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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Scientists have created a novel antimicrobial coating that, when mixed with paint, can be applied to a range of surfaces to destroy bacteria and viruses – including particularly persistent and difficult to kill strains like MRSA, flu virus and SARS-CoV-2. The development potentially paves the way for substantial improvements in scientific, commercial and clinical hygiene.

The University of Nottingham-led team made the material by combining chlorhexidine digluconate (CHX) – a disinfectant commonly used by dentists to treat mouth infections and by clinicians for cleaning before surgery – with everyday paint-on epoxy resin. Using this material, the team worked with staff at Birmingham-based specialist coating company Indestructible Paint to create a prototype antimicrobial paint. They found that, when dried, the coating can kill a wide range of pathogens.

The findings of the study, which was funded by the Royal Academy of Engineering Industrial Fellowship Scheme, were published in Scientific Reports.

Persistent antimicrobial protection

As part of the project, the researchers painted the antimicrobial coating onto a surface and used a range of scientific techniques to analyse the distribution of the biocide in the paint, to confirm that it remained uniformly distributed at a molecular level.

According to project leader Felicity de Cogan, the new paint can be used to provide antimicrobial protection on a wide array of plastic and hard non-porous surfaces. Crucially, it could be effective in a range of clinical environments, where surfaces like hospital beds and toilet seats can act as a breeding ground for bacteria for extended periods of time – even after the introduction of stringent cleaning regimes.

The team, based at the University’s School of Pharmacy, is also investigating the material’s use in the transport and aerospace industries, especially on frequently touched surfaces in public spaces such as aeroplane seats and tray tables.

“The antimicrobial in the paint is chlorhexidine – a biocide commonly used in products like mouthwash. Once it is added, the paint works in exactly the same way as all other paint and the addition of the antimicrobial doesn’t affect its application or durability on the surface,” says de Cogan.

The researchers also note that adding CHX to the epoxy resin did not affect its optical transparency.

According to de Cogan, the novel concoction has a range of potential scientific, clinical and commercial applications.

“We have shown that it is highly effective against a range of different pathogens like E. coli and MRSA. We have also shown that it is effective against bacteria even when they are already resistant to antibiotics and biocides,” she says. “This means the technology could be a useful tool to circumvent the global problem of antimicrobial resistance.”

In de Cogan’s view, there are also number of major advantages to using the new coating to tackle bacterial infection – especially when compared to existing approaches – further boosting the prospects of future applications.

The key advantage of the technology is that the paint is “self-cleaning” – meaning that it would no longer be necessary to carry out the arduous task of repeatedly cleaning a surface to remove harmful microbes. Instead, after a single application, the simple presence of the paint on the surface would actively and continuously kill bacteria and viruses whenever they come into contact with it.

“This means that you can be sure a surface won’t pass on infections when you touch it,” says de Cogan.

“We are looking at more extensive testing in harsher environments and long-term durability testing over months and years. This work is ongoing and we will be following up with another publication shortly,” she adds.

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Peer review is a cornerstone of academic publishing. It is how we ensure that published science is valid. Peer review, by which researchers judge the quality of papers submitted to journals, stops pseudoscience from being peddled as equivalent to rigorous research. At the same time, the peer-review system is under considerable strain as the number of journal articles published each year increases, jumping from 1.9 million in 2016 to 2.8 million in 2022, according to Scopus and Web of Science.

All these articles require experienced peer reviewers, with papers typically taking months to go through peer review. This cannot be blamed alone on the time taken to post manuscripts and reviews back and forth between editors and reviewers, but instead is a result of high workloads and, fundamentally, how busy everyone is. Given peer reviewers need to be expert in their field, the pool of potential reviewers is inherently limited. A bottleneck is emerging as the number of papers grows quicker than the number of researchers in academia.

Scientific publishers have long been central to managing the process of peer review. For anyone outside academia, the concept of peer review may seem illogical given that researchers spend their time on it without much acknowledgement. While initiatives are in place to change this such as outstanding-reviewer awards and the Web of Science recording reviewer data, there is no promise that such recognition will be considered when looking for permanent positions or applying for promotion.

The impact of open access

Why, then, do we agree to review? As an active researcher myself in quantum physics, I peer-reviewed more than 40 papers last year and I’ve always viewed it as a duty. It’s a necessary time-sink to make our academic system function, to ensure that published research is valid and to challenge questionable claims. However, like anything people do out of a sense of duty, inevitably there are those who will seek to exploit it for profit.

Many journals today are open access, in which fees, known as article-processing charges, are levied to make the published work freely available online. It makes sense that costs need to be imposed – staff working at publishing companies need paying; articles need editing and typesetting; servers need be maintained and web-hosting fees have to be paid. Recently, publishers have invested heavily in digital technology and developed new ways to disseminate research to a wider audience.

Open access, however, has encouraged some publishers to boost revenues by simply publishing as many papers as possible. At the same time, there has been an increase in retractions, especially of fabricated or manipulated manuscripts sold by “paper mills”. The rise of retractions isn’t directly linked to the emergence of open access, but it’s not a good sign, especially when the academic publishing industry reports profit margins of roughly 40% – higher than many other industries. Elsevier, for instance, publishes nearly 3000 journals and in 2023 its parent company, Relx, recorded a profit of £1.79bn. This is all money that was either paid in open-access fees or by libraries (or private users) for journal subscriptions but ends up going to shareholders rather than science.

It’s important to add that not all academic publishers are for-profit. Some, like the American Physical Society (APS), IOP Publishing, Optica, AIP Publishing and the American Association for the Advancement of Science – as well as university presses – are wings of academic societies and universities. Any profit they make is reinvested into research, education or the academic community. Indeed, IOP Publishing, AIP Publishing and the APS have formed a new “purpose-led publishing” coalition, in which the three publishers confirm that they will continue to reinvest the funds generated from publishing back into research and “never” have shareholders that result in putting “profit above purpose”.

But many of the largest publishers – the likes of Springer Nature, Elsevier, Taylor and Francis, MDPI and Wiley – are for-profit companies and are making massive sums for their shareholders. Should we just accept that this is how the system is? If not, what can we do about it and what impact can we as individuals have on a multi-billion-dollar industry? I have decided that I will no longer review for, nor submit my articles (when corresponding author) to, any for-profit publishers.

I’m lucky in my field that I have many good alternatives such as the arXiv overlay journal Quantum, IOP Publishing’s Quantum Science and Technology, APS’s Physical Review X Quantum and Optica Quantum. If your field doesn’t, then why not push for them to be created? We may not be able to dismantle the entire for-profit publishing industry, but we can stop contributing to it (especially those who have a permanent job in academia and are not as tied down by the need to publish in high impact factor journals). Such actions may seem small, but together can have an effect and push to make academia the environment we want to be contributing to. It may sound radical to take change into your own hands, but it’s worth a try. You never know, but it could help more money make its way back into science.

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Researchers from four universities in Shanghai, China, are developing a practical visual assistance system to help blind and partially sighted people navigate. The prototype system combines lightweight camera headgear, rapid-response AI-facilitated software and artificial “skins” worn on the wrists and finger that provide physiological sensing. Functionality testing suggests that the integration of visual, audio and haptic senses can create a wearable navigation system that overcomes current designs’ adoptability and usability concerns.

Worldwide, 43 million people are blind, according to 2021 estimates by the International Agency for the Prevention of Blindness. Millions more are so severely visually impaired that they require the use of a cane to navigate.

Visual assistance systems offer huge potential as navigation tools, but current designs have many drawbacks and challenges for potential users. These include limited functionality with respect to the size and weight of headgear, battery life and charging issues, slow real-time processing speeds, audio command overload, high system latency that can create safety concerns, and extensive and sometimes complex learning requirements.

Innovations in miniaturized computer hardware, battery charge longevity, AI-trained software to decrease latency in auditory commands, and the addition of lightweight wearable sensory augmentation material providing near-real-time haptic feedback are expected to make visual navigation assistance viable.

The team’s prototype visual assistance system, described in Nature Machine Intelligence, incorporates an RGB-D (red, green, blue, depth) camera mounted on a 3D-printed glasses frame, ultrathin artificial skins, a commercial lithium-ion battery, a wireless bone-conducting earphone and a virtual reality training platform interfaced via triboelectric smart insoles. The camera is connected to a microcontroller via USB, enabling all computations to be performed locally without the need for a remote server.

When a user sets a target using a voice command, AI algorithms process the RGB-D data to estimate the target’s orientation and determine an obstacle-free direction in real time. As the user begins to walk to the target, bone conduction earphones deliver spatialized cues to guide them, and the system updates the 3D scene in real time.

The system’s real-time visual recognition incorporates changes in distance and perspective, and can compensate for low ambient light and motion blur. To provide robust obstacle avoidance, it combines a global threshold method with a ground interval approach to accurately detect overhead hanging, ground-level and sunken obstacles, as well as sloping or irregular ground surfaces.

