one of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities – one in Hanford, Washington and the other in Livingston, Louisiana – would have to close down after the budget proposes a 39.6% cut to LIGO operations. Having one LIGO facility would significantly cut its ability to identify and localize events that produce gravitational waves.

“This level of cut, if enacted, would drastically reduce the science coming out of LIGO and have long-term negative consequences for gravitational-wave astrophysics,” notes LIGO executive director David Reitze. LIGO officials told Physics World that the cuts would be “extremely punishing to US gravitational wave science” and would mean “layoffs to staff, reduced scientific output, and the loss of scientific leadership in a field that made first detections just under 10 years ago”.

NASA’s science funding, meanwhile, would reduce by 47% year on year, and the agency as a whole would see more than 5500 staff lose their jobs as its workforce gets slashed from 17 391 to just 11 853. NASA would also lose planned missions to Venus, Mars, Jupiter and the asteroid Apophis that will pass close to Earth in 2029. Several scientific missions focusing on planet Earth, meanwhile, would also be axed.

The American Astronomical Society expressed “grave concern” that the cuts to NASA and the NSF “would result in an historic decline of American investment in basic scientific research”. The Planetary Society called the proposed NASA budget “an extinction-level event for the space agency’s most productive, successful and broadly supported activity”. Before the cuts were announced, the Trump administration pulled its nomination of billionaire industrialist Jared Isaacman for NASA administrator after his supporter Elon Musk left his post as head of the “Department of Government Efficiency.”

‘The elephant in the room’

The Department of Energy, meanwhile, will receive a slight increase in its defence-related budget, from the current $34.0bn to next year’s proposed $33.8bn. But its non-defence budget will fall by 26% from $16.83bn to $12.48bn. Michael Kratsios, Trump’s science adviser and head of the White House Office of Science and Technology Policy, sought to justify the administration’s planned cuts in a meeting at the National Academy of Sciences (NAS) on 19 May.

“Spending more money on the wrong things is far worse than spending less money on the right things,” Kratsios noted, adding that the country had received “diminishing returns” on its investments in science over the past four decades and that it now requires “new methods and approaches to supporting research”. He also suggested that research now undertaken at US universities falls short of what he called “gold standard science”, citing “political biases [that] have displaced the vital search for truth”. Universities, he stated, have lost public trust because they have “promoted diversity, equity and inclusion”.

The US science community, however, is unconvinced. “The elephant in the room right now is whether the drastic reductions in research budgets and new research policies across the federal agencies will allow us to remain a research and development powerhouse,” says Marcia McNutt, president of the National Academy of Sciences. “Thus, we are embarking on a radical new experiment in what conditions promote science leadership – with the US being the ‘treatment’ group, and China as the control.”

Former presidential science adviser Neal Lane, now at Rice University, told Physics World that while the US administration appears to value some aspects of scientific research such as AI, quantum, nuclear and biotechnologies, it “doesn’t seem to understand or acknowledge that technological advances and innovation often come from basic research in unlikely fields of science“. He expects the science community to “continue to push back” by writing and visiting members of Congress, many of whom support science, and “by speaking out to the public and encouraging various organizations to do that same”.

Indeed, an open letter by the group Stand Up for Science dated 26 May calls the administration’s stated commitment to “gold standard science” an approach “that will actually undermine scientific rigor and the transparent progress of science”. It would “introduce stifling limits on intellectual freedom in our nation’s laboratories and federal funding agencies”, the letter adds.

As of 13 June, the letter had more than 9250 signatures. Another letter, sent to Jay Bhattachayra, director of the National Institutes of Health (NIH), from some 350 NIH members, almost 100 of whom identified themselves, asserted that they “remain pressured to implement harmful measures” such as halting clinical trials midstream. In the budget request, the NIH would lose about 40%, leaving it with $27.5bn next year. The administration also plans to consolidate the NIH’s 27 institutes into just eight.

A political divide

On the day that the budget was announced, 16 states run by Democratic governors called on a federal court to block cuts in programmes and funding for the NSF. They point out that universities in their states could lose significant income if the cuts go ahead. In fact, the administration’s budget proposal is just that: a proposal. Congress will almost certainly make changes to it before presenting it to Trump for his signature. And while Republicans in the Senate and House of Representatives find it difficult to oppose the administration, science has historically enjoyed support by both Democrats and Republicans.

Despite that, scientists are gearing up for a difficult summer of speculation about financial support. “We are gaming matters at the moment because we are looking at the next budget cycle,” says Peter Littlewood, chair of the University of Chicago’s physics department. “The principal issues now are to bridge postdocs and graduating PhD students, who are in limbo because offers are drying up.” Littlewood says that, while alternative sources of funding such as philanthropic contributions can help, if the proposed government cuts are approved then philanthropy can’t replace federal support. “I’m less worried about whether this or that piece of research gets done than in stabilizing the pipeline, so all our discussions centre around that,” adds Littlewood.

Lane fears the cuts will put people off from careers in science, even in the unlikely event that all the cuts get reversed. “The combination of statements by the president and other administrative officials do considerable harm by discouraging young people born in the US and other parts of the world from pursuing their education and careers in [science] in America,” he says. “That’s a loss for all Americans.”

The post US astronomy facing ‘extinction level’ event following Trump’s 2026 budget request appeared first on Physics World.

As the world celebrates the 2025 International Year of Quantum Science and Technology, it’s natural that we should focus on the exciting applications of quantum physics in computing, communication and cryptography. But quantum physics is also set to have a huge impact on medicine and healthcare. Quantum sensors, in particular, can help us to study the human body and improve medical diagnosis – in fact, several systems are close to being commercialized.

Quantum computers, meanwhile, could one day help us to discover new drugs by providing representations of atomic structures with greater accuracy and by speeding up calculations to identify potential drug reactions. But what other technologies and projects are out there? How can we forge new applications of quantum physics in healthcare and how can we help discover new potential use cases for the technology?

Those are the some of the questions tackled in a recent report, on which this Physics World article is based, published by Innovate UK in October 2024. Entitled Quantum for Life, the report aims to kickstart new collaborations by raising awareness of what quantum physics can do for the healthcare sector. While the report says quite a bit about quantum computing and quantum networking, this article will focus on quantum sensors, which are closer to being deployed.

Sense about sensors

The importance of quantum science to healthcare isn’t new. In fact, when a group of academics and government representatives gathered at Chicheley Hall back in 2013 to hatch plans for the UK’s National Quantum Technologies Programme, healthcare was one of the main applications they identified. The resulting £1bn programme, which co-ordinated the UK’s quantum-research efforts, was recently renewed for another decade and – once again – healthcare is a key part of the remit.

As it happens, most major hospitals already use quantum sensors in the form of magnetic resonance imaging (MRI) machines. Pioneered in the 1970s, these devices manipulate the quantum spin states of hydrogen atoms using magnetic fields and radio waves. By measuring how long those states take to relax, MRI can image soft tissues, such as the brain, and is now a vital part of the modern medicine toolkit.

While an MRI machine measures the quantum properties of atoms, the sensor itself is classical, essentially consisting of electromagnetic coils that detect the magnetic flux produced when atomic spins change direction. More recently, though, we’ve seen a new generation of nanoscale quantum sensors that are sensitive enough to detect magnetic fields emitted by a target biological system. Others, meanwhile, consist of just a single atom and can monitor small changes in the environment.

There are lots of different quantum-based companies and institutions working in the healthcare sector

As the Quantum for Life report shows, there are lots of different quantum-based companies and institutions working in the healthcare sector. There are also many promising types of quantum sensors, which use photons, electrons or spin defects within a material, typically diamond. But ultimately what matters is what quantum sensors can achieve in a medical environment.

Quantum diagnosis

While compiling the report, it became clear that quantum-sensor technologies for healthcare come in five broad categories. The first is what the report labels “lab diagnostics”, in which trained staff use quantum sensors to observe what is going on inside the human body. By monitoring everything from our internal temperature to the composition of cells, the sensors can help to identify diseases such as cancer.

Currently, the only way to definitively diagnose cancer is to take a sample of cells – a biopsy – and examine them under a microscope in a laboratory. Biopsies are often done with visual light but that can damage a sample, making diagnosis tricky. Another option is to use infrared radiation. By monitoring the specific wavelengths the cells absorb, the compounds in a sample can be identified, allowing molecular changes linked with cancer to be tracked.

Unfortunately, it can be hard to differentiate these signals from background noise. What’s more, infrared cameras are much more expensive than those operating in the visible region. One possible solution is being explored by Digistain, a company that was spun out of Imperial College, London, in 2019. It is developing a product called EntangleCam that uses two entangled photons – one infrared and one visible (figure 1).

If the infrared photon is absorbed by, say, a breast cancer cell, that immediately affects the visible photon with which it is entangled. So by measuring the visible light, which can be done with a cheap, efficient detector, you can get information about the infrared photon – and hence the presence of a potential cancer cell (Phys. Rev. 108 032613). The technique could therefore allow cancer to be quickly diagnosed before a tumour has built up, although an oncologist would still be needed to identify the area for the technique to be applied.

Point of care

The second promising application of quantum sensors lies in “point-of-care” diagnostics. We all became familiar with the concept during the COVID-19 pandemic when lateral-flow tests proved to be a vital part of the worldwide response to the virus. The tests could be taken anywhere and were quick, simple, reliable and relatively cheap. Something that had originally been designed to be used in a lab was now available to most people at home.

Quantum technology could let us miniaturize such tests further and make them more accurate, such that they could be used at hospitals, doctor’s surgeries or even at home. At the moment, biological indicators of disease tend to be measured by tagging molecules with fluorescent markers and measuring where, when and how much light they emit. But because some molecules are naturally fluorescent, those measurements have to be processed to eliminate the background noise.

One emerging quantum-based alternative is to characterize biological samples by measuring their tiny magnetic fields. This can be done, for example, using diamond specially engineered with nitrogen-vacancy (NV) defects. Each is made by removing two carbon atoms from the lattice and implanting a nitrogen atom in one of the gaps, leaving a vacancy in the other. Behaving like an atom with discrete energy levels, each defect’s spin state is influenced by the local magnetic field and can be “read out” from the way it fluoresces.

One UK company working in this area is Element Six. It has joined forces with the US-based firm QDTI to make a single-crystal diamond-based device that can quickly identify biomarkers in blood plasma, cerebrospinal fluid and other samples extracted from the body. The device detects magnetic fields produced by specific proteins, which can help identify diseases in their early stages, including various cancers and neurodegenerative conditions like Alzheimer’s. Another firm using single-crystal diamond to detect cancer cells is Germany-based Quantum Total Analysis Systems (QTAS).

Matthew Markham, a physicist who is head of quantum technologies at Element Six, thinks that healthcare has been “a real turning point” for the company. “A few years ago, this work was mostly focused on academic problems,” he says. “But now we are seeing this technology being applied to real-world use cases and that it is transitioning into industry with devices being tested in the field.”

An alternative approach involves using tiny nanometre-sized diamond particles with NV centres, which have the advantage of being highly biocompatible. QT Sense of the Netherlands, for example, is using these nanodiamonds to build nano-MRI scanners that can measure the concentration of molecules that have an intrinsic magnetic field. This equipment has already been used by biomedical researchers to investigate single cells (figure 2).

Australian firm FeBI Technologies, meanwhile, is developing a device that uses nanodiamonds to measure the magnetic properties of ferritin – a protein that stores iron in the body. The company claims its technology is nine orders of magnitude more sensitive than traditional MRI and will allow patients to monitor the amount of iron in their blood using a device that is accurate and cheap.

Wearable healthcare

The third area in which quantum technologies are benefiting healthcare is what’s billed in the Quantum for Life report as “consumer medical monitoring and wearable healthcare”. In other words, we’re talking about devices that allow people to monitor their health in daily life on an ongoing basis. Such technologies are particularly useful for people who have a diagnosed medical condition, such as diabetes or high blood pressure.

NIQS Tech, for example, was spun off from the University of Leeds in 2022 and is developing a highly accurate, non-invasive sensor for measuring glucose levels. Traditional glucose-monitoring devices are painful and invasive because they basically involve sticking a needle in the body. While newer devices use light-based spectroscopic measurements, they tend to be less effective for patients with darker skin tones.

The sensor from NIQS Tech instead uses a doped silica platform, which enables quantum interference effects. When placed in contact with the skin and illuminated with laser light, the device fluoresces, with the lifetime of the fluorescence depending on the amount of glucose in the user’s blood, regardless of skin tone. NIQS has already demonstrated proof of concept with lab-based testing and now wants to shrink the technology to create a wearable device that monitors glucose levels continuously.

Body imaging

The fourth application of quantum tech lies in body scanning, which allows patients to be diagnosed without needing a biopsy. One company leading in this area is Cerca Magnetics, which was spun off from the University of Nottingham. In 2023 it won the inaugural qBIG prize for quantum innovation from the Institute of Physics, which publishes Physics World, for developing wearable optically pumped magnetometers for magnetoencephalography (MEG), which measure magnetic fields generated by neuronal firings in the brain. Its devices can be used to scan patients’ brains in a comfortable seated position and even while they are moving.