First author Jian Tang of Shanghai Jiao Tong University and colleagues tested three audio feedback approaches: spatialized cues, 3D sounds and verbal instructions. They determined that spatialized cues are the most rapid to convey and be understood and provide precise direction perception.

Real-world testing A visually impaired person navigates through a cluttered conference room. (Courtesy: Tang et al. Nature Machine Intelligence)

To complement the audio feedback, the researchers developed stretchable artificial skin – an integrated sensory-motor device that provides near-distance alerting. The core component is a compact time-of-flight sensor that vibrates to stimulate the skin when the distance to an obstacle or object is smaller than a predefined threshold. The actuator is designed as a slim, lightweight polyethylene terephthalate cantilever. A gap between the driving circuit and the skin promotes air circulation to improve skin comfort, breathability and long-term wearability, as well as facilitating actuator vibration.

Users wear the sensor on the back of an index or middle finger, while the actuator and driving circuit are worn on the wrist. When the artificial skin detects a lateral obstacle, it provides haptic feedback in just 18 ms.

The researchers tested the trained system in virtual and real-world environments, with both humanoid robots and 20 visually impaired individuals who had no prior experience of using visual assistance systems. Testing scenarios included walking to a target while avoiding a variety of obstacles and navigating through a maze. Participants’ navigation speed increased with training and proved comparable to walking with a cane. Users were also able to turn more smoothly and were more efficient at pathfinding when using the navigation system than when using a cane.

“The proficient completion of tasks mirroring real-world challenges underscores the system’s effectiveness in meeting real-life challenges,” the researchers write. “Overall, the system stands as a promising research prototype, setting the stage for the future advancement of wearable visual assistance.”

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Subtle quantum effects within atomic nuclei can dramatically affect how some nuclei break apart. By studying 100 isotopes with masses below that of lead, an international team of physicists uncovered a previously unknown region in the nuclear landscape where fragments of fission split in an unexpected way. This is driven not by the usual forces, but by shell effects rooted in quantum mechanics.

“When a nucleus splits apart into two fragments, the mass and charge distribution of these fission fragments exhibits the signature of the underlying nuclear structure effect in the fission process,” explains Pierre Morfouace of Université Paris-Saclay, who led the study. “In the exotic region of the nuclear chart that we studied, where nuclei do not have many neutrons, a symmetric split was previously expected. However, the asymmetric fission means that a new quantum effect is at stake.”

This unexpected discovery not only sheds light on the fine details of how nuclei break apart but also has far-reaching implications. These range from the development of safer nuclear energy to understanding how heavy elements are created during cataclysmic astrophysical events like stellar explosions.

Quantum puzzle

Fission is the process by which a heavy atomic nucleus splits into smaller fragments. It is governed by a complex interplay of forces. The strong nuclear force, which binds protons and neutrons together, competes with the electromagnetic repulsion between positively charged protons. The result is that certain nuclei are unstable and typically leads to a symmetric fission.

But there’s another, subtler phenomenon at play: quantum shell effects. These arise because protons and neutrons inside the nucleus tend to arrange themselves into discrete energy levels or “shells,” much like electrons do in atoms.

“Quantum shell effects [in atomic electrons] play a major role in chemistry, where they are responsible for the properties of noble gases,” says Cedric Simenel of the Australian National University, who was not involved in the study. “In nuclear physics, they provide extra stability to spherical nuclei with so-called ‘magic’ numbers of protons or neutrons. Such shell effects drive heavy nuclei to often fission asymmetrically.”

In the case of very heavy nuclei, such as uranium or plutonium, this asymmetry is well documented. But in lighter, neutron-deficient nuclei – those with fewer neutrons than their stable counterparts – researchers had long expected symmetric fission, where the nucleus breaks into two roughly equal parts. This new study challenges that view.

New fission landscape

To investigate fission in this less-explored part of the nuclear chart, scientists from the R3B-SOFIA collaboration carried out experiments at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. They focused on nuclei ranging from iridium to thorium, many of which had never been studied before. The nuclei were fired at high energies into a lead target to induce fission.

The fragments produced in each fission event were carefully analysed using a suite of high-resolution detectors. A double ionization chamber captured the number of protons in each product, while a superconducting magnet and time-of-flight detectors tracked their momentum, enabling a detailed reconstruction of how the split occurred.

Using this method, the researchers found that the lightest fission fragments were frequently formed with 36 protons, which is the atomic number of krypton. This pattern suggests the presence of a stabilizing shell effect at that specific proton number.

“Our data reveal the stabilizing effect of proton shells at Z=36,” explains Morfouace. “This marks the identification of a new ‘island’ of asymmetric fission, one driven by the light fragment, unlike the well-known behaviour in heavier actinides. It expands our understanding of how nuclear structure influences fission outcomes.”

Future prospects

“Experimentally, what makes this work unique is that they provide the distribution of protons in the fragments, while earlier measurements in sub-lead nuclei were essentially focused on the total number of nucleons,” comments Simenel.

Since quantum shell effects are tied to specific numbers of protons or neutrons, not just the overall mass, these new measurements offer direct evidence of how proton shell structure shapes the outcome of fission in lighter nuclei. This makes the results particularly valuable for testing and refining theoretical models of fission dynamics.

“This work will undoubtedly lead to further experimental studies, in particular with more exotic light nuclei,” Simenel adds. “However, to me, the ball is now in the camp of theorists who need to improve their modelling of nuclear fission to achieve the predictive power required to study the role of fission in regions of the nuclear chart not accessible experimentally, as in nuclei formed in the astrophysical processes.”

The research is described in Nature.

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A new type of coronagraph that could capture images of dim exoplanets that are extremely close to bright stars has been developed by a team led by Nico Deshler at the University of Arizona in the US. As well as boosting the direct detection of exoplanets, the new instrument could support advances in areas including communications, quantum sensing, and medical imaging.

Astronomers have confirmed the existence of nearly 6000 exoplanets, which are planets that orbit stars other as the Sun. The majority of these were discovered based on their effects on their companion stars, rather than being observed directly. This is because most exoplanets are too dim and too close to their companion stars for the exoplanet light to be differentiated from starlight. That is where a coronagraph can help.

A coronagraph is an astronomical instrument that blocks light from an extremely bright source to allow the observation of dimmer objects in the nearby sky. Coronagraphs were first developed a century ago to allow astronomers to observe the outer atmosphere (corona) of the Sun , which would otherwise be drowned out by light from the much brighter photosphere.

At the heart of a coronagraph is a mask that blocks the light from a star, while allowing light from nearby objects into a telescope. However, the mask (and the telescope aperture) will cause the light to interfere and create diffraction patterns that blur tiny features. This prevents the observation of dim objects that are closer to the star than the instrument’s inherent diffraction limit.

Off limits

Most exoplanets lie within the diffraction limit of today’s coronagraphs and Deshler’s team addressed this problem using two spatial mode sorters. The first device uses a sequence of optical elements to separate starlight from light originating from the immediate vicinity of the star. The starlight is then blocked by a mask while the rest of the light is sent through a second spatial mode sorter, which reconstructs an image of the region surrounding the star.

As well as offering spatial resolution below the diffraction limit, the technique approaches the fundamental limit on resolution that is imposed by quantum mechanics.

“Our coronagraph directly captures an image of the surrounding object, as opposed to measuring only the quantity of light it emits without any spatial orientation,” Deshler describes. “Compared to other coronagraph designs, ours promises to supply more information about objects in the sub-diffraction regime – which lie below the resolution limits of the detection instrument.”

To test their approach, Deshler and colleagues simulated an exoplanet orbiting at a sub-diffraction distance from a host star some 1000 times brighter. After passing the light through the spatial mode sorters, they could resolve the exoplanet’s position – which would have been impossible with any other coronagraph.

Context and composition

The team believe that their technique will improve astronomical images. “These images can provide context and composition information that could be used to determine exoplanet orbits and identify other objects that scatter light from a star, such as exozodiacal dust clouds,” Deshler says.

The team’s coronagraph could also have applications beyond astronomy. With the ability to detect extremely faint signals close to the quantum limit, it could help to improve the resolution of quantum sensors. This could to lead to new methods for detecting tiny variations in magnetic or gravitational fields.

Elsewhere, the coronagraph could help to improve non-invasive techniques for imaging living tissue on the cellular scale – with promising implications in medical applications such as early cancer detection and the imaging of neural circuits. Another potential use could be new multiplexing techniques for optical communications. This would see the coronagraph being used to differentiate between overlapping signals. This has the potential of boosting the rate at which data could be transferred between satellites and ground-based receivers.

The research is described in Optica.

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Five-body recombination, in which five identical atoms form a tetramer molecule and a single free atom, could be the largest contributor to loss from ultracold atom traps at specific “Efimov resonances”, according to calculations done by physicists in the US. The process, which is less well understood than three- and four-body recombination, could be useful for building molecules, and potentially for modelling nuclear fusion.

A collision involving trapped atoms can be either elastic – in which the internal states of the atoms and their total kinetic energy remain unchanged – or inelastic, in which there is an interchange between the kinetic energy of the system and the internal energy states of the colliding atoms.

Most collisions in a dilute quantum gas involve only two atoms, and when physicists were first studying Bose-Einstein condensates (the ultralow-temperature state of some atomic gases), they suppressed inelastic two-body collisions, keeping the atoms in the desired state and preserving the condensate. A relatively small number of collisions, however, involve three or more bodies colliding simultaneously.

“They couldn’t turn off three body [inelastic collisions], and that turned out to be the main reason atoms leaked out of the condensate,” says theoretical physicist Chris Greene of Purdue University in the US.

Something remarkable

While attempting to understand inelastic three-body collisions, Greene and colleagues made the connection to work done in the 1970s by the Soviet theoretician Vitaly Efimov. He showed that at specific “resonances” of the scattering length, quantum mechanics allowed two colliding particles that could otherwise not form a bound state to do so in the presence of a third particle. While Efimov first considered the scattering of nucleons (protons and neutrons) or alpha particles, the effect applies to atoms and other quantum particles.

In the case of trapped atoms, the bound dimer and free atom are then ejected from the trap by the energy released from the binding event. “There were signatures of this famous Efimov effect that had never been seen experimentally,” Greene says. This was confirmed in 2005 by experiments from Rudolf Grimm’s group at the University of Innsbruck in Austria.

Hundreds of scientific papers have now been written about three-body recombination. Greene and colleagues subsequently predicted resonances at which four-body Efimov recombination could occur, producing a trimer. These were observed almost immediately by Grimm and colleagues. “Five was just too hard for us to do at the time, and only now are we able to go that next step,” says Greene.

Principal loss channel

In the new work, Greene and colleague Michael Higgins modelled collisions between identical caesium atoms in an optical trap. At specific resonances, five-body recombination – in which four atoms combine to produce a tetramer and a free particle – is not only enhanced but becomes the principal loss channel. The researchers believe these resonances should be experimentally observable using today’s laser box traps, which hold atomic gases in a square-well potential.

“For most ultracold experiments, researchers will be avoiding loss as much as possible – they would stay away from these resonances,” says Greene; “But for those of us in the few-body community interested in how atoms bind and resonate and how to describe complicated rearrangement, it’s really interesting to look at these points where the loss becomes resonant and very strong.” This is one technique that can be used to create new molecules, for example.

In future, Greene hopes to apply the model to nucleons themselves. “There have been very few people in the few-body theory community willing to tackle a five-particle collision – the Schrödinger equation has so many dimensions,” he says.

Fusion reactions

He hopes it may be possible to apply the researchers’ toolkit to nuclear reactions. “The famous one is the deuterium/tritium fusion reaction. When they collide they can form an alpha particle and a neutron and release a ton of energy, and that’s the basis of fusion reactors…There’s only one theory in the world from the nuclear community, and it’s such an important reaction I think it needs to be checked,” he says.

The researchers also wish to study the possibility of even larger bound states. However, they foresee a problem because the scattering length of the ground state resonance gets shorter and shorter with each additional particle. “Eventually the scattering length will no longer be the dominant length scale in the problem, and we think between five and six is about where that border line occurs,” Greene says. Nevertheless, higher-lying, more loosely-bound six-body Efimov resonances could potentially be visible at longer scattering lengths.

The research is described in Proceedings of the National Academy of Sciences.

Theoretical physicist Ravi Rau of Louisiana State University in the US is impressed by Greene and Higgins’ work. “For quite some time Chris Greene and a succession of his students and post-docs have been extending the three-body work that they did, using the same techniques, to four and now five particles,” he says. “Each step is much more complicated, and that he could use this technique to extend it to five bosons is what I see as significant.” Rau says, however, that “there is a vast gulf” between five atoms and the number treated by statistical mechanics, so new theoretical approaches may be required to bridge the gap.

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In the quantum world, observing a particle is not a passive act. If you shine light on a quantum object to measure its position, photons scatter off it and disturb its motion. This disturbance is known as quantum backaction noise, and it limits how precisely physicists can observe or control delicate quantum systems.

Physicists at Swansea University have now proposed a technique that could eliminate quantum backaction noise in optical traps, allowing a particle to remain suspended in space undisturbed. This would bring substantial benefits for quantum sensors, as the amount of noise in a system determines how precisely a sensor can measure forces such as gravity; detect as-yet-unseen interactions between gravity and quantum mechanics; and perhaps even search for evidence of dark matter.

There’s just one catch: for the technique to work, the particle needs to become invisible.

Levitating nanoparticles

Backaction noise is a particular challenge in the field of levitated optomechanics, where physicists seek to trap nanoparticles using light from lasers. “When you levitate an object, the whole thing moves in space and there’s no bending or stress, and the motion is very pure,” explains James Millen, a quantum physicist who studies levitated nanoparticles at Kings College, London, UK. “That’s why we are using them to detect crazy stuff like dark matter.”

While some noise is generally unavoidable, Millen adds that there is a “sweet spot” called the Heisenberg limit. “This is where you have exactly the right amount of measurement power to measure the position optimally while causing the least noise,” he explains.

The problem is that laser beams powerful enough to suspend a nanoparticle tend to push the system away from the Heisenberg limit, producing an increase in backaction noise.

Blocking information flow

The Swansea team’s method avoids this problem by, in effect, blocking the flow of information from the trapped nanoparticle. Its proposed setup uses a standing-wave laser to trap a nanoparticle in space with a hemispherical mirror placed around it. When the mirror has a specific radius, the scattered light from the particle and its reflection interfere so that the outgoing field no longer encodes any information about the particle’s position.

At this point, the particle is effectively invisible to the observer, with an interesting consequence: because the scattered light carries no usable information about the particle’s location, quantum backaction disappears. “I was initially convinced that we wanted to suppress the scatter,” team leader James Bateman tells Physics World. “After rigorous calculation, we arrived at the correct and surprising answer: we need to enhance the scatter.”

In fact, when scattering radiation is at its highest, the team calculated that the noise should disappear entirely. “Even though the particle shines brighter than it would in free space, we cannot tell in which direction it moves,” says Rafał Gajewski, a postdoctoral researcher at Swansea and Bateman’s co-author on a paper in Physical Review Research describing the technique.

Gajewski and Bateman’s result flips a core principle of quantum mechanics on its head. While it’s well known that measuring a quantum system disturbs it, the reverse is also true: if no information can be extracted, then no disturbance occurs, even when photons continuously bombard the particle. If physicists do need to gain information about the trapped nanoparticle, they can use a different, lower-energy laser to make their measurements, allowing experiments to be conducted at the Heisenberg limit with minimal noise.

Putting it into practice

For the method to work experimentally, the team say the mirror needs a high-quality surface and a radius that is stable with temperature changes. “Both requirements are challenging, but this level of control has been demonstrated and is achievable,” Gajewski says.

Positioning the particle precisely at the center of the hemisphere will be a further challenge, he adds, while the “disappearing” effect depends on the mirror’s reflectivity at the laser wavelength. The team is currently investigating potential solutions to both issues.

If demonstrated experimentally, the team says the technique could pave the way for quieter, more precise experiments and unlock a new generation of ultra-sensitive quantum sensors. Millen, who was not involved in the work, agrees. “I think the method used in this paper could possibly preserve quantum states in these particles, which would be very interesting,” he says.

Because nanoparticles are far more massive than atoms, Millen adds, they interact more strongly with gravity, making them ideal candidates for testing whether gravity follows the strange rules of quantum theory.  “Quantum gravity – that’s like the holy grail in physics!” he says.

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Researchers on the AEgIS collaboration at CERN have designed an experiment that could soon boost our understanding of how antimatter falls under gravity. Created by a team led by Francesco Guatieri at the Technical University of Munich, the scheme uses modified smartphone camera sensors to improve the spatial resolution of measurements of antimatter annihilations. This approach could be used in rigorous tests of the weak equivalence principle (WEP).

The WEP is a key concept of Albert Einstein’s general theory of relativity, which underpins our understanding of gravity. It suggests that within a gravitational field, all objects of should be accelerated at the same rate, regardless of their mass or whether they are matter or antimatter. Therefore, if matter and antimatter accelerate at different rates in freefall, it would reveal serious problems with the WEP.

In 2023 the ALPHA-g experiment at CERN was the first to observe how antimatter responds to gravity. They found that it falls down, with the tantalizing possibility that antimatter’s gravitational response is weaker than matter’s. Today, there are several experiments that are seeking to improve on this observation.