Quantum-based scanning techniques could also help diagnose breast cancer, which is usually done by exposing a patient’s breast tissue to low doses of X-rays. The trouble with such mammograms is that all breasts contain a mix of low-density fatty and other, higher-density tissue. The latter creates a “white blizzard” effect against the dark background, making it challenging to differentiate between healthy tissue and potential malignancies.

That’s a particular problem for the roughly 40% of women who have a higher concentration of higher-density tissue. One alternative is to use molecular breast imaging (MBI), which involves imaging the distribution of a radioactive tracer that has been intravenously injected into a patient. This tracer, however, exposes patients to a higher (albeit still safe) dose of radiation than with a mammogram, which means that patients have to be imaged for a long time to get enough signal.

A solution could lie with the UK-based firm Kromek, which is using cadmium zinc telluride (CZT) semiconductors that produce a measurable voltage pulse from just a single gamma-ray photon. As well as being very efficient over a broad range of X-ray and gamma-ray photon energies, CZTs can be integrated onto small chips operating at room temperature. Preliminary results with Kromek’s ultralow-dose and ultrafast detectors show they work with barely one-eighth of the amount of tracer as traditional MBI techniques.

“Our prototypes have shown promising results,” says Alexander Cherlin, who is principal physicist at Kromek. The company is now designing and building a full-size prototype of the camera as part of Innovate UK’s £2.5m “ultralow-dose” MBI project, which runs until the end of 2025. It involves Kromek working with hospitals in Newcastle along with researchers at University College London and the University of Newcastle.

Microscopy matters

The final application of quantum sensors to medicine lies in microscopy, which these days no longer just means visible light but everything from Raman and two-photon microscopy to fluorescence lifetime imaging and multiphoton microscopy. These techniques allow samples to be imaged at different scales and speeds, but they are all reaching various technological limits.

Quantum technologies can help us break the technological limits of microscopy

Quantum technologies can help us break those limits. Researchers at the University of Glasgow, for example, are among those to have used pairs of entangled photons to enhance microscopy through “ghost imaging”. One photon in each pair interacts with a sample, with the image built up by detecting the effect on its entangled counterpart. The technique avoids the noise created when imaging with low levels of light (Sci. Adv. 6 eaay2652).

Researchers at the University of Strathclyde, meanwhile, have used nanodiamonds to get around the problem that dyes added to biological samples eventually stop fluorescing. Known as photobleaching, the effect prevents samples from being studied after a certain time (Roy. Soc. Op. Sci. 6 190589). In the work, samples could be continually imaged and viewed using two-photon excitation microscopy with a 10-fold increase in resolution.

Looking to the future

But despite the great potential of quantum sensors in medicine, there are still big challenges before the technology can be deployed in real, clinical settings. Scalability – making devices reliably, cheaply and in sufficient numbers – is a particular problem. Fortunately, things are moving fast. Even since the Quantum for Life report came out late in 2024, we’ve seen new companies being founded to address these problems.

One such firm is Bristol-based RobQuant, which is developing solid-state semiconductor quantum sensors for non-invasive magnetic scanning of the brain. Such sensors, which can be built with the standard processing techniques used in consumer electronics, allow for scans on different parts of the body. RobQuant claims its sensors are robust and operate at ambient temperatures without requiring any heating or cooling.

Agnethe Seim Olsen, the company’s co-founder and chief technologist, believes that making quantum sensors robust and scalable is vital if they are to be widely adopted in healthcare. She thinks the UK is leading the way in the commercialization of such sensors and will benefit from the latest phase of the country’s quantum hubs. Bringing academia and businesses together, they include the £24m Q-BIOMED biomedical-sensing hub led by University College London and the £27.5m QuSIT hub in imaging and timing led by the University of Birmingham.

Q-BIOMED is, for example, planning to use both single-crystal diamond and nanodiamonds to develop and commercialize sensors that can diagnose and treat diseases such as cancer and Alzheimer’s at much earlier stages of their development. “These healthcare ambitions are not restricted to academia, with many startups around the globe developing diamond-based quantum technology,” says Markham at Element Six.

As with the previous phases of the hubs, allowing for further research encourages start-ups – researchers from the forerunner of the QuSIT hub, for example, set up Cerca Magnetics. The growing maturity of some of these quantum sensors will undoubtedly attract existing medical-technology companies. The next five years will be a busy and exciting time for the burgeoning use of quantum sensors in healthcare.

The post How quantum sensors could improve human health and wellbeing appeared first on Physics World.

When I’m sitting in my armchair, eating chocolate and finding it hard to motivate myself to exercise, a little voice in my head starts singing “You’ve got to move it, move it” to the tune of will.i.am’s “I like to move it”. The positive reinforcement and joy of this song as it plays on a loop in my mind propels me out of my seat and onto the tennis court.

Songs like this are earworms – catchy pieces of music that play on repeat in your head long after you’ve heard them. Some tunes are more likely to become earworms than others, and there are a few reasons for this.

To truly hook you in, the music must be repetitive so that the brain can easily finish it. Generally, it is also simple, and has a rising and falling pitch shape. While you need to hear a song several times for it to stick, once it’s wormed its way into your head, some lyrics become impossible to escape – “I just can’t get you out of my head”, as Kylie would say.

In his book Musicophilia, neurologist Oliver Sacks describes these internal music loops as “the brainworms that arrive unbidden and leave only on their own time”. They can fade away, but they tend to lie in wait, dormant until an association sets them off again – like when I need to exercise. But for me as a physics teacher for 16–18 year olds, this fact is more than just of passing interest: I use it in the classroom.

There are some common mistakes students make in physics, so I play songs in class that are linked (sometimes tenuously) to the syllabus to remind them to check their work. Before I continue, I should add that I’m not advocating rote learning without understanding – the explanation of the concept must always come first. But I have found the right earworm can be a great memory aid.

I’ve been a physics teacher for a while, and I’ll admit to a slight bias towards the music of the 1980s and 1990s. I play David Bowie’s “Changes” (which the students associate with the movie Shrek) when I ask the class to draw a graph, to remind them to check if they need to process – or change – the data before plotting. The catchy “Ch…ch…ch…changes” is now the irritating tune they hear when I look over their shoulders to check if they have found, for example, the sine values for Snell’s law, or the square root of tension if looking at the frequency of a stretched wire.

When describing how to verify the law of conservation of momentum, students frequently leave out the mechanism that makes the two trollies stick together after the collision. Naturally, this is an opportunity for me to play Roxy Music’s “Let’s stick together”.

Meanwhile, “Ice ice baby” by Vanilla Ice is obviously the perfect earworm for calculating the specific latent heat of fusion of ice, which is when students often drop parts of the equations because they forget that the ice both melts and changes temperature.

In the experiment where you charge a gold leaf electroscope by induction, pupils often fail to do the four steps in the correct order. I therefore play Shirley Bassey’s “Goldfinger” to remind pupils to earth the disc with their finger. Meanwhile, Spandau Ballet’s bold and dramatic “Gold” is reserved for Rutherford’s gold leaf experiment.

“Pump up the volume” by M|A|R|R|S or Ireland’s 1990 football song “Put ‘em under pressure” are obvious candidates for investigating Boyle’s law. I use “Jump around” by House of Pain when causing a current-carrying conductor in a magnetic field to experience a force.

Some people may think that linking musical lyrics and physics in this way is a waste of time. However, it also introduces some light-hearted humour into the classroom – and I find teenagers learn better with laughter. The students enjoy mocking my taste in music and coming up with suitable (more modern) songs, and we laugh together about the tenuous links I’ve made between lyrics and physics.

More importantly, this is how my memory works. I link phrases or lyrics to the important things I need to remember. Auditory information functions as a strong mnemonic. I am not saying that this works for everyone, but I have heard my students sing the lyrics to each other while studying in pairs or groups. I smile to myself as I circulate the room when I hear them saying phrases like, “No you forgot mass × specific latent heat – remember it’s ‘Ice, ice baby!’ ”.

On their last day of school – after two years of playing these tunes in class – I hold a quiz where I play a song and the students have to link it to the physics. It turns into a bit of a sing-along, with chocolate for prizes, and there are usually a few surprises in there too. Have a go yourself with the quiz below.

The post ‘Can’t get you out of my head’: using earworms to teach physics appeared first on Physics World.

Year 12 students (aged 16 or 17) often do work experience while studying for their A-levels. It can provide valuable insights into what the working world is like and showcase what potential career routes are available. And that’s exactly why I requested to do my week of work experience at the Institute of Physics (IOP).

I’m studying maths, chemistry and physics, with a particular interest in the latter. I’m hoping to study physics or chemical physics at university so was keen to find out how the subject can be applied to business, and get a better understanding of what I want to do in the future. The IOP was therefore a perfect placement for me and here are a few highlights of what I did.

Monday

My week at the IOP’s headquarters in London began with a brief introduction to the Institute with the head of science and innovation, Anne Crean, and Katherine Platt, manager for the International Year of Quantum Science and Technology (IYQ). Platt, who planned and supervised my week of activities, then gave me a tour of the building and explained more about the IOP’s work, including how it aims to nurture upcoming physics innovation and projects, and give businesses and physicists resources and support.

My first task was working with Jenny Lovell, project manager in the science and innovation team. While helping her organize the latest round of the IOP’s medals and awards, she explained why the IOP honours the physics community in this way and described the different degrees of achievement that it recognizes.

Next I got to meet the IOP’s chief executive officer Tom Grinyer, and unexpectedly the president-elect Michele Dougherty, who is a space physicist at Imperial College London. They are both inspiring people, who gave me some great advice about how I might go about my future in physics.  They talked about the exciting opportunities available as a woman in physics, and how no matter where I start, I can go into many different sectors as the subject is so applicable.

To round off the day, I sat in a meeting about how the science and innovation team can increase engagement, before starting on a presentation I was due to make on Thursday about quantum physics and young people.

Tuesday

My second day began with a series of meetings. First up was the science and innovation team’s weekly stand-up meeting. I then attended a larger staff meeting with most of IOP’s employees, which proved informative and gave me a chance to see how different teams interact with each other. Next was the science and innovation managers’ meeting, where I took the minutes as they spoke.

I then met data science lead, Robert Cocking, who went through his work on data insights. He talked about IOP membership statistics in the UK and Ireland, as well as age and gender splits, and how he can do similar breakdowns for the different areas of special interest (such as quantum physics or astronomy). I found the statistics around the representation of girls in the physics community, specifically at A-level, particularly fascinating as it applies to me. Notably, although a lower percentage of girls take A-level physics compared to boys, a higher proportion of those girls go on to study it at university.

The day ended with some time to work on my presentation and research different universities and pathways I could take once I have finished my A-levels.

Wednesday

It was a steady start to Wednesday as I continued with my presentation and research with Platt’s help. Later in the morning, I attended a meeting with the public engagement team about Mimi’s Tiny Adventure, a children’s book written by Toby Shannon-Smith, public programmes manager at IOP, and illustrated by Pauline Gregory. The book, which is the third in the Mimi’s Adventures series, is part of the IOP’s Limit Less campaign to engage young people in physics, and will be published later this year to coincide with the IYQ. It was interesting to see how the IOP advertises physics to a younger audience and makes it more engaging for them.

Platt and I then had a video call with the Physics World team at IOP Publishing in Bristol, joining for their daily news meeting before having an in-depth chat with the editor-in-chief, Matin Durrani, and feature editors, Tushna Commissariat and Sarah Tesh. After giving me a brief introduction to the magazine, website and team structure, we discussed physics careers. It was good hear the editors’ insights as they cover a broad range of jobs in Physics World and all have a background in physics. It was particularly good to hear from Durrani as he studied chemical physics, which combines my three subjects and my passions.

Thursday

On Thursday I met David Curry, founder of Quantum Base Alpha – a start-up using quantum-inspired algorithms to solve issues facing humanity. We talked about physics in a business context, what he and his company do, and what he hopes for the future of quantum.

I then gave my presentation on “Why should young people care about quantum?”. I detailed the importance of quantum physics, the major things happening in the field and what it can become, as well as the careers quantum will offer in the future. I also discussed diversity and representation in the physics community, and how that is translated to what I see in everyday life, such as in my school and class. As a woman of colour going into science, technology, engineering and mathematics (STEM), I think it is important for me to have conversations around diversity of both gender and race, and the combination of two. After my presentation, Curry gave me some feedback, and we discussed what I am aiming to do at university and beyond.

Friday

For my final day, I visited the University of Sussex, where I toured the campus with Curry’s daughter Kitty, an undergraduate student studying social sciences. I then met up again with Curry, who introduced me to Thomas Clarke, a PhD student in Sussex’s ion quantum technologies group. We went to the physics and maths building, where he explained the simple process of quantum computing to me, and the struggles they have implementing that on a larger scale.

Clarke then gave us a tour of the lab that he shares with other PhD students, and showed us his experiments, which consisted of multiple lasers that made up their trapped ion quantum computing platform. As we read off his oscilloscope attached to the laser system, it was interesting to hear that a lot of his work involved trial and error, and the visit helped me realize that I am probably more interested in the experimental side of physics rather than pure theory.

My work experience week at the IOP has been vital in helping me to understand how physics can be applied in both business and academia. Thanks to the IOP’s involvement in the IYQ, I now have a deeper understanding of quantum science and how it might one day be applied to almost every aspect of physics – including chemical physics – as the sector grows in interest and funding. It’s been an eye-opening week, and I’ve returned to school excited and better informed about my potential next career steps.

The post Beyond the classroom: a high-school student’s week at the Institute of Physics appeared first on Physics World.

Join us to learn about the development and application of a 3-Electrode setup for the operando detection of side reactions in Li-Ion batteries.

Detecting parasitic side reactions originating both from the cathode active materials (CAMs) and the electrolyte is paramount for developing more stable cell chemistries for Li-ion batteries. This talk will present a method for the qualitative analysis of oxidative electrolyte oxidation, as well as the quantification of released lattice oxygen and transition metal ions (TM ions) from the CAM. It is based on a 3-electrode cell design employing a Vulcan carbon-based sense electrode (SE) that is held at a controlled voltage against a partially delithiated lithium iron phosphate (LFP) counter electrode (CE). At this SE, reductive currents can be measured while polarizing a CAM or carbon working electrode (WE) against the same LFP CE. In voltametric scans, we show how the SE potential can be selected to specifically detect a given side reaction during CAM charge/discharge, allowing, e.g., to discriminate between lattice oxygen, protons, and dissolved TMs. Furthermore, it is shown via On-line Electrochemical Mass Spectrometry (OEMS) that O₂ reduction in the here-used LP₄₇ electrolyte consumes ~2.3 electrons/O₂. Using this value, the lattice oxygen release deduced from the 3-electrode setup upon charging of the NCA WE is in good agreement with OEMS measurements up to NCA potentials >4.65 V_Li. At higher potentials, the contributions from the reduction of TM ions can be quantified by comparing the integrated SE current with the O₂ evolution from OEMS

Lennart Reuter is a PhD student in the group of Prof Hubert A Gasteiger at the Chair of Technical Electrochemistry at TUM. His research focused on the interfacial processes in lithium-ion batteries that govern calendar life, cycle stability, and rate capability. He advanced the on-line electrochemical mass spectrometry (OEMS) technique to investigate gas evolution mechanisms from interfacial side reactions at the cathode and anode. His work also explored how SEI formation and graphite structural changes affect Li⁺ transport, using impedance spectroscopy and complementary analysis techniques.

 

Leonhard J Reinschluessel is currently a PhD candidate at at the Chair of Technical Electrochemistry in the Gasteiger research group at the Technical University of Munich (TUM). His current work encompasses an in-depth understanding of the complex interplay of cathode- and electrolyte degradation mechanisms in lithium-ion batteries using operando lab-based and synchrotron techniques. He received his MSc in chemistry from TUM, where he investigated the mitigation of aging of FeNC-based cathode catalyst layers in PEMFCs in his thesis at the Gasteiger group Electrochemistry at TUM. His research focused on the interfacial processes in lithium-ion batteries that govern calendar life, cycle stability, and rate capability. He advanced the on-line electrochemical mass spectrometry (OEMS) technique to investigate gas evolution mechanisms from interfacial side reactions at the cathode and anode. His work also explored how SEI formation and graphite structural changes affect Li⁺ transport, using impedance spectroscopy and complementary analysis techniques.

The post Development and application of a 3-electrode setup for the operando detection of side reactions in Li-Ion batteries appeared first on Physics World.

Developing quantum computing systems with high operational fidelity, enhanced processing capabilities plus inherent (and rapid) scalability is high on the list of fundamental problems preoccupying researchers within the quantum science community. One promising R&D pathway in this regard is being pursued by the Superconducting Quantum Materials and Systems (SQMS) National Quantum Information Science Research Center at the US Department of Energy’s Fermi National Accelerator Laboratory, the pre-eminent US particle physics facility on the outskirts of Chicago, Illinois.

The SQMS approach involves placing a superconducting qubit chip (held at temperatures as low as 10–20 mK) inside a three-dimensional superconducting radiofrequency (3D SRF) cavity – a workhorse technology for particle accelerators employed in high-energy physics (HEP), nuclear physics and materials science. In this set-up, it becomes possible to preserve and manipulate quantum states by encoding them in microwave photons (modes) stored within the SRF cavity (which is also cooled to the millikelvin regime).

Put another way: by pairing superconducting circuits and SRF cavities at cryogenic temperatures, SQMS researchers create environments where microwave photons can have long lifetimes and be protected from external perturbations – conditions that, in turn, make it possible to generate quantum states, manipulate them and read them out. The endgame is clear: reproducible and scalable realization of such highly coherent superconducting qubits opens the way to more complex and scalable quantum computing operations – capabilities that, over time, will be used within Fermilab’s core research programme in particle physics and fundamental physics more generally.

Fermilab is in a unique position to turn this quantum technology vision into reality, given its decadal expertise in developing high-coherence SRF cavities. In 2020, for example, Fermilab researchers demonstrated record coherence lifetimes (of up to two seconds) for quantum states stored in an SRF cavity.

“It’s no accident that Fermilab is a pioneer of SRF cavity technology for accelerator science,” explains Sir Peter Knight, senior research investigator in physics at Imperial College London and an SQMS advisory board member. “The laboratory is home to a world-leading team of RF engineers whose niobium superconducting cavities routinely achieve very high quality factors (Q) from 10¹⁰ to above 10¹¹ – figures of merit that can lead to dramatic increases in coherence time.”

Moreover, Fermilab offers plenty of intriguing HEP use-cases where quantum computing platforms could yield significant research dividends. In theoretical studies, for example, the main opportunities relate to the evolution of quantum states, lattice-gauge theory, neutrino oscillations and quantum field theories in general. On the experimental side, quantum computing efforts are being lined up for jet and track reconstruction during high-energy particle collisions; also for the extraction of rare signals and for exploring exotic physics beyond the Standard Model.

Cavities and qubits

SQMS has already notched up some notable breakthroughs on its quantum computing roadmap, not least the demonstration of chip-based transmon qubits (a type of charge qubit circuit exhibiting decreased sensitivity to noise) showing systematic and reproducible improvements in coherence, record-breaking lifetimes of over a millisecond, and reductions in performance variation.

Key to success here is an extensive collaborative effort in materials science and the development of novel chip fabrication processes, with the resulting transmon qubit ancillas shaping up as the “nerve centre” of the 3D SRF cavity-based quantum computing platform championed by SQMS. What’s in the works is essentially a unique quantum analogue of a classical computing architecture: the transmon chip providing a central logic-capable quantum information processor and microwave photons (modes) in the 3D SRF cavity acting as the random-access quantum memory.

As for the underlying physics, the coupling between the transmon qubit and discrete photon modes in the SRF cavity allows for the exchange of coherent quantum information, as well as enabling quantum entanglement between the two. “The pay-off is scalability,” says Alexander Romanenko, a senior scientist at Fermilab who leads the SQMS quantum technology thrust. “A single logic-capable processor qubit, such as the transmon, can couple to many cavity modes acting as memory qubits.”

In principle, a single transmon chip could manipulate more than 10 qubits encoded inside a single-cell SRF cavity, substantially streamlining the number of microwave channels required for system control and manipulation as the number of qubits increases. “What’s more,” adds Romanenko, “instead of using quantum states in the transmon [coherence times just crossed into milliseconds], we can use quantum states in the SRF cavities, which have higher quality factors and longer coherence times [up to two seconds].”

In terms of next steps, continuous improvement of the ancilla transmon coherence times will be critical to ensure high-fidelity operation of the combined system – with materials breakthroughs likely to be a key rate-determining step. “One of the unique differentiators of the SQMS programme is this ‘all-in’ effort to understand and get to grips with the fundamental materials properties that lead to losses and noise in superconducting qubits,” notes Knight. “There are no short-cuts: wide-ranging experimental and theoretical investigations of materials physics – per the programme implemented by SQMS – are mandatory for scaling superconducting qubits into industrial and scientifically useful quantum computing architectures.”

Laying down a marker, SQMS researchers recently achieved a major milestone in superconducting quantum technology by developing the longest-lived multimode superconducting quantum processor unit (QPU) ever built (coherence lifetime >20 ms). Their processor is based on a two-cell SRF cavity and leverages its exceptionally high quality factor (~10¹⁰) to preserve quantum information far longer than conventional superconducting platforms (typically 1 or 2 ms for rival best-in-class implementations).

Coupled with a superconducting transmon, the two-cell SRF module enables precise manipulation of cavity quantum states (photons) using ultrafast control/readout schemes (allowing for approximately 10⁴ high-fidelity operations within the qubit lifetime). “This represents a significant achievement for SQMS,” claims Yao Lu, an associate scientist at Fermilab and co-lead for QPU connectivity and transduction in SQMS. “We have demonstrated the creation of high-fidelity [>95%] quantum states with large photon numbers [20 photons] and achieved ultra-high-fidelity single-photon entangling operations between modes [>99.9%]. It’s work that will ultimately pave the way to scalable, error-resilient quantum computing.”

Fast scaling with qudits

There’s no shortage of momentum either, with these latest breakthroughs laying the foundations for SQMS “qudit-based” quantum computing and communication architectures. A qudit is a multilevel quantum unit that can be more than two states and, in turn, hold a larger information density – i.e. instead of working with a large number of qubits to scale information processing capability, it may be more efficient to maintain a smaller number of qudits (with each holding a greater range of values for optimized computations).

Scale-up to a multiqudit QPU system is already underway at SQMS via several parallel routes (and all with a modular computing architecture in mind). In one approach, coupler elements and low-loss interconnects integrate a nine-cell multimode SRF cavity (the memory) to a two-cell SRF cavity quantum processor. Another iteration uses only two-cell modules, while yet another option exploits custom-designed multimodal cavities (10+ modes) as building blocks.

One thing is clear: with the first QPU prototypes now being tested, verified and optimized, SQMS will soon move to a phase in which many of these modules will be assembled and put together in operation. By extension, the SQMS effort also encompasses crucial developments in control systems and microwave equipment, where many devices must be synchronized optimally to encode and analyse quantum information in the QPUs.

Along a related coordinate, complex algorithms can benefit from fewer required gates and reduced circuit depth. What’s more, for many simulation problems in HEP and other fields, it’s evident that multilevel systems (qudits) – rather than qubits – provide a more natural representation of the physics in play, making simulation tasks significantly more accessible. The work of encoding several such problems into qudits – including lattice-gauge-theory calculations and others – is similarly ongoing within SQMS.

Taken together, this massive R&D undertaking – spanning quantum hardware and quantum algorithms – can only succeed with a “co-design” approach across strategy and implementation: from identifying applications of interest to the wider HEP community to full deployment of QPU prototypes. Co-design is especially suited to these efforts as it demands sustained alignment of scientific goals with technological implementation to drive innovation and societal impact.

In addition to their quantum computing promise, these cavity-based quantum systems will play a central role in serving both as the “adapters” and low-loss channels at elevated temperatures for interconnecting chip or cavity-based QPUs hosted in different refrigerators. These interconnects will provide an essential building block for the efficient scale-up of superconducting quantum processors into larger quantum data centres.

 “The SQMS collaboration is ploughing its own furrow – in a way that nobody else in the quantum sector really is,” says Knight. “Crucially, the SQMS partners can build stuff at scale by tapping into the phenomenal engineering strengths of the National Laboratory system. Designing, commissioning and implementing big machines has been part of the ‘day job’ at Fermilab for decades. In contrast, many quantum computing start-ups must scale their R&D infrastructure and engineering capability from a far-less-developed baseline.”

The last word, however, goes to Romanenko. “Watch this space,” he concludes, “because SQMS is on a roll. We don’t know which quantum computing architecture will ultimately win out, but we will ensure that our cavity-based quantum systems will play an enabling role.”

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By adapting mathematical techniques used in particle physics, researchers in Germany have developed an approach that could boost our understanding of the gravitational waves that are emitted when black holes collide. Led by Jan Plefka at The Humboldt University of Berlin, the team’s results could prove vital to the success of future gravitational-wave detectors.

Nearly a decade on from the first direct observations of gravitational waves, physicists are hopeful that the next generation of ground- and space-based observatories will soon allow us to study these ripples in space–time with unprecedented precision. But to ensure the success of upcoming projects like the LISA space mission, the increased sensitivity offered by these detectors will need to be accompanied with a deeper theoretical understanding of how gravitational waves are generated through the merging of two black holes.