Falling beam

AEgIS’ approach is to create a horizontal beam of cold antihydrogen atoms and observe how the atoms fall under gravity. The drop will be measured by a moiré deflectometer in which a beam passes through two successive and aligned grids of horizontal slits before striking a position-sensitive detector. As the beam falls under gravity between the grids, the effect is similar to a slight horizontal misalignment of the grids. This creates a moiré pattern – or superlattice – that results in the particles making a distinctive pattern on the detector. By detecting a difference in the measured moiré pattern and that predicted by WEP, the AEgIS collaboration hopes to reveal a discrepancy with general relativity.

However, as Guatieri explains, a number of innovations are required for this to work. “For AEgIS to work, we need a detector with incredibly high spatial resolution. Previously, photographic plates were the only option, but they lacked real-time capabilities.”

AEgIS physicists are addressing this by developing a new vertexing detector. Instead of focussing on the antiparticles directly, their approach detects the secondary particles produced when the antimatter annihilates on contact with the detector. Tracing the trajectories of these particles back to their vertex gives the precise location of the annihilation.

Vertexing detector

Borrowing from industry, the team has created its vertexing detector using an array of modified mobile-phone camera sensors (see figure). Gautieri had already used this approach to measure the real-time positions of low-energy positrons (anti-electrons) with unprecedented precision.

“Mobile camera sensors have pixels smaller than 1 micron,” Guatieri describes. “We had to strip away the first layers of the sensors, which are made to deal with the advanced integrated electronics of mobile phones. This required high-level electronic design and micro-engineering.”

With these modifications in place, the team measured the positions of antiproton annihilations to within just 0.62 micron: making their detector some 35 times more precise than previous designs.

Many benefits

“Our solution, demonstrated for antiprotons and directly applicable to antihydrogen, combines photographic-plate-level resolution, real-time diagnostics, self-calibration and a good particle collection surface, all in one device,” Gautieri says.

With some further improvements, the AEgIS team is confident that their vertexing detector with boost the resolution of the freefall of horizontal antihydrogen beams – allowing rigorous tests of the WEC.

AEgIS team member Ruggero Caravita of Italy’s University of Trento adds, “This game-changing technology could also find broader applications in experiments where high position resolution is crucial, or to develop high-resolution trackers”. He says, “Its extraordinary resolution enables us to distinguish between different annihilation fragments, paving the way for new research on low-energy antiparticle annihilation in materials”.

The research is described in Science Advances.

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Chatbots could boost students’ interest in maths and physics and make learning more enjoyable. So say researchers in Germany, who have compared the emotional response of students using artificial intelligence (AI) texts to learn physics compared to those who only read traditional textbooks. The team, however, found no difference in test performance between the two groups.

The study has been led by Julia Lademann, a physics-education researcher from the University of Cologne, who wanted to see if AI could boost students’ interested in physics. They did this by creating a customized chatbot using OpenAI’s ChatGPT model with a tone and language that was considered accessible to second-year high-school students in Germany.

After testing the chatbot for factual accuracy and for its use of motivating language, the researchers prompted it to generate explanatory text on proportional relationships in physics and mathematics. They then split 214 students, who had an average age of 11.7, into two groups. One was given textbook material on the topic along with chatbot text, while the control group only got the textbook .

The researchers first surveyed the students’ interest in mathematics and physics and then gave them 15 minutes to review the learning material. Their interest was assessed again afterwards along with the students’ emotional state and “cognitive load” – the mental effort required to do the work – through a series of questionnaires.

Higher confidence

The chatbot was found to significantly enhance students’ positive emotions – including pleasure and satisfaction, interest in the learning material and self-belief in their understanding of the subject — compared with those who only used textbook text. “The text of the chatbot is more human-like, more conversational than texts you will find in a textbook,” explains Lademann. “It is more chatty.”

Chatbot text was also found to reduce cognitive load. “The group that used the chatbot explanation experience higher positive feelings about the subject [and] they also had a higher confidence in their learning comprehension,” adds Lademann.

Tests taken within 30 minutes of the “learning phase” of the experiment, however, found no difference in performance between students that received the AI-generated explanatory text and the control group, despite the former receiving more information. Lademann says this could be due to the short study time of 15 minutes.

The researchers say that while their findings suggest that AI could provide a superior learning experience for students, further research is needed to assess its impact on learning performance and long-term outcomes. “It is also important that this improved interest manifests in improved learning performance,” Lademann adds.

Lademann would now like to see “longer term studies with a lot of participants and with children actually using the chatbot”. Such research would explore the potential key strength of chatbots; their ability to respond in real time to student’s queries and adapt their learning level to each individual student.

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What is the main role of the European Centre for Medium-Range Weather Forecasts (ECMWF)?

Making weather forecasts more accurate is at the heart of what we do at the ECMWF, working in close collaboration with our member states and their national meteorological services (see box below). That means enhanced forecasting for the weeks and months ahead as well as seasonal and annual predictions. We also have a remit to monitor the atmosphere and the environment – globally and regionally – within the context of a changing climate.

How does the ECMWF produce its weather forecasts?

Our task is to get the best representation, in a 3D sense, of the current state of the atmosphere versus key metrics like wind, temperature, humidity and cloud cover. We do this via a process of reanalysis and data assimilation: combining the previous short-range weather forecast, and its component data, with the latest atmospheric observations – from satellites, ground stations, radars, weather balloons and aircraft. Unsurprisingly, using all this observational data is a huge challenge, with the exploitation of satellite measurements a significant driver of improved forecasting over the past decade.

In what ways do satellite measurements help?

Consider the EarthCARE satellite that was launched in May 2024 by the European Space Agency (ESA) and is helping ECMWF to improve its modelling of clouds, aerosols and precipitation. EarthCARE has a unique combination of scientific instruments – a cloud-profiling radar, an atmospheric lidar, a multispectral imager and a broadband radiometer – to infer the properties of clouds and how they interact with solar radiation as well as thermal-infrared radiation emitted by different layers of the atmosphere.

How are you combining such data with modelling?

The ECMWF team is learning how to interpret and exploit the EarthCARE data to directly initiate our models. Put simply, mathematical models that better represent clouds and, in turn, yield more accurate forecasts. Indirectly, EarthCARE is also revealing a clearer picture of  the fundamental physics governing cloud formation, distribution and behaviour. This is just one example of numerous developments taking advantage of new satellite data. We are looking forward, in particular, to fully exploiting next-generation satellite programmes from the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) – including the EPS-SG polar-orbiting system and the Meteosat Third Generation geostationary satellite for continuous monitoring over Europe, Africa and the Indian Ocean.

What other factors help improve forecast accuracy?

We talk of “a day, a decade” improvement in weather forecasting, such that a five-day forecast now is as good as a three-day forecast 20 years ago. A richer and broader mix of observational data underpins that improvement, with diverse data streams feeding into bigger supercomputers that can run higher-resolution models and better algorithms. Equally important is ECMWF’s team of multidisciplinary scientists, whose understanding of the atmosphere and climate helps to optimize our models and data assimilation methods. A case study in this regard is Destination Earth, an ambitious European Union initiative to create a series of “digital twins” – interactive computer simulations – of our planet by 2030. Working with ESA and EUMETSTAT, the ECMWF is building the software and data environment for Destination Earth as well as developing the first two digital twins.

What are these two twins?

Our Digital Twin on Weather-Induced and Geophysical Extremes will assess and predict environmental extremes to support risk assessment and management. Meanwhile, in collaboration with others, the Digital Twin on Climate Change Adaptation complements and extends existing capabilities for the analysis and testing of “what if” scenarios – supporting sustainable development and climate adaptation and mitigation policy-making over multidecadal timescales.

Progress in machine learning and AI has been dramatic over the past couple of years

What kind of resolution will these models have?

Both digital twins integrate sea, atmosphere, land, hydrology and sea ice and their deep connections with a resolution currently impossible to reach. Right now, for example, the ECMWF’s operational forecasts cover the whole globe in a 9 km grid – effectively a localized forecast every 9 km. With Destination Earth, we’re experimenting with 4 km, 2 km, and even 1 km grids.

In February, the ECMWF unveiled a 10-year strategy to accelerate the use of machine learning and AI. How will this be implemented?

The new strategy prioritizes growing exploitation of data-driven methods anchored on established physics-based modelling – rapidly scaling up our previous deployment of machine learning and AI. There are also a variety of hybrid approaches combining data-driven and physics-based modelling.

What will this help you achieve?

On the one hand, data assimilation and observations will help us to directly improve as well as initialize our physics-based forecasting models – for example, by optimizing uncertain parameters or learning correction terms. We are also investigating the potential of applying machine-learning techniques directly on observations – in effect, to make another step beyond the current state-of-the-art and produce forecasts without the need for reanalysis or data assimilation.

How is machine learning deployed at the moment?