In particular, they will need to predict more accurately the physical properties of gravitational waves produced by any given colliding pair and account for factors including their respective masses and orbital velocities. For this to happen, physicists will need to develop more precise solutions to the relativistic two-body problem. This problem is a key application of the Einstein field equations, which relate the geometry of space–time to the distribution of matter within it.

No exact solution

“Unlike its Newtonian counterpart, which is solved by Kepler’s Laws, the relativistic two-body problem cannot be solved exactly,” Plefka explains. “There is an ongoing international effort to apply quantum field theory (QFT) – the mathematical language of particle physics – to describe the classical two-body problem.”

In their study, Plefka’s team started from state-of-the-art techniques used in particle physics for modelling the scattering of colliding elementary particles, while accounting for their relativistic properties. When viewed from far away, each black hole can be approximated as a single point which, much like an elementary particle, carries a single mass, charge, and spin.

Taking advantage of this approximation, the researchers modified existing techniques in particle physics to create a framework called worldline quantum field theory (WQFT). “The advantage of WQFT is a clean separation between classical and quantum physics effects, allowing us to precisely target the classical physics effects relevant for the vast distances involved in astrophysical observables,” Plefka describes

Ordinarily, doing calculations with such an approach would involve solving millions of integrals that sum-up every single contribution to the black hole pair’s properties across all possible ways that the interaction between them could occur. To simplify the problem, Plefka’s team used a new algorithm that identified relationships between the integrals. This reduced the problem to just 250 “master integrals”, making the calculation vastly more manageable.

With these master integrals, the team could finally produce expressions for three key physical properties of black hole binaries within WQFT. These includes the changes in momentum during the gravity-mediated scattering of two black holes and the total energy radiated by both bodies over the course of the scattering.

Genuine physical process

Altogether, the team’s WQFT framework produced the most accurate solution to the Einstein field equations ever achieved to date. “In particular, the radiated energy we found contains a new class of mathematical functions known as ‘Calabi–Yau periods’,” Plefka explains. “While these functions are well-known in algebraic geometry and string theory, this marks the first time they have been shown to describe a genuine physical process.”

With its unprecedented insights into the structure of the relativistic two-body problem, the team’s approach could now be used to build more precise models of gravitational-wave formation, which could prove invaluable for the next generation of gravitational-wave detectors.

More broadly, however, Plefka predicts that the appearance of Calabi–Yau periods in their calculations could lead to an entirely new class of mathematical functions applicable to many areas beyond gravitational waves.

“We expect these periods to show up in other branches of physics, including collider physics, and the mathematical techniques we employed to calculate the relevant integrals will no doubt also apply there,” he says.

The research is described in Nature.

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CP Snow’s classic The Two Cultures lecture, published in book form in 1959, is the usual go-to reference when exploring the divide between the sciences and humanities. It is a culture war that was raging long before the term became social-media shorthand for today’s tribal battles over identity, values and truth.

While Snow eloquently lamented the lack of mutual understanding between scientific and literary elites, the 21st-century version of the two-cultures debate often plays out with a little less decorum and a lot more profanity. Hip hop duo Insane Clown Posse certainly didn’t hold back in their widely memed 2010 track “Miracles”, which included the lyric “And I don’t wanna talk to a scientist / Y’all motherfuckers lying and getting me pissed”. An extreme example to be sure, but it hammers home the point: Snow’s two-culture concerns continue to resonate strongly almost 70 years after his influential lecture and writings.

A Perfect Harmony: Music, Mathematics and Science by David Darling is the latest addition to a growing genre that seeks to bridge that cultural rift. Like Peter Pesic’s Music and the Making of Modern Science, Susan Rogers and Ogi Ogas’ This Is What It Sounds Like, and Philip Ball’s The Music Instinct, Darling’s book adds to the canon that examines the interplay between musical creativity and the analytical frameworks of science (including neuroscience) and mathematics.

I’ve also contributed, in a nanoscopically small way, to this music-meets-science corpus with an analysis of the deep and fundamental links between quantum physics and heavy metal (When The Uncertainty Principle Goes To 11), and have a long-standing interest in music composed from maths and physics principles and constants (see my Lateral Thoughts articles from September 2023 and July 2024). Darling’s book, therefore, struck a chord with me.

Darling is not only a talented science writer with an expansive back-catalogue to his name but he is also an accomplished musician (check out his album Songs Of The Cosmos ), and his enthusiasm for all things musical spills off the page. Furthermore, he is a physicist, with a PhD in astronomy from the University of Manchester. So if there’s a writer who can genuinely and credibly inhabit both sides of the arts–science cultural divide, it’s Darling.

But is A Perfect Harmony in tune with the rest of the literary ensemble, or marching to a different beat? In other words, is this a fresh new take on the music-meets-maths (meets pop sci) genre or, like too many bands I won’t mention, does it sound suspiciously like something you’ve heard many times before? Well, much like an old-school vinyl album, Darling’s work has the feel of two distinct sides. (And I’ll try to make that my final spin on groan-worthy musical metaphors. Promise.)

Not quite perfect pitch

Although the subtitle for A Perfect Harmony is “Music, Mathematics and Science”, the first half of the book is more of a history of the development and evolution of music and musical instruments in various cultures, rather than a new exploration of the underpinning mathematical and scientific principles. Engaging and entertaining though this is – and all credit to Darling for working in a reference to Van Halen in the opening lines of chapter 1 – it’s well-worn ground: Pythagorean tuning, the circle of fifths, equal temperament, Music of the Spheres (not the Coldplay album, mercifully), resonance, harmonics, etc. I found myself wishing, at times, for a take that felt a little more off the beaten track.

One case in point is Darling’s brief discussion of the theremin. If anything earns the title of “The Physicist’s Instrument”, it’s the theremin – a remarkable device that exploits the innate electrical capacitance of the human body to load a resonant circuit and thus produce an ethereal, haunting tone whose pitch can be varied, without, remarkably, any physical contact.

While I give kudos to Darling for highlighting the theremin, the brevity of the description is arguably a lost opportunity when put in the broader context of the book’s aim to explain the deeper connections between music, maths and science. This could have been a novel and fascinating take on the links between electrical and musical resonance that went well beyond the familiar territory mapped out in standard physics-of-music texts.

Using the music of the eclectic Australian band King Gizzard and the Lizard Wizard to explain microtonality is nothing short of inspired

As the book progresses, however, Darling moves into more distinctive territory, choosing a variety of inventive examples that are often fascinating and never short of thought-provoking. I particularly enjoyed his description of orbital resonance in the system of seven planets orbiting the red dwarf TRAPPIST-1, 41 light-years from Earth. The orbital periods have ratios, which, when mapped to musical intervals, correspond to a minor sixth, a major sixth, two perfect fifths, a perfect fourth and another perfect fifth. And it’s got to be said that using the music of the eclectic Australian band King Gizzard and the Lizard Wizard to explain microtonality is nothing short of inspired.

A Perfect Harmony doesn’t entirely close the cultural gap highlighted by Snow all those years ago, but it does hum along pleasantly in the space between. Though the subject matter occasionally echoes well-trodden themes, Darling’s perspective and enthusiasm lend it freshness. There’s plenty here to enjoy, especially for physicists inclined to tune into the harmonies of the universe.

• 2025 Oneworld Publications 288pp £10.99pb/£6.99ebook

The post Harmonious connections: bridging the gap between music and science appeared first on Physics World.

If a tree fell in a forest almost 4000 years ago, did it make a sound? Well, in the case of an Eastern red cedar in what is now Quebec, Canada, it’s certainly still making noise today.

That’s because in 2013, a team of scientists were digging a trench when they came across the 3775-year-old log. Despite being buried for nearly four millennia, the wood wasn’t rotten and useless. In fact, recent analysis unearthed an entirely different story.

The team, led by atmospheric scientist Ning Zeng of the University of Maryland in the US, found that the wood had only lost 5% of its carbon compared with a freshly cut Eastern red cedar log. “The wood is nice and solid – you could probably make a piece of furniture out of it,” says Zeng. The log had been preserved in such remarkable shape because the clay soil it was buried in was highly impermeable. That limited the amount of oxygen and water reaching the wood, suppressing the activity of micro-organisms that would otherwise have made it decompose.

This ancient log is a compelling example of “biomass burial”. When plants decompose or are burnt, they release the carbon dioxide (CO₂) they had absorbed from the atmosphere. One idea to prevent this CO₂ being released back into the atmosphere is to bury the waste biomass under conditions that prevent or slow decomposition, thereby trapping the carbon underground for centuries.

In fact, Zeng and his colleagues discovered the cedar log while they were digging a huge trench to bury 35 tonnes of wood to test this very idea. Nine years later, when they dug up some samples, they found that the wood had barely decomposed. Further analysis suggested that if the logs had been left buried for a century, they would still hold 97% of the carbon that was present when they were felled.

Digging holes

To combat climate change, there is often much discussion about how to remove carbon from the atmosphere. As well as conventional techniques like restoring peatland and replanting forests, there are a variety of more technical methods being developed (figure 1). These include direct air capture (DAC) and ocean alkalinity enhancement, which involves tweaking the chemistry of oceans so that they absorb more CO₂. But some scientists – like Sinéad Crotty, a managing director at the Carbon Containment Lab in Connecticut, US – think that biomass burial could be a simpler and cheaper way to sequester carbon.

The 3775-year-old log shows that carbon can be stored for centuries underground, but the wood has to be buried under specific conditions. “People tend to think, ‘Who doesn’t know how to dig a hole and bury some wood?’” Zeng says. “But think about how many wooden coffins were buried in human history. How many of them survived? For a timescale of hundreds or thousands of years, we need the right conditions.”

The key for scientists seeking to test biomass burial is to create dry, low-oxygen environments, similar to those in the Quebec clay soil. Last year, for example, Crotty and her colleagues dug more than 100 pits at a site in Colorado, in the US, filled them with woody material and then covered them up again. In five years’ time they plan to dig the biomass back out of the pits to see how much it has decomposed.

The pits vary in depth, and have been refilled and packed in different ways, to test how their build impacts carbon storage. The researchers will also be calculating the carbon emissions of processes such as transporting and burying the biomass – including the amount of carbon released from the soil when the pits are dug. “What we are trying to do here is build an understanding of what works and what doesn’t, but also how we can measure, report and verify that what we are doing is truly carbon negative,” Crotty says.

Over the next five years the team will continuously measure surface CO₂ and methane fluxes from several of the pits, while every pit will have its CO₂ and methane emissions measured monthly. There are also moisture sensors and oxygen probes buried in the pits, plus a full weather station on the site.

Crotty says that all this data will allow them to assess how different depths, packing styles and the local environment alter conditions in the chambers. When the samples are excavated in five years, the researchers will also explore what types of decomposition the burial did and did not suppress. This will include tests to identify different fungal and bacterial signatures, to uncover the micro-organisms involved in any decay.

The big questions

Experiments like Crotty’s will help answer one of the key concerns about terrestrial storage of biomass: how long can the carbon be stored?

In 2023 a team led by Lawrence Livermore National Laboratory (LLNL) did a large-scale analysis of the potential for CO₂ removal in the US. The resulting Road to Removal report outlined how CO₂ removal could be used to help the US achieve its net zero goals (these have since been revoked by the Trump administration), focusing on techniques like direct air capture (DAC), increasing carbon uptake in forests and agricultural lands, and converting waste biomass into fuels and CO₂.

The report did not, however, look at biomass burial. One of the report authors, Sarah Baker – an expert in decarbonization and CO₂ removal at LLNL – told Physics World that this was because of a lack of evidence around the durability of the carbon stored. The report’s minimum requirement for carbon storage was at least 100 years, and there were not enough data available to show how much carbon stored in biomass would remain after that period, Baker explains.

The US Department of Energy is also working to address this question. It has funded a set of projects, which Baker is involved with, to bridge some of the knowledge gaps on carbon-removal pathways. This includes one led by the National Renewable Energy Lab, measuring how long carbon in buried biomass remains stored under different conditions.

Bury the problem

Crotty’s Colorado experiment is also addressing another question: are all forms of biomass equally appropriate for burial? To test this, Crotty’s team filled the pits with a range of woody materials, including different types of wood and wood chip as well as compressed wood, and “slash” – small branches, leaves, bark and other debris created by logging and other forestry work.

Indeed, Crotty and her colleagues see biomass storage as crucial for those managing our forests. The western US states, in particular, have seen an increased risk of wildfires through a mix of climate change and aggressive fire-suppression policies that do not allow smaller fires to burn and thereby produce overgrown forests. “This has led to a build-up of fuels across the landscape,” Crotty says. “So, in a forest that would typically have a high number of low-severity fires, it’s changed the fire regime into a very high-intensity one.”

These concerns led the US Forest Service to announce a 10-year wildfire crisis plan in 2022 that seeks to reduce the risk of fires by thinning and clearing 50 million acres of forest land, in addition to 20 million acres already slated for treatment. But this creates a new problem.

“There are currently very few markets for the types of residues that need to come out of these forests – it is usually small-diameter, low-value timber,” explains Crotty. “They typically can’t pay their way out of the forests, so business as usual in many areas is to simply put them in a pile and burn them.”

A recent study Crotty co-authored suggests that every year “pile burning” in US National Forests emits greenhouse gases equivalent to almost two million tonnes of CO₂, and more than 11 million tonnes of fine particulate matter – air pollution that is linked to a range of health problems. Conservative estimates by the Carbon Containment Lab indicate that the material scheduled for clearance under the Forest Service’s 10-year crisis plan will contain around two gigatonnes (Gt) of CO₂ equivalents. This is around 5% of current annual global CO₂ emissions.

There are also cost implications. Crotty’s recent analysis found that piling and burning forest residue costs around $700 to $1300 per acre. By adding value to the carbon in the forest residues and keeping it out of the atmosphere, biomass storage may offer a solution to these issues, Crotty says.

As an incentive to remove carbon from the atmosphere, trading mechanisms exist whereby individuals, companies and governments can buy and sell carbon emissions. In essence, carbon has a price attached to it, meaning that someone who has emitted too much, say, can pay someone else to capture and store the equivalent amount of emissions, with an often-touted figure being $100 per tonne of CO₂ stored. For a long time, this has been seen as the price at which carbon capture becomes affordable, enabling scale up to the volumes needed to tackle climate change.

“There is only so much capital that we will ever deploy towards [carbon removal] and thus the cheaper the solution, the more credits we’ll be able to generate, the more carbon we will be able to remove from the atmosphere,” explains Justin Freiberg, a managing director of the Carbon Containment Lab. “$100 is relatively arbitrary, but it is important to have a target and aim low on pricing for high quality credits.”

DAC has not managed to reach this magical price point. Indeed, the Swiss firm Climeworks – which is one of the biggest DAC companies – has stated that its costs might be around $300 per tonne by 2030.

A tomb in a mine

Another carbon-removal company, however, claims it has hit this benchmark using biomass burial. “We’re selling our first credits at $100 per tonne,” says Hannah Murnen, chief technology officer at Graphyte – a US firm backed by Bill Gates.

Graphyte is confident that there is significant potential in biomass burial. Based in Pine Bluff, Arkansas, the firm dries and compresses waste biomass into blocks before storage. “We dry it to below a level at which life can exist,” says Murnen, which effectively halts decomposition.

The company claims that it will soon be storing 50,000 tonnes of CO₂ per year and is aiming for five million tonnes per year by 2030. Murnen acknowledges that these are “really significant figures”, particularly compared with what has been achieved in carbon capture so far. Nevertheless, she adds, if you look at the targets around carbon capture “this is the type of scale we need to get to”.

Graphyte is currently working with sawmill residue and rice hulls, but in the future Murnen says it plans to accept all sorts of biomass waste. “One of the great things about biomass for the purpose of carbon removal is that, because we are not doing any sort of chemical transformation on the biomass, we’re very flexible to the type of biomass,” Murnen adds.

And there appears to be plenty available. Estimates by researchers in the UK and India (NPJ Climate and Atmospheric Science 2 35) suggest that every year around 140 Gt of biomass waste is generated globally from forestry and agriculture. Around two-thirds of the agricultural residues are from cereals, like wheat, rice, barley and oats, while sugarcane stems and leaves are the second largest contributors. The rest is made up of things like leaves, roots, peels and shells from other crops. Like forest residues, much of this waste ends up being burnt or left to rot, releasing its carbon.

Currently, Graphyte has one storage site about 30 km from Pine Bluff, where its compressed biomass blocks are stored underground, enclosed in an impermeable layer that prevents water ingress. “We took what used to be an old gravel mine – so basically a big hole in the ground – and we’ve created a lined storage tomb where we are placing the biomass and then sealing it closed,” says Murnen.

Once sealed, Graphyte monitors the CO₂ and methane concentrations in the headspace of the vaults, to check for any decomposition of the biomass. The company also analyses biomass as it enters the facility, to track how much carbon it is storing. Wood residues, like sawmill waste are generally around 50% carbon, says Murnen, but rice hulls are closer to 35% carbon.

Graphyte is confident that its storage is physically robust and could avoid any re-emission for what Murnen calls “a very long period of time”. However, it is also exploring how to prevent accidental disturbance of the biomass in the future – possibly long after the company ceases to exist. One option is to add a conservation easement to the site, a well-established US legal mechanism for adding long-term protection to land.

“We feel pretty strongly that the way we are approaching [carbon removal] is one of the most scalable ways,” Murnen says. “In as far as impediments or barriers to scale, we have a much easier permitting pathway, we don’t need pipelines, we are pretty flexible on the type of land that we can use for our storage sites, and we have a huge variety of feedstocks that we can take into the process.”

A simple solution

Back at LLNL, Baker says that although she hasn’t “run the numbers”, and there are a lot caveats, she suspects that biomass burial is “true carbon removal because it is so simple”.

Once associated upstream and downstream emissions are taken into account, many techniques that people call carbon removal are probably not, she says, because they emit more fossil CO₂ than they store.

Biomass burial is also cheap. As the Road to Removal analysis found, “thermal chemical” techniques, like pyrolysis, have great potential for removing and storing carbon while converting biomass into hydrogen and sustainable aviation fuel. But they require huge investment, with larger facilities potentially costing hundreds of millions of dollars. Biomass burial could even act as temporary storage until facilities are ready to convert the carbon into sustainable fuels. “Buy ourselves time and then use it later,” says Baker.

Either way, biomass burial has great potential for the future of carbon storage, and therefore our environment. “The sooner we can start doing these things the greater the climate impact,” Baker says.

We just need to know that the storage is durable – and if that 3775-year-old log is any indication, there’s the potential to store biomass for hundreds, maybe thousands of years.

The post Bury it, don’t burn it: turning biomass waste into a carbon solution appeared first on Physics World.

Since 1912, we’ve known that the Andromeda galaxy is racing towards our own Milky Way at about 110 kilometres per second. A century later, in 2012, astrophysicists at the Space Telescope Science Institute (STScI) in Maryland, US came to a striking conclusion. In four billion years, they predicted, a collision between the two galaxies was a sure thing.

Now, it’s not looking so sure.

Using the latest data from the European Space Agency’s Gaia astrometric mission, astrophysicists led by Till Sawala of the University of Helsinki, Finland re-modelled the impending crash, and found that it’s 50/50 as to whether a collision happens or not.

This new result differs from the 2012 one because it considers the gravitational effect of an additional galaxy, the Large Magellanic Cloud (LMC), alongside the Milky Way, Andromeda and the nearby Triangulum spiral galaxy, M33. While M33’s gravity, in effect, adds to Andromeda’s motion towards us, Sawala and colleagues found that the LMC’s gravity tends to pull the Milky Way out of Andromeda’s path.

“We’re not predicting that the merger is not going to happen within 10 billion years, we’re just saying that from the data we have now, we can’t be certain of it,” Sawala tells Physics World.

“A step in the right direction”

While the LMC contains only around 10% of the Milky Way’s mass, Sawala and colleagues’ work indicates that it may nevertheless be massive enough to turn a head-on collision into a near-miss. Incorporating its gravitational effects into simulations is therefore “a step in the right direction”, says Sangmo Tony Sohn, a support scientist at the STScI and a co-author of the 2012 paper that predicted a collision.

Even with more detailed simulations, though, uncertainties in the motion and masses of the galaxies leave room for a range of possible outcomes. According to Sawala, the uncertainty with the greatest effect on merger probability lies in the so-called “proper motion” of Andromeda, which is its motion as it appears on our night sky. This motion is a mixture of Andomeda’s radial motion towards the centre of the Milky Way and the two galaxies’ transverse motion perpendicular to one another.

If the combined transverse motion is large enough, Andromeda will pass the Milky Way at a distance greater than 200 kiloparsecs (652,000 light years). This would avert a collision in the next 10 billion years, because even when the two galaxies loop back on each other, their next pass would still be too distant, according to the models.

Conversely, a smaller transverse motion would limit the distance at closest approach to less than 200 kiloparsecs. If that happens, Sawala says the two galaxies are “almost certain to merge” because of the dynamical friction effect, which arises from the diffuse halo of old stars and dark matter around galaxies. When two galaxies get close enough, these haloes begin interacting with each other, generating tidal and frictional heating that robs the galaxies of orbital energy and makes them fall ever closer.

The LMC itself is an excellent example of how this works. “The LMC is already so close to the Milky Way that it is losing its orbital energy, and unlike [Andromeda], it is guaranteed to merge with the Milky Way,” Sawala says, adding that, similarly, M33 stands a good chance of merging with Andromeda.

“A very delicate task”

Because Andromeda is 2.5 million light years away, its proper motion is very hard to measure. Indeed, no-one had ever done it until the STScI team spent 10 years monitoring the galaxy, which is also known as M31, with the Hubble Space Telescope – something Sohn describes as “a very delicate task” that continues to this day.

Another area where there is some ambiguity is in the mass estimate of the LMC. “If the LMC is a little more massive [than we think], then it pulls the Milky Way off the collision course with M31 a little more strongly, reducing the possibility of a merger between the Milky Way and M31,” Sawala explains.

The good news is that these ambiguities won’t be around forever. Sohn and his team are currently analysing new Hubble data to provide fresh constraints on the Milky Way’s orbital trajectory, and he says their results have been consistent with the Gaia analyses so far. Sawala agrees that new data will help reduce uncertainties. “There’s a good chance that we’ll know more about what is going to happen fairly soon, within five years,” he says.

Even if the Milky Way and Andromeda don’t collide in the next 10 billion years, though, that won’t be the end of the story. “I would expect that there is a very high probability that they will eventually merge, but that could take tens of billions of years,” Sawala says.

The research is published in Nature Astronomy.

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Scientists who switch research fields suffer a drop in the impact of their new work – a so-called “pivot penalty”. That is according to a new analysis of scientific papers and patents, which finds that the pivot penalty increases the further away a researcher shifts from their previous topic of research.

The analysis has been carried out by a team led by Dashun Wang and Benjamin Jones of Northwestern University in Illinois. They analysed more than 25 million scientific papers published between 1970 and 2015 across 154 fields as well as 1.7 million US patents across 127 technology classes granted between 1985 and 2020.

To identify pivots and quantify how far a scientist moves from their existing work, the team looked at the scientific journals referenced in a paper and compared them with those cited by previous work. The more the set of journals referenced in the main work diverged from those usually cited, the larger the pivot. For patents, the researchers used “technological field codes” to measure pivots.

Larger pivots are associated with fewer citations and a lower propensity for high-impact papers, defined as those in the top 5% of citations received in their field and publication year. Low-pivot work – moving only slightly away from the typical field of research – led to a high-impact paper 7.4% of the time, yet the highest-pivot shift resulted in a high-impact paper only 2.2% of the time. A similar trend was seen for patents.

When looking at the output of an individual researcher, low-pivot work was 2.1% more likely to have a high-impact paper while high-pivot work was 1.8% less likely to do so. The study found the pivot penalty to be almost universal across scientific fields and it persists regardless of a scientist’s career stage, productivity and collaborations.

COVID impact

The researchers also studied the impact of COVID-19, when many researchers pivoted to research linked to the pandemic. After analysing 83,000 COVID-19 papers and 2.63 million non-COVID papers published in 2020, they found that COVID-19 research was not immune to the pivot penalty. Such research had a higher impact than average, but the further a scientist shifted from their previous work to study COVID-19 the less impact the research had.

“Shifting research directions appears both difficult and costly, at least initially, for individual researchers,” Wang told Physics World. He thinks, however, that researchers should not avoid change but rather “approach it strategically”. Researchers should, for example, try anchoring their new work in the conventions of their prior field or the one they are entering.

To help researchers pivot, Wang says research institutions should “acknowledge the friction” and not “assume that a promising researcher will thrive automatically after a pivot”. Instead, he says, institutions need to design support systems, such as funding or protected time to explore new ideas, or pairing researchers with established scholars in the new field.

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Firm evidence of Majorana bound states in quantum dots has been reported by researchers in the Netherlands. Majorana modes appeared at both edges of a quantum dot chain when an energy gap suppressed them in the centre, and the experiment could allow researchers to investigate the unique properties of these particles in hitherto unprecedented detail. This could bring topologically protected quantum bits (qubits) for quantum computing one step closer.

Majorana fermions were first proposed in 1937 by the Italian physicist Ettore Majorana. They were imagined as elementary particles that would be their own antiparticles. However, such elementary particles have never been definitively observed. Instead, physicists have worked to create Majorana quasiparticles (particle-like collective excitations) in condensed matter systems.

In 2001, the theoretical physicist Alexei Kitaev  at Microsoft Research, proposed that “Majorana bound states” could be produced in nanowires comprising topological superconductors. The Majorana quasiparticle would exist as a single nonlocal mode at either end of a wire, while being zero-valued in the centre. Both ends would be constrained by the laws of physics to remain identical despite being spatially separated. This phenomenon could produce “topological qubits” robust to local disturbance.