Progress in machine learning and AI has been dramatic over the past couple of years – so much so that we launched our Artificial Intelligence Forecasting System (AIFS) back in February. Trained on many years of reanalysis and using traditional data assimilation, AIFS is already an important addition to our suite of forecasts, though still working off the coat-tails of our physics-based predictive models. Another notable innovation is our Probability of Fire machine-learning model, which incorporates multiple data sources beyond weather prediction to identify regional and localized hot-spots at risk of ignition. Those additional parameters – among them human presence, lightning activity as well as vegetation abundance and its dryness – help to pinpoint areas of targeted fire risk, improving the model’s predictive skill by up to 30%.

What do you like most about working at the ECMWF?

Every day, the ECMWF addresses cutting-edge scientific problems – as challenging as anything you’ll encounter in an academic setting – by applying its expertise in atmospheric physics, mathematical modelling, environmental science, big data and other disciplines. What’s especially motivating, however, is that the ECMWF is a mission-driven endeavour with a straight line from our research outcomes to wider societal and economic benefits.

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“The universe began with a Big Bang.”

I’ve said this neat line more times than I can count at the start of a public lecture. It summarizes one of the most incomprehensible ideas in science: that the universe began in an extreme, hot, dense and compact state, before expanding and evolving into everything we now see around us. The certainty of the simple statement is reassuring, and it is an easy way of quickly setting the background to any story in astronomy.

But what if it isn’t just an oversimplified summary? What if it is misleading, perhaps even wholly inaccurate?

The Battle of the Big Bang: the New Tales of Our Cosmic Origin aims to dismantle the complacency many of us have fallen into when it comes to our knowledge of the earliest time. And it succeeds – if you push through the opening pages.

When a theory becomes so widely accepted that it is immune to question, we’ve moved from science supported by evidence to belief upheld by faith

Early on, authors Niayesh Afshordi and Phil Halper say “in some sense the theory of the Big Bang cannot be trusted”, which caused me to raise an eyebrow and wonder what I had let myself in for. After all, for many astronomers, myself included, the Big Bang is practically gospel. And therein lies the problem. When a theory becomes so widely accepted that it is immune to question, we’ve moved from science supported by evidence to belief upheld by faith.

It is easy to read the first few pages of The Battle of the Big Bang with deep scepticism but don’t worry, your eyebrows will eventually lower. That the universe has evolved from a “hot Big Bang” is not in doubt – observations such as the measurements of the cosmic microwave background leave no room for debate. But the idea that the universe “began” as a singularity – a region of space where the curvature of space–time becomes infinite – is another matter. The authors argue that no current theory can describe such a state, and there is no evidence to support it.

An astronomical crowbar

Given the confidence with which we teach it, many might have assumed the Big Bang theory beyond any serious questioning, thereby shutting the door on their own curiosity. Well, Afshordi and Halper have written the popular science equivalent of a crowbar, gently prising that door back open without judgement, keen only to share the adventure still to be had.

A cosmologist at the University of Waterloo, Canada, Afshordi is obsessed with finding observational ways of solving problems in fundamental physics, and is known for his creative alternative theories, such as a non-constant speed of light. Meanwhile Halper, a science popularizer, has carved out a niche by interviewing leading voices in early universe cosmology on YouTube, often facilitating fierce debates between competing thinkers. The result is a book that is both authoritative and accessible – and refreshingly free from ego.

Over 12 chapters, the book introduces more than two dozen alternatives to the Big Bang singularity, with names as tongue-twisting as the theories are mind-bending. For most readers, and even this astrophysicist, the distinctions between the theories quickly blur. But that’s part of the point. The focus isn’t on convincing you which model is correct, it’s about making clear that many alternatives exist that are all just as credible (give or take). Reading this book feels like walking through an art gallery with a knowledgeable and thoughtful friend explaining each work’s nuance. They offer their own opinions in hushed tones, but never suggest that their favourite should be yours too, or even that you should have a favourite.

If you do find yourself feeling dizzy reading about the details of holographic cosmology or eternal inflation, then it won’t be long before an insight into the nature of scientific debate or a crisp analogy brings you up for air. This is where the co-authorship begins to shine: Halper’s presence is felt in the moments when complicated theories are reduced to an idea anyone can relate to; while Afshordi brings deep expertise and an insider’s view of the cosmological community. These vivid and sometimes gossipy glimpses into the lives and rivalries of his colleagues paint a fascinating picture. It is a huge cast of characters – including Roger Penrose, Alan Guth and Hiranya Peiris – most of whom appear only for a page. But even though you won’t remember all the names, you are left with the feeling that Big Bang cosmology is a passionate, political and philosophical side of science very much still in motion.

Keep the door open

The real strength of this book is its humility and lack of defensiveness. As much as reading about the theory behind a multiverse is interesting, as a scientist, I’m always drawn to data. A theory that cannot be tested can feel unscientific, and the authors respect that instinct. Surprisingly, some of the most fantastical ideas, like pre-Big Bang cosmologies, are testable. But the tools required are almost science fiction themselves – such as a fleet of gravitational-wave detectors deployed in space. It’s no small task, and one of the most delightful moments in the book is a heartfelt thank you to taxpayers, for funding the kind of fundamental research that might one day get us to an answer.

In the concluding chapters, the authors pre-emptively respond to scepticism, giving real thought to discussing when thinking outside the box becomes going beyond science altogether. There are no final answers in this book, and it does not pretend to offer any. In fact, it actively asks the reader to recognize that certainty does not belong at the frontiers of science. Afshordi doesn’t mind if his own theories are proved wrong, the only terror for him is if people refuse to ask questions or pursue answers simply because the problem is seen as intractable.

Curiosity, unashamed and persistent, is far more scientific than shutting the door for fear of the uncertain

A book that leaves you feeling like you understand less about the universe than when you started it might sound like it has failed. But when that “understanding” was an illusion based on dogma, and a book manages to pry open a long-sealed door in your mind, that’s a success.

The Battle of the Big Bang offers both intellectual humility and a reviving invitation to remain eternally open-minded. It reminded me of how far I’d drifted from being one of the fearless schoolchildren who, after I declare with certainty that the universe began with a Big Bang, ask, “But what came before it?”. That curiosity, unashamed and persistent, is far more scientific than shutting the door for fear of the uncertain.

• May 2025 University of Chicago Press 360pp $32.50/£26.00 hb

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The first strong evidence for an exoplanet with an orbit perpendicular to that of the binary system it orbits has been observed by astronomers in the UK and Portugal. Based on observations from the ESO’s Very Large Telescope (VLT), researchers led by Tom Baycroft, a PhD student at the University of Birmingham, suggest that such an exoplanet is required to explain the changing orientation in the orbit of a pair of brown dwarfs – objects that are intermediate in mass between the heaviest gas-giant planets and the lightest stars.

The Milky Way is known to host a diverse array of planetary systems, providing astronomers with extensive insights into how planets form and systems evolve. One thing that is evident is that most exoplanets (planets that orbit stars other than the Sun) and systems that have been observed so far bear little resemblance to Earth and the solar system.

Among the most interesting planets are the circumbinaries, which orbit two stars in a binary system. So far, 16 of these planets have been discovered. In each case, they have been found to orbit in the same plane as the orbits of their binary host stars. In other words, the planetary system is flat. This is much like the solar system, where each planet orbits the Sun within the same plane.

“But there has been evidence that planets might exist in a different configuration around a binary star,” Baycroft explains. “Inclined at 90° to the binary, these polar orbiting planets have been theorized to exist, and discs of dust and gas have been found in this configuration.”

Especially interesting

Baycroft’s team had set out to investigate a binary pair of brown dwarfs around 120 light–years away. The system is called 2M1510 and each brown dwarf is only about 45 million years old and they have masses about 18 times that of Jupiter. The pair are especially interesting because they are eclipsing: periodically passing in front of each other from our line of sight. When observed by the VLT, this unique vantage allowed the astronomers to determine the masses and radii of the stars and the nature of their orbit.

“This is a rare object, one of only two eclipsing binary brown dwarfs, which is useful for understanding how brown dwarfs form and evolve,” Baycroft explains. “In our study, we were not looking for a planet, only aiming to improve our understanding of the brown dwarfs.”

Yet as they analysed the VLT’s data, the team noticed something strange about pair’s orbit. Doppler shifts in the light they emitted revealed that their elliptical orbit was slowly changing orientation in an apsidal precession.

Not unheard of

This behaviour is not unheard of. In its orbit around the Sun, Mercury undergoes apsidal precession, which is explained by Albert Einstein’s general theory of relativity. But Baycroft says that the precession must have had an entirely different cause in the brown-dwarf pair.

“Unlike Mercury, this precession is going backwards, in the opposite direction to the orbit,” he explains. “Ruling out any other causes for this, we find that the best explanation is that there is a companion to the binary on a polar orbit, inclined at close to 90° relative to the binary.” As it exerts its gravitational pull on the binary pair, the inclination of this third, smaller body induces a gradual rotation in the orientation of the binary’s elliptical orbit.

For now, the characteristics of this planet are difficult to pin down and the team believe its mass could lie anywhere between 10–100 Earths. All the same, the astronomers are confident that their results now confirm the possibility of polar exoplanets existing in circumbinary orbits – providing valuable guidance for future observations.

“This result exemplifies how the many different configurations of planetary systems continue to astound us,” Baycroft comments. “It also paves the way for more studies aiming to find out how common such polar orbits may be.”

The observations are described in Science Advances.

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In his debut book, Einstein’s Tutor: the Story of Emmy Noether and the Invention of Modern Physics, Lee Phillips champions the life and work of German mathematician Emmy Noether (1882–1935). Despite living a life filled with obstacles, injustices and discrimination as a Jewish mathematician, Noether revolutionized the field and discovered “the single most profound result in all of physics”. Phillips’ book weaves the story of her extraordinary life around the central subject of “Noether’s theorem”, which itself sits at the heart of a fascinating era in the development of modern theoretical physics.

Noether grew up at a time when women had few rights. Unable to officially register as a student, she was instead able to audit courses at the University of Erlangen in Bavaria, with the support of her father who was a mathematics professor there. At the time, young Noether was one of only two female auditors in the university of 986 students. Just two years previously, the university faculty had declared that mixed-sex education would “overthrow academic order”. Despite going against this formidable status quo, she was able to graduate in 1903.

Noether continued her pursuit of advanced mathematics, travelling to the “[world’s] centre of mathematics” – the University of Göttingen. Here, she was able to sit in the lectures of some of the brightest mathematical minds of the time – Karl Schwarzschild, Hermann Minkowski, Otto Blumenthal, Felix Klein and David Hilbert. While there, the law finally changed: women were, at last, allowed to enrol as students at university. In 1904 Noether returned to the University of Erlangen to complete her postgraduate dissertation under the supervision of Paul Gordan. At the time, she was the only woman to matriculate alongside 46 men.

Despite being more than qualified, Noether was unable to secure a university position after graduating from her PhD in 1907. Instead, she worked unpaid for almost a decade – teaching her father’s courses and supervising his PhD students. As of 1915, Noether was the only woman in the whole of Europe with a PhD in mathematics. She had worked hard to be recognized as an expert on symmetry and invariant theory, and eventually accepted an invitation from Klein and Hilbert to work alongside them in Göttingen. Here, the three of them would meet Albert Einstein to discuss his latest project – a general theory of relativity.

Infiltrating the boys’ club

In Einstein’s Tutor, Phillips paints an especially vivid picture of Noether’s life at Göttingen, among colleagues including Klein, Hilbert and Einstein, who loom large and bring a richness to the story. Indeed, much of the first three chapters are dedicated to these men, setting the scene for Noether’s arrival in Göttingen. Phillips makes it easy to imagine these exceptionally talented and somewhat eccentric individuals working at the forefront of mathematics and theoretical physics together. And it was here, when supporting Einstein with the development of general relativity (GR), that Noether discovered a profound result: for every symmetry in the universe, there is a corresponding conservation law.

Throughout the book, Phillips makes the case that, without Noether, Einstein would never have been able to get to the heart of GR. Einstein himself “expressed wonderment at what happened to his equations in her hands, how he never imagined that things could be expressed with such elegance and generality”. Phillips argues that Einstein should not be credited as the sole architect of GR. Indeed, the contributions of Grossman, Klein, Besso, Hilbert, and crucially, Noether, remain largely unacknowledged – a wrong that Phillips is trying to right with this book.

Phillips makes the case that, without Noether, Einstein would never have been able to get to the heart of general relativity

A key theme running through Einstein’s Tutor is the importance of the support and allyship that Noether received from her male contemporaries. While at Göttingen, there was a battle to allow Noether to receive her habilitation (eligibility for tenure). Many argued in her favour but considered her an exception, and believed that in general, women were not suited as university professors. Hilbert, in contrast, saw her sex as irrelevant (famously declaring “this is not a bath house”) and pointed out that science requires the best people, of which she was one. Einstein also fought for her on the basis of equal rights for women.

Eventually, in 1919 Noether was allowed to habilitate (as an exception to the rule) and was promoted to professor in 1922. However, she was still not paid for her work. In fact, her promotion came with the specific condition that she remained unpaid, making it clear that Noether “would not be granted any form of authority over any male employee”. Hilbert however, managed to secure a contract with a small salary for her from the university administration.

Her allies rose to the cause again in 1933, when Noether was one of the first Jewish academics to be dismissed under the Nazi regime. After her expulsion, German mathematician Helmut Hasse convinced 14 other colleagues to write letters advocating for her importance, asking that she be allowed to continue as a teacher to a small group of advanced students – the government denied this request.

When the time came to leave Germany, many colleagues wrote testimonials in her support for immigration, with one writing “She is one of the 10 or 12 leading mathematicians of the present generation in the entire world.” Rather than being placed at a prestigious university or research institute (Hermann Weyl and Einstein were both placed at “the men’s university”, the Institute for Advanced Study in Princeton), it was recommended she join Bryn Mawr, a women’s college in Pennsylvania, US. Her position there would “compete with no-one… the most distinguished feminine mathematician connected with the most distinguished feminine university”. Phillips makes clear his distaste for the phrasing of this recommendation. However, all accounts show that she was happy at Bryn Mawr and stayed there until her unexpected death in 1935 at the age of 53.

Noether’s legacy

With a PhD in theoretical physics, Phillips has worked for many years in both academia and industry. His background shows itself clearly in some unusual writing choices. While his writing style is relaxed and conversational, it includes the occasional academic turn of phrase (e.g. “In this chapter I will explain…”), which feels out of place in a popular-science book. He also has a habit of piling repetitive and overly sincere praise onto Noether. I personally prefer stories that adopt the “show, don’t tell” approach – her abilities speak for themselves, so it should be easy to let the reader come to their own conclusions.

Phillips has made the ambitious choice to write a popular-science book about complex mathematical concepts such as symmetries and conservation laws that are challenging to explain, especially to general readers. He does his best to describe the mathematics and physics behind some of the key concepts around Noether’s theorem. However, in places, you do need to have some familiarity with university-level physics and maths to properly follow his explanations. The book also includes a 40-page appendix filled with additional physics content, which I found unnecessary.

Einstein’s Tutor does achieve its primary goal of familiarizing the reader with Emmy Noether and the tremendous significance of her work. The final chapter on her legacy breezes quickly through developments in particle physics, astrophysics, quantum computers, economics and XKCD Comics to highlight the range and impact this single theorem has had. Phillips’ goal was to take Noether into the mainstream, and this book is a small step in the right direction. As cosmologist and author Katie Mack summarizes perfectly: “Noether’s theorem is to theoretical physics what natural selection is to biology.”

• 2024 Hachette UK £25hb 368pp

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Sending an email, typing a text message, streaming a movie. Many of us do these activities every day. But what if you couldn’t move your muscles and navigate the digital world? This is where brain–computer interfaces (BCIs) come in.

BCIs that are implanted in the brain can bypass pathways damaged by illness and injury. They analyse neural signals and produce an output for the user, such as interacting with a computer.

A major focus for scientists developing BCIs has been to interpret brain activity associated with movements to control a computer cursor. The user drives the BCI by imagining arm and hand movements, which often originate in the dorsal motor cortex. Speech BCIs, which restore communication by decoding attempted speech from neural activity in sensorimotor cortical areas such as the ventral precentral gyrus, have also been developed.

Researchers at the University of California, Davis recently found that the same part of the brain that supported a speech BCI could also support computer cursor control for an individual with amyotrophic lateral sclerosis (ALS). ALS is progressive neurodegenerative disease affecting the motor neurons in the brain and spinal cord.

“Once that capability [to control a computer mouse] became reliably achievable roughly a decade ago, it stood to reason that we should go after another big challenge, restoring speech, that would help people unable to speak. And from there – and this is where this new paper comes in – we recognized that patients would benefit from both of these capabilities [speech and computer cursor control],” says Sergey Stavisky, who co-directs the UC Davis Neuroprosthetics Lab with David Brandman.

Their clinical case study suggests that computer cursor control may not be as body-part-specific as scientists previously believed. If results are replicable, this could enable the creation of multi-modal BCIs that restore communication and movement to people with paralysis. The researchers share information about their cursor BCI and the case study in the Journal of Neural Engineering.

The study participant, a 45-year-old man with ALS, had previous success working with a speech BCI. The researchers recorded neural activity from the participant’s ventral precentral gyrus while he imagined controlling a computer cursor, and built a BCI to interpret that neural activity and predict where and when he wanted to move and click the cursor. The participant then used the new cursor BCI to send texts and emails, watch Netflix, and play The New York Times Spelling Bee game on his personal computer.