Microsoft and others continue to research Majorana modes using this platform to this day.  Multiple groups claim to have observed them, but this remains controversial. “It’s still a matter of debate in these extended 1D systems: have people seen them? Have they not seen them?”, says Srijit Goswami of QuTech in Delft.

 Controlling disorder

 In 2012, theoretical physicists Jay Sau, then of Harvard University and Sankar Das Sarma of the University of Maryland proposed looking for Majorana bound states in quantum dots. “We looked at [the nanowires] and thought ‘OK, this is going to be a while given the amount of disorder that system has – what are the ways this disorder could be controlled?’ and this is exactly one of the ways we thought it could work,” explains Sau. The research was not taken seriously at the time, however, Sau says, partly because people underestimated the problem of disorder.

Goswami and others have previously observed “poor man’s Majoranas” (PMMs) in two quantum dots. While they share some properties with Majorana modes, PMMs lack topological protection. Last year the group coupled two spin-polarized quantum dots connected by a semiconductor–superconductor hybrid material. At specific points, the researchers found zero-bias conductance peaks.

“Kitaev says that if you tune things exactly right you have one Majorana on one dot and another Majorana on another dot,” says Sau. “But if you’re slightly off then they’re talking to each other. So it’s an uncomfortable notion that they’re spatially separated if you just have two dots next to each other.”

Recently, a group that included Goswami’s colleagues at QuTech found that the introduction of a third quantum dot stabilized the Majorana modes. However, they were unable to measure the energy levels in the quantum dots.

Zero energy

In new work, Goswami’s team used systems of three electrostatically-gated, spin-polarized quantum dots in a 2D electron gas joined by hybrid semiconductor–superconductor regions. The quantum dots had to be tuned to zero energy. The dots exchanged charge in two ways: by standard electron hopping through the semiconductor and by Cooper-pair mediated coupling through the superconductor.

“You have to change the energy level of the superconductor–semiconductor hybrid region so that these two processes have equal probability,” explains Goswami. “Once you satisfy these conditions, then you get Majoranas at the ends.”

In addition to more topological protection, the addition of a third qubit provided the team with crucial physical insight. “Topology is actually a property of a bulk system,” he explains; “Something special happens in the bulk which gives rise to things happening at the edges. Majoranas are something that emerge on the edges because of something happening in the bulk.” With three quantum dots, there is a well-defined bulk and edge that can be probed separately: “We see that when you have what is called a gap in the bulk your Majoranas are protected, but if you don’t have that gap your Majoranas are not protected,” Goswami says.

To produce a qubit will require more work to achieve the controllable coupling of four Majorana bound states and the integration of a readout circuit to detect this coupling. In the near-term, the researchers are investigating other phenomena, such as the potential to swap Majorana bound states.

Sau is now at the University of Maryland and says that an important benefit of the experimental platform is that it can be determined unambiguously whether or not Majorana bound states have been observed. “You can literally put a theory simulation next to the experiment and they look very similar.”

 The research is published in Nature.

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Powerful flares on highly-magnetic neutron stars called magnetars could produce up to 10% of the universe’s gold, silver and platinum, according to a new study. What is more, astronomers may have already observed this cosmic alchemy in action.

Gold, silver, platinum and a host of other rare heavy nuclei are known as rapid-process (r-process) elements. This is because astronomers believe that these elements are produced by the rapid capture of neutrons by lighter nuclei. Neutrons can only exist outside of an atomic nucleus for about 15 min before decaying (except in the most extreme environments). This means that the r-process must be fast and take place in environments rich in free neutrons.

In August 2017, an explosion resulting from the merger of two neutron stars was witnessed by telescopes operating across the electromagnetic spectrum and by gravitational-wave detectors. Dubbed a kilonova, the explosion produced approximately 16,000 Earth-masses worth of r-process elements, including about ten Earth masses of gold and platinum.

While the observations seem to answer the question of where precious metals came from, there remains a suspicion that neutron-star mergers cannot explain the entire abundance of r-process elements in the universe.

Giant flares

Now researchers led by Anirudh Patel, who is a PhD student at New York’s Columbia University, have created a model that describes how flares on the surface of magnetars can create r-process elements.

Patel tells Physics World that “The rate of giant flares is significantly greater than mergers.” However, given that one merger “produces roughly 10,000 times more r-process mass than a single magnetar flare”, neutron-star mergers are still the dominant factory of rare heavy elements.

A magnetar is an extreme type of neutron star with a magnetic field strength of up to a thousand trillion gauss. This makes magnetars the most magnetic objects in the universe. Indeed, if a magnetar were as close to Earth as the Moon, its magnetic field would wipe your credit card.

Astrophysicists believe that when a magnetar’s powerful magnetic fields are pulled taut, the magnetic tension will inevitably snap. This would result in a flare, which is an energetic ejection of neutron-rich material from the magnetar’s surface.

Mysterious mechanism

However, the physics isn’t entirely understood, according to Jakub Cehula of Charles University in the Czech Republic, who is a member of Patel’s team. “While the source of energy for a magnetar’s giant flares is generally agreed to be the magnetic field, the exact mechanism by which this energy is released is not fully understood,” he explains.

One possible mechanism is magnetic reconnection, which creates flares on the Sun. Flares could also be produced by energy released during starquakes following a build-up of magnetic stress. However, neither satisfactorily explains the giant flares, of which only nine have thus far been detected.

In 2024 Cehula led research that attempted to explain the flares by combining starquakes with magnetic reconnection. “We assumed that giant flares are powered by a sudden and total dissipation of the magnetic field right above a magnetar’s surface,” says Cehula.

This sudden release of energy drives a shockwave into the magnetar’s neutron-rich crust, blasting a portion of it into space at velocities greater than a tenth of the speed of light, where in theory heavy elements are formed via the r-process.

Gamma-ray burst

Remarkably, astronomers may have already witnessed this in 2004, when a giant magnetar flare was spotted as a half-second gamma-ray burst that released more energy than the Sun does in a million years. What happened next remained unexplained until now. Ten minutes after the initial burst, the European Space Agency’s INTEGRAL satellite detected a second, weaker signal that was not understood.

Now, Patel and colleagues have shown that the r-process in this flare created unstable isotopes that quickly decayed into stable heavy elements – creating the gamma-ray signal.

Patel calculates that the 2004 flare resulted in the creation of two million billion billion kilograms of r-process elements, equivalent to about the mass of Mars.

Extrapolating, Patel calculates that giant flares on magnetars contribute between 1–10% of all the r-process elements in the universe.

Lots of magnetars

“This estimate accounts for the fact that these giant flares are rare,” he says, “But it’s also important to note that magnetars have lifetimes of 1000 to 10,000 years, so while there may only be a couple of dozen magnetars known to us today, there have been many more magnetars that have lived and died over the course of the 13 billion-year history of our galaxy.”

Magnetars would have been produced early in the universe by the supernovae of massive stars, whereas it can take a billion years or longer for two neutron stars to merge. Hence, magnetars would have been a more dominant source of r-process elements in the early universe. However, they may not have been the only source.

“If I had to bet, I would say there are other environments in which r-process elements can be produced, for example in certain rare types of core-collapse supernovae,” says Patel.

Either way, it means that some of the gold and silver in your jewellery was forged in the violence of immense magnetic fields snapping on a dead star.

The research is described in Astrophysical Journal Letters.

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Everyday life is three dimensional, with even a sheet of paper having a finite thickness. Shengxi Huang from Rice University in the US, however, is attracted by 2D materials, which are usually just one atomic layer thick. Graphene is perhaps the most famous example — a single layer of carbon atoms arranged in a hexagonal lattice. But since it was first created in 2004, all sorts of other 2D materials, notably boron nitride, have been created.

An electrical engineer by training, Huang did a PhD at the Massachusetts Institute of Technology and postdoctoral research at Stanford University before spending five years as an assistant professor at the Pennsylvania State University. Huang has been at Rice since 2022, where she is now an associate professor in the Department of Electrical and Computer Engineering, the Department of Material Science and NanoEngineering, and the Department of Bioengineering.

Her group at Rice currently has 12 people, including eight graduate students and four postdocs. Some are physicists, some are engineers, while others have backgrounds in material science or chemistry. But they all share an interest in understanding the optical and electronic properties of quantum materials and seeing how they can be used, for example, as biochemical sensors. Lab equipment from Picoquant is vital in helping in that quest, as Huang explains in an interview with Physics World.

Why are you fascinated by 2D materials?

I’m an electrical engineer by training, which is a very broad field. Some electrical engineers focus on things like communication and computing, but others, like myself, are more interested in how we can use fundamental physics to build useful devices, such as semiconductor chips. I’m particularly interested in using 2D materials for optoelectronic devices and as single-photon emitters.

What kinds of 2D materials do you study?

The materials I am particularly interested in are transition metal dichalcogenides, which consist of a layer of transition-metal atoms sandwiched between two layers of chalcogen atoms – sulphur, selenium or tellurium. One of the most common examples is molybdenum disulphide, which in its monolayer form has a layer of sulphur on either side of a layer of molybdenum. In multi-layer molybdenum disulphide, the van der Waals forces between the tri-layers are relatively weak, meaning that the material is widely used as a lubricant – just like graphite, which is a many-layer version of graphene.

Why do you find transition metal dichalcogenides interesting?

Transition metal dichalcogenides have some very useful optoelectronic properties. In particular, they emit light whenever the electron and hole that make up an “exciton” recombine. Now because these dichalcogenides are so thin, most of the light they emit can be used. In a 3D material, in contrast, most light is generated deep in the bulk of the material and doesn’t penetrate beyond the surface. Such 2D materials are therefore very efficient and, what’s more, can be easily integrated onto chip-based devices such as waveguides and cavities.

Transition metal dichalcogenide materials also have promising electronic applications, particularly as the active material in transistors. Over the years, we’ve seen silicon-based transistors get smaller and smaller as we’ve followed Moore’s law, but we’re rapidly reaching a limit where we can’t shrink them any further, partly because the electrons in very thin layers of silicon move so slowly. In 2D transition metal dichalcogenides, in contrast, the electron mobility can actually be higher than in silicon of the same thickness, making them a promising material for future transistor applications.

What can such sources of single photons be used for?

Single photons are useful for quantum communication and quantum cryptography. Carrying information as zero and one, they basically function as a qubit, providing a very secure communication channel. Single photons are also interesting for quantum sensing and even quantum computing. But it’s vital that you have a highly pure source of photons. You don’t want them mixed up with “classical photons”, which — like those from the Sun — are emitted in bunches as otherwise the tasks you’re trying to perform cannot be completed.

What approaches are you taking to improve 2D materials as single-photon emitters?

What we do is introduce atomic defects into a 2D material to give it optical properties that are different to what you’d get in the bulk. There are several ways of doing this. One is to irradiate a sample with ions or electrons, which can bombard individual atoms out to generate “vacancy defects”. Another option is to use plasmas, whereby atoms in the sample get replaced by atoms from the plasma.

So how do you study the samples?

We can probe defect emission using a technique called photoluminescence, which basically involves shining a laser beam onto the material. The laser excites electrons from the ground state to an excited state, prompting them to emit light. As the laser beam is about 500-1000 nm in diameter, we can see single photon emission from an individual defect if the defect density is suitable.

What sort of experiments do you do in your lab?

We start by engineering our materials at the atomic level to introduce the correct type of defect. We also try to strain the material, which can increase how many single photons are emitted at a time. Once we’ve confirmed we’ve got the correct defects in the correct location, we check the material is emitting single photons by carrying out optical measurements, such as photoluminescence. Finally, we characterize the purity of our single photons – ideally, they shouldn’t be mixed up with classical photons but in reality, you never have a 100% pure source. As single photons are emitted one at a time, they have different statistical characteristics to classical light. We also check the brightness and lifetime of the source, the efficiency, how stable it is, and if the photons are polarized. In fact, we have a feedback loop: what improvements can we do at the atomic level to get the properties we’re after?

Is it difficult adding defects to a sample?

It’s pretty challenging. You want to add just one defect to an area that might be just one micron square so you have to control the atomic structure very finely. It’s made harder because 2D materials are atomically thin and very fragile. So if you don’t do the engineering correctly, you may accidentally introduce other types of defects that you don’t want, which will alter the defects’ emission.

What techniques do you use to confirm the defects are in the right place?

Because the defect concentration is so low, we cannot use methods that are typically used to characterise materials, such as X-ray photo-emission spectroscopy or scanning electron microscopy. Instead, the best and most practical way is to see if the defects generate the correct type of optical emission predicted by theory. But even that is challenging because our calculations, which we work on with computational groups, might not be completely accurate.

How do your PicoQuant instruments help in that regard?

We have two main pieces of equipment – a MicroTime 100 photoluminescence microscope and a FluoTime 300 spectrometer. These have been customized to form a Hanbury Brown Twiss interferometer, which measures the purity of a single photon source. We also use the microscope and spectrometer to characterise photoluminescence spectrum and lifetime. Essentially, if the material emits light, we can then work out how long it takes before the emission dies down.