“This finding, that the tiny region of the brain we record from has a lot more than just speech information, has led to the participant also being able to control his own computer on a daily basis, and get back some independence for him and his family,” says first author Tyler Singer-Clark, a graduate student in biomedical engineering at UC Davis.

The researchers found that most of the information driving cursor control came from one of the participant’s four implanted microelectrode arrays, while click information was available on all four of the BCI arrays.

“The neural recording arrays are the same ones used in many prior studies,” explains Singer-Clark. “The result that our cursor BCI worked well given this choice makes it all the more convincing that this brain area (speech motor cortex) has untapped potential for controlling BCIs in multiple useful ways.”

The researchers are working to incorporate more computer actions into their cursor BCI, to make the control faster and more accurate, and to reduce calibration time. They also note that it’s important to replicate these results in more people to understand how generalizable the results of their case study may be.

The research was conducted as part of the BrainGate2 clinical trial.

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Most of us have heard of Schrödinger’s eponymous cat, but it is not the only feline in the quantum physics bestiary. Quantum Cheshire cats may not be as well known, yet their behaviour is even more insulting to our classical-world common sense.

These quantum felines get their name from the Cheshire cat in Lewis Carroll’s Alice’s Adventures in Wonderland, which disappears leaving its grin behind. As Alice says: “I’ve often seen a cat without a grin, but a grin without a cat! It’s the most curious thing I ever saw in my life!”

Things are curiouser in the quantum world, where the property of a particle seems to be in a different place from the particle itself. A photon’s polarization, for example, may exist in a totally different location from the photon itself: that’s a quantum Cheshire cat.

While the prospect of disembodied properties might seem disturbing, it’s a way of interpreting the elegant predictions of quantum mechanics. That at least was the thinking when quantum Cheshire cats were first put forward by Yakir Aharonov, Sandu Popescu, Daniel Rohrlich and Paul Skrzypczyk in an article published in 2013 (New J. Phys. 15 113015).

Strength of a measurement

To get to grips with the concept, remember that making a measurement on a quantum system will “collapse” it into one of its eigenstates – think of opening the box and finding Schrödinger’s cat either dead or alive. However, by playing on the trade-off between the strength of a measurement and the uncertainty of the result, one can gain a tiny bit of information while disturbing the system as little as possible. If such a measurement is done many times, or on an ensemble of particles, it is possible to average out the results, to obtain a precise value.

First proposed in the 1980s, this method of teasing out information from the quantum system by a series of gentle pokes is known as weak measurement. While the idea of weak measurement in itself does not appear a radical departure from quantum formalism, “an entire new world appeared” as Popescu puts it. Indeed, Aharonov and his collaborators have spent the last four decades investigating all kinds of scenarios in which weak measurement can lead to unexpected consequences, with the quantum Cheshire cat being one they stumbled upon.

In their 2013 paper, Aharonov and colleagues imagined a simple optical interferometer set-up, in which the “cat” is a photon that can be in either the left or the right arm, while the “grin” is the photon’s circular polarization. The cat (the photon) is first prepared in a certain superposition state, known as pre-selection. After it enters the set-up, the cat can leave via several possible exits. The disembodiment between particle and property appears in the cases in which the particle emerges in a particular exit (post-selection).

Certain measurements, analysing the properties of the particle, are performed while the particle is in the interferometer (in between the pre- and post-selection). Being weak measurements, they have to be carried out many times to get the average. For certain pre- and post-selection, one finds the cat will be in the left arm while the grin is in the right. It’s a Cheshire cat disembodied from its grin.

The mathematical description of this curious state of affairs was clear, but the interpretation seemed preposterous and the original article spent over a year in peer review, with its eventual publication still sparking criticism. Soon after, experiments with polarized neutrons (Nature Comms 5 4492) and photons (Phys. Rev. A 94 012102) tested the original team’s set-up. However, these experiments and subsequent tests, despite confirming the theoretical predictions, did not settle the debate – after all, the issue was with the interpretation.

A quantum of probabilities

To come to terms with this perplexing notion, think of the type of pre- and post-selected set-up as a pachinko machine, in which a ball starts at the top in a single pre-selected slot and goes down through various obstacles to end up in a specific point (post-selection): the jackpot hole. If you count how many balls hit the jackpot hole, you can calculate the probability distribution. In the classical world, measuring the position and properties of the ball at different points, say with a camera, is possible.

This observation will not affect the trajectory of the ball, or the probability of the jackpot. In a quantum version of the pachinko machine, the pre- and post-selection will work in a similar way, except you could feed in balls in superposition states. A weak measurement will not disturb the system so multiple measurements can tease out the probability of certain outcomes. The measurement result will not yield an eigenvalue, which corresponds to a physical property of the system, but weak values, and the way one should interpret these is not clear-cut.

To make sense of this in a quantum sense, we need an intuitive mental image, even a limited one. This is why quantum Cheshire cats are a powerful metaphor, but they are also more than that, guiding researchers into new directions. Indeed, since the initial discovery, Aharonov, Popescu and colleagues have stumbled  upon more surprises.

In 2021 they generalized the quantum Cheshire cat effect to a dynamical picture in which the “disembodied” property can propagate in space (Nature Comms 12 4770). For example, there could be a flow of angular momentum without anything carrying it (Phys. Rev. A 110 L030201). In another generalization, Aharonov imagined a massive particle with a mass that could be measured in one place with no momentum, while its momentum could be measured in another place without its mass (Quantum 8 1536). A gedankenexperiment to test this effect would involve a pair of nested Mach–Zehnder interferometers with moving mirrors and beam splitters.

Provocative interpretations

If you find these ideas bewildering, you’re in good company. “They’re brain teasers,” explains Jonte Hance, a researcher in quantum foundations at Newcastle University, UK. In fact, Hance thinks that quantum Cheshire cats are a great way to get people interested in the foundations of quantum mechanics.

Physicists were too busy applying quantum mechanics to various problems to be bothered with foundational questions

Sure, the early years of quantum physics saw famous debates between Niels Bohr and Albert Einstein, culminating in the criticism in the Einstein–Podolski–Rosen (EPR) paradox (Phys. Rev. 47 777) in 1935. But after that, physicists were too busy applying quantum mechanics to various problems to be bothered with foundational questions.

This lack of interest in quantum fundamentals is perfectly illustrated by two anecdotes, the first involving Aharonov himself. When he was studying physics at Technion in Israel in the 1950s, he asked Nathan Rosen (the R of the EPR) about working on the foundations of quantum mechanics. The topic was deemed so unfashionable that Rosen advised him to focus on applications. Luckily, Aharonov ignored the advice and went on to work with American quantum theorist David Bohm.

The other story concerns Alain Aspect, who in 1975 visited CERN physicist John Bell to ask for advice on his plans to do an experimental test of Bell’s inequalities to settle the EPR paradox. Bell’s very first question was not about the details of the experiment – but whether Aspect had a permanent position (Nature Phys. 3 674). Luckily, Aspect did, so he carried out the test, which went on to earn him a share of the 2022 Nobel Prize for Physics.

As quantum computing and quantum information began to emerge, there was a brief renaissance in quantum foundations culminating in the early 2010s. But over the past decade, with many of aspects of quantum physics reaching commercial fruition, research interest has shifted firmly once again towards applications.

Despite popular science’s constant reminder of how “weird” quantum mechanics is, physicists often take the pragmatic “shut up and calculate” approach. Hance says that researchers “tend to forget how weird quantum mechanics is, and to me you need that intuition of it being weird”. Indeed, paradoxes like Schrödinger’s cat and EPR have attracted and inspired generations of physicists and have been instrumental in the development of quantum technologies.

The point of the quantum Cheshire cat, and related paradoxes, is to challenge our intuition and provoke us to think outside the box. That’s important even if applications may not be immediately in sight. “Most people agree that although we know the basic laws of quantum mechanics, we don’t really understand what quantum mechanics is all about,” says Popescu.

Aharonov and colleagues’ programme is to develop a correct intuition that can guide us further. “We strongly believe that one can find an intuitive way of thinking about quantum mechanics,” adds Popescu. That may, or may not, involve felines.

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India must intensify its efforts in quantum technologies as well as boost private investment if it is to become a leader in the burgeoning field. That is according to the first report from India’s National Quantum Mission (NQM), which also warns that the country must improve its quantum security and regulation to make its digital infrastructure quantum-safe.

Approved by the Indian government in 2023, the NQM is an eight-year $750m (60bn INR) initiative that aims to make the country a leader in quantum tech. Its new report focuses on developments in four aspects of NQM’s mission: quantum computing; communication; sensing and metrology; and materials and devices.

Entitled India’s International Technology Engagement Strategy for Quantum Science, Technology and Innovation, the report finds that India’s research interests include error-correction algorithms for quantum computers. It is also involved in building quantum hardware with superconducting circuits, trapped atoms/ions and engineered quantum dots.