Did you buy the equipment off-the-shelf?

It’s more of a customised instrument with different components – lasers, microscopes, detectors and so on — connected together so we can do multiple types of measurement. I put in a request to Picoquant, who discussed my requirements with me to work out how to meet my needs. The equipment has been very important for our studies as we can carry out high-throughput measurements over and over again. We’ve tailored it for our own research purposes basically.

So how good are your samples?

The best single-photon source that we currently work with is boron nitride, which has a single-photon purity of 98.5% at room temperature. In other words, for every 200 photons only three are classical. With transition-metal dichalcogenides, we get a purity of 98.3% at cryogenic temperatures.

What are your next steps?

There’s still lots to explore in terms of making better single-photon emitters and learning how to control them at different wavelengths. We also want to see if these materials can be used as high-quality quantum sensors. In some cases, if we have the right types of atomic defects, we get a high-quality source of single photons, which we can then entangle with their spin. The emitters can therefore monitor the local magnetic environment with better performance than is possible with classical sensing methods.

• More information about the author’s work can be found in Sci. Adv. (11 2899,  ACS Nano (16 7428) and J. Phys. Chem. Lett. (14 3274).

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Comics are regarded as an artform in France, where they account for a quarter of all book sales. Nevertheless, the graphic novel World Without End: an Illustrated Guide to the Climate Crisis was a surprise French bestseller when it first came out in 2022. Taking the form of a Socratic dialogue between French climate expert Jean-Marc Jancovici and acclaimed comic artist Christophe Blain, it’s serious, scientific stuff.

Now translated into English by Edward Gauvin, the book follows the conventions of French-language comic strips or bandes dessinées. Jancovici is drawn with a small nose – denoting seriousness – while Blain’s larger nose signals humour. The first half explores energy and consumption, with the rest addressing the climate crisis and possible solutions.

Overall, this is a Trojan horse of a book: what appears to be a playful comic is packed with dense, academic content. Though marketed as a graphic novel, it reads more like illustrated notes from a series of sharp, provocative university lectures. It presents a frightening vision of the future and the humour doesn’t always land.

The book spans a vast array of disciplines – not just science and economics but geography and psychology too. In fact, there’s so much to unpack that, had I Blain’s skills, I might have reviewed it in the form of a comic strip myself. The old adage that “a picture is worth a thousand words” has never rung more true.

Absurd yet powerful visual metaphors feature throughout. We see a parachutist with a flaming main chute that represents our dependence on fossil fuels. The falling man jettisons his reserve chute – nuclear power – and tries to knit an alternative using clean energy, mid-fall. The message is blunt: nuclear may not be ideal, but it works.

World Without End is bold, arresting, provocative and at times polemical

The book is bold, arresting, provocative and at times polemical. Charts and infographics are presented to simplify complex issues, even if the details invite scrutiny. Explanations are generally clear and concise, though the author’s claim that accidents like Chernobyl and Fukushima couldn’t happen in France smacks of hubris.

Jancovici makes plenty of attention-grabbing statements. Some are sound, such as the notion that fossil fuels spared whales from extinction as we didn’t need this animal’s oil any more. Others are dubious – would a 4 °C temperature rise really leave a third of humanity unable to survive outdoors?

But Jancovici is right to say that the use of fossil fuels makes logical sense. Oil can be easily transported and one barrel delivers the equivalent of five years of human labour. A character called Armor Man (a parody of Iron Man) reminds us that fossil fuels are like having 200 mechanical slaves per person, equivalent to an additional 1.5 trillion people on the planet.

Fossil fuels brought prosperity – but now threaten our survival. For Jancovici, the answer is nuclear power, which is perhaps not surprising as it produces 72% of electricity in the author’s homeland. But he cherry picks data, accepting – for example – the United Nations figure that only about 50 people died from the Chernobyl nuclear accident.

While acknowledging that many people had to move following the disaster, the author downplays the fate of those responsible for “cleaning up” the site, the long-term health effects on the wider population and the staggering economic impact – estimated at €200–500bn. He also sidesteps nuclear-waste disposal and the cost and complexity of building new plants.

While conceding that nuclear is “not the whole answer”, Jancovici dismisses hydrogen and views renewables like wind and solar as too intermittent – they require batteries to ensure electricity is supplied on demand – and diffuse. Imagine blanketing the Earth in wind turbines.

Still, his views on renewables seem increasingly out of step. They now supply nearly 30% of global electricity – 13% from wind and solar, ahead of nuclear at 9%. Renewables also attract 70% of all new investment in electricity generation and (unlike nuclear) continue to fall in price. It’s therefore disingenuous of the author to say that relying on renewables would be like returning to pre-industrial life; today’s wind turbines are far more efficient than anything back then.

Beyond his case for nuclear, Jancovici offers few firm solutions. Weirdly, he suggests “educating women” and providing pensions in developing nations – to reduce reliance on large families – to stabilize population growth. He also cites French journalist Sébastien Bohler, who thinks our brains are poorly equipped to deal with long-term threats.

But he says nothing about the need for more investment in nuclear fusion or for “clean” nuclear fission via, say, liquid fluoride thorium reactors (LFTRs), which generate minimal waste, won’t melt down and cannot be weaponized.

Perhaps our survival depends on delaying gratification, resisting the lure of immediate comfort, and adopting a less extravagant but sustainable world. We know what changes are needed – yet we do nothing. The climate crisis is unfolding before our eyes, but we’re paralysed by a global-scale bystander effect, each of us hoping someone else will act first. Jancovici’s call for “energy sobriety” (consuming less) seems idealistic and futile.

Still, World Without End is a remarkable and deeply thought-provoking book that deserves to be widely read. I fear that it will struggle to replicate its success beyond France, though Raymond Briggs’ When the Wind Blows – a Cold War graphic novel about nuclear annihilation – was once a British bestseller. If enough people engaged with the book, it would surely spark discussion and, one day, even lead to meaningful action.

• 2024 Particular Books £25.00hb 196pp

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“Welcome to this special issue of Physics World, marking the 200th anniversary of quantum mechanics. In this double-quantum edition, the letters in this text are stored using qubits. As you read, you project the letters into a fixed state, and that information gets copied into your mind as the article that you are reading. This text is actually in a superposition of many different articles, but only one of them gets copied into your memory. We hope you enjoy the one that you are reading.”

That’s how I imagine the opening of the 2125 Physics World quantum special issue, when fully functional quantum computers are commonplace, and we have even figured out how to control individual qubits on display screens. If you are lucky enough to experience reading such a magazine, you might be disappointed as you can read only one of the articles the text gets projected into. The problem is that by reading the superposition of articles, you made them decohere, because you copied the information about each letter into your memory. Can you figure out a way to read the others too? After all, more Physics World articles is always better.

A possible solution may be if you could restore the coherence of the text by just erasing your memory of the particular article you read. Once you no longer have information identifying which article your magazine was projected into, there is then no fundamental reason for it to remain decohered into a single state. You could then reread it to enjoy a different article.

While this thought experiment may sound fantastical, the concept is closely connected to a mind-bending twist on the famous double-slit experiment, known as the delayed-choice quantum eraser. It is often claimed to exhibit a radical phenomenon: where measurements made in the present alter events that occurred in the past. But is such a paradoxical suggestion real, even in the notoriously strange quantum realm?

A double twist on the double slit

In a standard double-slit experiment, photons are sent one by one through two slits to create an interference pattern on a screen, illustrating the wave-like behaviour of light. But if we add a detector that can spot which of the two slits the photon goes through, the interference disappears and we see only two distinct clumps on the screen, signifying particle-like behaviour. Crucially, gaining information about which path the photon took changes the photon’s quantum state, from the wave-like interference pattern to the particle-like clumps.

The first twist on this thought experiment is attributed to proposals from physicist John Wheeler in 1978, and a later collaboration with Wojciech Zurek in 1983. Wheeler’s idea was to delay the measurement of which slit the photon goes through. Instead of measuring the photon as it passes through the double-slit, the measurement could be delayed until just before the photon hits the screen. Interestingly, the delayed detection of which slit the photon goes through still determines whether or not it displays the wave-like or particle-like behaviour. In other words, even a detection done long after the photon has gone through the slit determines whether or not that photon is measured to have interfered with itself.

If that’s not strange enough, the delayed-choice quantum eraser is a further modification of this idea. First proposed by American physicists Marlan Scully and Kai Drühl in 1982 (Phys. Rev. A 25 2208), it was later experimentally implemented by Yoon-Ho Kim and collaborators using photons in 2000 (Phys. Rev. Lett. 84 1). This variation adds a second twist: if recording which slit the photon passes through causes it to decohere, then what happens if we were to erase that information? Imagine shrinking the detector to a single qubit that becomes entangled with the photon: “left” slit might correlate to the qubit being 0, “right” slit to 1. Instead of measuring whether the qubit is a 0 or 1 (revealing the path), we could measure it in a complementary way, randomising the 0s and 1s (erasing the path information).

Strikingly, while the screen still shows particle-like clumps overall, these complementary measurements of the single-qubit detector can actually be used to extract a wave-like interference pattern. This works through a sorting process: the two possible outcomes of the complementary measurements are used to separate out the photon detections on the screen. The separated patterns then each individually show bright and dark fringes.

I like to visualize this using a pair of 3D glasses, with one blue and one red lens. Each colour lens reveals a different individual image, like the two separate interference patterns. Without the 3D glasses, you see only the overall sum of the images. In the quantum eraser experiment, this sum of the images is a fully decohered pattern, with no trace of interference. Having access to the complementary measurements of the detector is like getting access to the 3D glasses: you now get an extra tool to filter out the two separate interference patterns.

Rewriting the past – or not?

If erasing the information at the detector lets us extract wave-like patterns, it may seem like we’ve restored wave-like behaviour to an already particle-like photon. That seems truly head-scratching. However, Jonte Hance, a quantum physicist at Newcastle University in the UK, highlights a different conclusion, focused on how the individual interference patterns add up to show the usual decohered pattern. “They all feel like they shouldn’t be able to fit together,” Hance explains. “It’s really showing that the correlations you get through entanglement have to be able to fit every possible way you could measure a system.” The results therefore reveal an intriguing aspect of quantum theory – the rich, counterintuitive structure of quantum correlations from entanglement – rather than past influences.

Even Wheeler himself did not believe the thought experiment implies backward-in-time influence, as explained by Lorenzo Catani, a researcher at the International Iberian Nanotechnology Laboratory (INL) in Portugal. Commenting on the history of the thought experiment, Catani notes that “Wheeler concluded that one must abandon a certain type of realism – namely, the idea that the past exists independently of its recording in the present. As far as I know, only a minority of researchers have interpreted the experiment as evidence for retrocausality.”

Eraser vs Bell: a battle of the bizarre

One physicist who is attempting to unpack this problem is Johannes Fankhauser at the University of Innsbruck, Austria. “I’d heard about the quantum eraser, and it had puzzled me a lot because of all these bizarre claims of backwards-in-time influence”, he explains. “I see something that sounds counterintuitive and puzzling and bizarre and then I want to understand it, and by understanding it, it gets a bit demystified.”

Fankhauser realized that the quantum eraser set-up can be translated into a very standard Bell experiment. These experiments are based on entangling a pair of qubits, the idea being to rule out local “hidden-variable” models of quantum theory. This led him to see that there is no need to explain the eraser using backwards-in-time influence, since the related Bell experiments can be understood without it, as explained in his 2017 paper (Quanta 8 44). Fankhauser then further analysed the thought experiment using the de Broglie–Bohm interpretation of quantum theory, which gives a physical model for the quantum wavefunction (as particles are guided by a “pilot” wave). Using this, he showed explicitly that the outcomes of the eraser experiment can be fully explained without requiring backwards-in-time influences.

So does that mean that the eraser doesn’t tell us anything else beyond what Bell experiments already tell us? Not quite. “It turns different knobs than the Bell experiment,” explains Fankhauser. “I would say it asks the question ‘what do measurements signify?’, and ‘when can I talk about the system having a property?’. That’s an interesting question and I would say we don’t have a full answer to this.”

In particular, the eraser demonstrates the importance that the very act of observation has on outcomes, with the detector playing the role of an observer. “You measure some of its properties, you change another property,” says Fankhauser. “So the next time you measure it, the new property was created through the observation. And I’m trying to formalize this now more concretely. I’m trying to come up with a new approach and framework to study these questions.”

Meanwhile, Catani found an intriguing contrast between Bell experiments and the eraser in his research. “The implications of Bell’s theorem are far more profound,” says Catani. In the 2023 paper (Quantum 7 1119) he co-authored, Catani considers a model for classical physics, with an extra condition: there is a restriction on what you can know about the underlying physical states. Applying this model to the quantum eraser, he finds that its results can be reproduced by such a classical theory. By contrast, the classical model cannot reproduce the statistical violations of a Bell experiment. This shows that having incomplete knowledge of the physical state is not, by itself, enough to explain the strange results of the Bell experiment. It is therefore demonstrating a more powerful deviation from classical physics than the eraser. Catani also contrasts the mathematical rigour of the two cases. While Bell experiments are based on explicitly formulated assumptions, claims about backwards-in-time influence in the quantum eraser rely on a particular narrative – one that gives rise to the apparent paradox

The eraser as a brainteaser

Physicists therefore broadly agree that the mathematics of the quantum eraser thought experiment fits well within standard quantum theory. Even so, Hance argues that formal results alone are not the entire story: “This is something we need to pick apart, not just in terms of mathematical assumptions, but also in terms of building intuitions for us to be able to actually play around with what quantumness is.” Hance has been analysing the physical implications of different assumptions in the thought experiment, with some options discussed in his 2021 preprint (arXiv:2111.09347) with collaborators on the quantum eraser paradox.

It therefore provides a tool for understanding how quantum correlations match up in a way that is not described by classical physics. “It’s a great thinking aid – partly brainteaser, partly demonstration of the nature of this weirdness.”

Information, observers and quantum computers

Every quantum physicist takes something different from the quantum eraser, whether it is a spotlight on the open problems surrounding the properties of measured systems; a lesson from history in mathematical rigour; or a counterintuitive puzzle to make sense of. For a minority that deviate from standard approaches to quantum theory, it may even be some form of backwards-in-time influence.

For myself, as explained in my video on YouTube and my 2023 paper (IEEE International Conference on Quantum Computing and Engineering 10.1109/QCE57702.2023.20325) on quantum thought experiments, the most dramatic implication of the quantum eraser is explaining the role of observers in the double-slit experiment. The quantum eraser emphasizes that even a single entanglement between qubits will cause decoherence, whether or not it is measured afterwards – meaning that no mysterious macroscopic observer is required. This also explains why building a quantum computer is so challenging, as unwanted entanglement with even one particle can cause the whole computation to collapse into a random state.

The quantum eraser emphasizes that even a single entanglement between qubits will cause decoherence, whether or not it is measured afterwards – meaning that no mysterious macroscopic observer is required

Where does this leave the futuristic readers of our 200-year double-quantum special issue of Physics World? Simply erasing their memories is not enough to restore the quantum behaviour of the article. It is too late to change which article was selected. Though, following an eraser-type protocol, our futurists can do one better than those sneaky magazine writers: they can use the outcomes of complementary measurements on their memory, to sort the article into two individual smaller articles, each displaying their own quantum entanglement structure that was otherwise hidden. So even if you can’t use the quantum eraser to rewrite the past, perhaps it can rewrite what you read in the future.

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Head-and-neck cancers are difficult to treat with radiation therapy because they are often located close to organs that are vital for patients to maintain a high quality-of-life. Radiation therapy can also alter a person’s shape, through weight loss or swelling, making it essential to monitor such changes throughout the treatment to ensure effective tumour targeting.

Researchers from Corewell Health William Beaumont University Hospital have now used a new proton therapy technique called step-and-shoot proton arc therapy (a spot-scanning proton arc method) to treat head-and-neck cancer in a human patient – the first person in the US to receive this highly accurate treatment.

“We envisioned that this technology could significantly improve the quality of treatment plans for patients and the treatment efficiency compared with the current state-of-the-art technique of intensity-modulated proton therapy (IMPT),” states senior author Xuanfeng Ding.

Progression towards dynamic proton arc therapy

“The first paper on spot-scanning proton arc therapy was published in 2016 and the first prototype for it was built in 2018,” says Ding. However, step-and-shoot proton arc therapy is an interim solution towards a more advanced technique known as dynamic proton arc therapy – which delivered its first pre-clinical treatment in 2024. Dynamic proton arc therapy is still undergoing development and regulatory approval clearance, so researchers have chosen to use step-and-shoot proton arc therapy clinically in the meantime.

Other proton therapies are more manual in nature and require a lot of monitoring, but the step-and-shoot technology delivers radiation directly to a tumour in a more continuous and automated fashion, with less lag time between radiation dosages. “Step-and-shoot proton arc therapy uses more beam angles per plan compared to the current clinical practice using IMPT and optimizes the spot and energy layers sparsity level,” explains Ding.

The extra beam angles provide a greater degree-of-freedom to optimize the treatment plan and provide a better dose conformity, robustness and linear energy transfer (LET, the energy deposited by ionizing radiation) through a more automated approach. During treatment delivery, the gantry rotates to each beam angle and stops to deliver the treatment irradiation.

In the dynamic proton arc technique that is also being developed, the gantry rotates continuously while irradiating the proton spot or switching energy layer. The step-and-shoot proton arc therapy therefore acts as an interim stage that is allowing more clinical data to be acquired to help dynamic proton arc therapy become clinically approved. The pinpointing ability of these proton therapies enables tumours to be targeted more precisely without damaging surrounding healthy tissue and organs.

The first clinical treatment

The team trialled the new technique on a patient with adenoid cystic carcinoma in her salivary gland – a rare and highly invasive cancer that’s difficult to treat as it targets the nerves in the body. This tendency to target nerves also means that fighting such tumours typically causes a lot of side effects. Using the new step-and-shoot proton arc therapy, however, the patient experienced minimal side effects and no radiation toxicity to other areas of her body (including the brain) after 33 treatments. Since finishing her treatment in August 2024, she continues to be cancer-free.

“Radiation to the head-and-neck typically results in dryness of the mouth, pain and difficulty swallowing, abnormal taste, fatigue and difficulty with concentration,” says Rohan Deraniyagala, a Corewell Health radiation oncologist involved with this research. “Our patient had minor skin irritation but did not have any issues with eating or performing at her job during treatment and for the last year since she was diagnosed.”

Describing the therapeutic process, Ding tells Physics World that “we developed an in-house planning optimization algorithm to select spot and energy per beam angle so the treatment irradiation time could be reduced to four minutes. However, because the gantry still needs to stop at each beam angle, the total treatment time is about 16 minutes per fraction.”

On monitoring the progression of the tumour over time and developing treatment plans, Ding confirms that the team “implemented a machine-learning-based synthetic CT platform which allows us to track the daily dosage of radiation using cone-beam computed tomography (CBCT) so that we can schedule an adaptive treatment plan for the patient.”

On the back of this research, Ding says that the next step is to help further develop the dynamic proton arc technique – known as DynamicARC – in collaboration with industry partner IBA.

The research was published in the International Journal of Particle Therapy.

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Arrays of superconducting wires have been used to detect beams of high-energy charged particles. Much thinner wires are already used to detect single photons, but this latest incarnation uses thicker wires that can absorb the large amounts of energy carried by fast-moving protons, electrons, and pions. The new detector was created by an international team led by Cristián Peña at Fermilab.

In a single-photon detector, an array of superconducting nanowires is operated below the critical temperature for superconductivity – with current flowing freely through the nanowires. When a nanowire absorbs a photon it creates a hotspot that temporarily destroys superconductivity and boosts the electrical resistance. This creates a voltage spike across the nanowire, allowing the location and time of the photon detection to be determined very precisely.

“These detectors have emerged as the most advanced time-resolved single-photon sensors in a wide range of wavelengths,” Peña explains. “Applications of these photon detectors include quantum networking and computing, space-to-ground communication, exoplanet exploration and fundamental probes for new physics such as dark matter.”

A similar hotspot is created when a superconducting wire is impacted by a high-energy charged particle. In principle, this could be used to create particle detectors that could be used in experiments at labs such as Fermilab and CERN.

New detection paradigm

“As with photons, the ability to detect charged particles with high spatial and temporal precision, beyond what traditional sensing technologies can offer, has the potential to propel the field of high-energy physics towards a new detection paradigm,” Peña explains.

However, the nanowire single-photon detector design is not appropriate for detecting charged particles. Unlike photons, charged particles do not deposit all of their energy at a single point in a wire. Instead, the energy can be spread out along a track, which becomes longer as particle energy increases. Also, at the relativistic energies reached at particle accelerators, the nanowires used in single-photon detectors are too thin to collect the energy required to trigger a particle detection.

To create their new particle detector, Peña’s team used the latest advances in superconductor fabrication. On a thin film of tungsten silicide, they deposited an 8×8, 2 mm² array of micron-thick superconducting wires.

Tested at Fermilab

To test out their superconducting microwire single-photon detector (SMSPD), they used it to detect high-energy particle beams generated at the Fermilab Test Beam Facility. These included a 12 GeV beam of protons and 8 GeV beams of electrons and pions.

“Our study shows for the first time that SMSPDs are sensitive to protons, electrons, and pions,” Peña explains. “In fact, they behave very similarly when exposed to different particle types. We measured almost the same detection efficiency, as well as spatial and temporal properties.”

The team now aims to develop a deeper understanding of the physics that unfolds as a charged particle passes through a superconducting microwire. “That will allow us to begin optimizing and engineering the properties of the superconducting material and sensor geometry to boost the detection efficiency, the position and timing precision, as well as optimize for the operating temperature of the sensor,” Peña says. With further improvements SMSPDs to become an integral part of high-energy physics experiments – perhaps paving the way for a deeper understanding of fundamental physics.

The research is described in the Journal of Instrumentation.

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There are two main approaches to what we consider neuromorphic computing. The first involves emulating biological neural processing systems through the physics of computation of computational substrates that have similar properties and constraints as real neural systems, with potential for denser structures and advantages in energy cost. The other simulates neural processing systems on scalable architectures that allow the simulation of large neural networks, with higher degree of abstraction, arbitrary precision, high resolution, and no constraints imposed by the physics of the computing medium.

Both may be required to advance the field, but is either approach ‘better’? Hosted by Neuromorphic Computing and Engineering, this webinar will see teams of leading experts in the field of neuromorphic computing argue the case for either approach, overseen by an impartial moderator.

Team emulation:
Elisa Donati. Elisa’s research interests aim at designing neuromorphic circuits that are ideally suited for interfacing with the nervous system and show how they can be used to build closed-loop hybrid artificial and biological neural processing systems.  She is also involved in the development of neuromorphic hardware and software systems able to mimic the functions of biological brains to apply for medical and robotics applications.

Jennifer Hasler received her BSE and MS degrees in electrical engineering from Arizona State University in August 1991. She received her PhD in computation and neural systems from California Institute of Technology in February 1997. Jennifer is a professor at the Georgia Institute of Technology in the School of Electrical and Computer Engineering; Atlanta is the coldest climate in which she has lived. Jennifer founded the Integrated Computational Electronics (ICE) laboratory at Georgia Tech, a laboratory affiliated with the Laboratories for Neural Engineering. She is a member of Tau Beta P, Eta Kappa Nu, and the IEEE.

Team simulation:
Catherine (Katie) Schuman is an assistant professor in the Department of Electrical Engineering and Computer Science at the University of Tennessee (UT). She received her PhD in computer science from UT in 2015, where she completed her dissertation on the use of evolutionary algorithms to train spiking neural networks for neuromorphic systems. Katie previously served as a research scientist at Oak Ridge National Laboratory, where her research focused on algorithms and applications of neuromorphic systems. Katie co-leads the TENNLab Neuromorphic Computing Research Group at UT. She has written for more than 70 publications as well as seven patents in the field of neuromorphic computing. She received the Department of Energy Early Career Award in 2019. Katie is a senior member of the Association of Computing Machinery and the IEEE.

Emre Neftci received his MSc degree in physics from EPFL in Switzerland, and his PhD in 2010 at the Institute of Neuroinformatics at the University of Zurich and ETH Zurich. He is currently an institute director at the Jülich Research Centre and professor at RWTH Aachen. His current research explores the bridges between neuroscience and machine learning, with a focus on the theoretical and computational modelling of learning algorithms that are best suited to neuromorphic hardware and non-von Neumann computing architectures.

Discussion chair:
Giulia D’Angelo is currently a Marie Skłodowska-Curie postdoctoral fellow at the Czech Technical University in Prague, where she focuses on neuromorphic algorithms for active vision. She obtained a bachelor’s degree in biomedical engineering from the University of Genoa and a master’s degree in neuroengineering with honours. During her master’s, she developed a neuromorphic system for the egocentric representation of peripersonal visual space at King’s College London. She earned her PhD in neuromorphic algorithms at the University of Manchester, receiving the President’s Doctoral Scholar Award, in collaboration with the Event-Driven Perception for Robotics Laboratory at the Italian Institute of Technology. There, she proposed a biologically plausible model for event-driven, saliency-based visual attention. She was recently awarded the Marie Skłodowska-Curie Fellowship to explore sensorimotor contingency theories in the context of neuromorphic active vision algorithms.

About this journal

Neuromorphic Computing and Engineering is a multidisciplinary, open access journal publishing cutting-edge research on the design, development and application of artificial neural networks and systems from both a hardware and computational perspective.

Editor-in-chief: Giacomo Indiveri, University of Zurich, Switzerland

 

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