The NQM-supported Bengaluru-based startup QPiAI, for example, recently developed a 25-superconducting qubit quantum computer called “Indus”, although the qubits were fabricated abroad.

Ajay Sood, principal scientific advisor to the Indian government, told Physics World that while India is strong in “software-centric, theoretical and algorithmic aspects of quantum computing, work on completely indigenous development of quantum computing hardware is…at a nascent stage.”

Sood, who is a physicist by training, adds that while there are a few groups working on different platforms, these are at less than 10-qubit stage. “[It is] important for [India] to have indigenous capabilities for fabricating qubits and other ancillary hardware for quantum computers,” he says

India is also developing secure protocols and satellite-based systems and implementing quantum systems for precision measurements. QNu Labs – another Begalaru startup – is, for example, developing a quantum-safe communication-chip module to secure satellite and drone communications with built-in quantum randomness and security micro-stack.

Lagging behind

The report highlights the need for greater involvement of Indian industry in hardware-related activities. Unlike other countries, India struggles with limited industry funding, in which most comes from angel investors, with limited participation from institutional investors such as venture-capital firms, tech corporates and private equity funds.

There are many areas of quantum tech that are simply not being pursued in India

Arindam Ghosh

The report also calls for more indigenous development of essential sensors and devices such as single-photon detectors, quantum repeaters, and associated electronics, with necessary testing facilities for quantum communication. “There is also room for becoming global manufacturers and suppliers for associated electronic or cryogenic components,” says Sood. “Our industry should take this opportunity.”

India must work on its quantum security and regulation as well, according to the report. It warns that the Indian financial sector, which is one the major drivers for quantum tech applications, “risks lagging behind” in quantum security and regulation, with limited participation of Indian financial-service providers.

“Our cyber infrastructure, especially related to our financial systems, power grids, and transport systems, need to be urgently protected by employing the existing and evolving post quantum cryptography algorithms and quantum key distribution technologies,” says Sood.

India currently has about 50 educational programmes in various universities and institutions. Yet Arindam Ghosh, who runs the Quantum Technology Initiative at the India Institute of Science, Bangalore, says that the country faces a lack of people going into quantum-related careers.

“In spite of [a] very large number of quantum-educated graduates, the human resource involved in developing quantum technologies is abysmally small,” says Ghosh. “As a result, there are many areas of quantum tech that are simply not being pursued in India.”  Other problems, according to Ghosh, include “modest” government funding compared to other countries as well as “slow and highly bureaucratic” government machinery.

Sood, however, is optimistic, pointing out recent Indian initiatives such as setting up hardware fabrication and testing facilities, supporting start-ups as well as setting up a $1.2bn (100bn INR) fund to promote “deep-tech” startups. “[With such initiatives] there is every reason to believe that India would emerge even stronger in the field,” says Sood.

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The future of quantum communication and quantum computing technologies may well revolve around superconducting qubits and quantum circuits, which have already been shown to improve processing capabilities over classical supercomputers – even when there is noise within the system. This scenario could be one step closer with the development of a novel quantum transducer by a team headed up at the Harvard John A Paulson School of Engineering and Applied Sciences (SEAS).

Realising this future will rely on systems having hundreds (or more) logical qubits (each built from multiple physical qubits). However, because superconducting qubits require ultralow operating temperatures, large-scale refrigeration is a major challenge – there is no technology available today that can provide the cooling power to realise such large-scale qubit systems.

Superconducting microwave qubits are a promising option for quantum processor nodes, but they currently require bulky microwave components. These components create a lot of heat that can easily disrupt the refrigeration systems cooling the qubits.

One way to combat this cooling conundrum is to use a modular approach, with small-scale quantum processors connected via quantum links, and each processor having its own dilution refrigerator. Superconducting qubits can be accessed using microwave photons between 3 and 8 GHz, thus the quantum links could be used to transmit microwave signals. The downside of this approach is that it would require cryogenically cooled links between each subsystem.

On the other hand, optical signals at telecoms frequency (around 200 THz) can be generated using much smaller form factor components, leading to lower thermal loads and noise, and can be transmitted via low-loss optical fibres. The transduction of information between optical and microwave frequencies is therefore key to controlling superconducting microwave qubits without the high thermal cost.

The large energy gap between microwave and optical photons makes it difficult to control microwave qubits with optical signals and requires a microwave–optical quantum transducer (MOQT). These MOQTs provide a coherent, bidirectional link between microwave and optical frequencies while preserving the quantum states of the qubit. A team led by SEAS researcher Marko Lončar has now created such a device, describing it in Nature Physics.

Electro-optic transducer controls superconducting qubits

Lončar and collaborators have developed a thin-film lithium niobate (TFLN) cavity electro-optic (CEO)-based MOQT (clad with silica to aid thermal dissipation and mitigate optical losses) that converts optical frequencies into microwave frequencies with low loss. The team used the CEO-MOQT to facilitate coherent optical driving of a superconducting qubit (controlling the state of the quantum system by manipulating its energy).

The on-chip transducer system contains three resonators: a microwave LC resonator capacitively coupled to two optical resonators using the electro-optic effect. The device creates hybridized optical modes in the transducer that enables a resonance-enhanced exchange of energy between the microwave and optical modes.

The transducer uses a process known as difference frequency generation to create a new frequency output from two input frequencies. The optical modes – an optical pump in a classical red-pumping regime and an optical idler – interact to generate a microwave signal at the qubit frequency, in the form of a shaped, symmetric single microwave photon.

This microwave signal is then transmitted from the transducer to a superconducting qubit (in the same refrigerator system) using a coaxial cable. The qubit is coupled to a readout resonator that enables its state to be read by measuring the transmission of a readout pulse.

The MOQT operated with a peak conversion efficiency of 1.18% (in both microwave-to-optical and optical-to-microwave regimes), low microwave noise generation and the ability to drive Rabi oscillations in a superconducting qubit. Because of the low noise, the researchers state that stronger optical-pump fields could be used without affecting qubit performance.

Having effectively demonstrated the ability to control superconducting circuits with optical light, the researchers suggest a number of future improvements that could increase the device performance by orders of magnitude. For example, microwave and optical coupling losses could be reduced by fabricating a single-ended microwave resonator directly onto the silicon wafer instead of on silica. A flux tuneable microwave cavity could increase the optical bandwidth of the transducer. Finally, the use of improved measurement methods could improve control of the qubits and allow for more intricate gate operations between qubit nodes.

The researchers suggest this type of device could be used for networking superconductor qubits when scaling up quantum systems. The combination of this work with other research on developing optical readouts for superconducting qubit chips “provides a path towards forming all-optical interfaces with superconducting qubits…to enable large scale quantum processors,” they conclude.

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Nonlocal correlations that define quantum entanglement could be reconciled with Einstein’s theory of relativity if space–time had two temporal dimensions. That is the implication of new theoretical work that extends nonlocal hidden variable theories of quantum entanglement and proposes a potential experimental test.

Marco Pettini, a theoretical physicist at Aix Marseille University in France, says the idea arose from conversations with the mathematical physicist Roger Penrose – who shared the 2020 Nobel Prize for Physics for showing that the general theory of relativity predicted black holes. “He told me that, from his point of view, quantum entanglement is the greatest mystery that we have in physics,” says Pettini. The puzzle is encapsulated by Bell’s inequality, which was derived in the mid-1960s by the Northern Irish physicist John Bell.

Bell’s breakthrough was inspired by the 1935 Einstein–Podolsky–Rosen paradox, a thought experiment in which entangled particles in quantum superpositions (using the language of modern quantum mechanics) travel to spatially separated observers Alice and Bob. They make measurements of the same observable property of their particles. As they are superposition states, the outcome of neither measurement is certain before it is made. However, as soon as Alice measures the state, the superposition collapses and Bob’s measurement is now fixed.

Quantum scepticism

A sceptic of quantum indeterminacy could hypothetically suggest that the entangled particles carried hidden variables all along, so that when Alice made her measurement, she simply found out the state that Bob would measure rather than actually altering it. If the observers are separated by a distance so great that information about the hidden variable’s state would have to travel faster than light between them, then hidden variable theory violates relativity. Bell derived an inequality showing the maximum degree of correlation between the measurements possible if each particle carried such a “local” hidden variable, and showed it was indeed violated by quantum mechanics.

A more sophisticated alternative investigated by the theoretical physicists David Bohm and his student Jeffrey Bub, as well as by Bell himself, is a nonlocal hidden variable. This postulates that the particle – including the hidden variable – is indeed in a superposition and defined by an evolving wavefunction. When Alice makes her measurement, this superposition collapses. Bob’s value then correlates with Alice’s. For decades, researchers believed the wavefunction collapse could travel faster than light without allowing superliminal exchange of information – therefore without violating the special theory of relativity. However, in 2012 researchers showed that any finite-speed collapse propagation would enable superluminal information transmission.

“I met Roger Penrose several times, and while talking with him I asked ‘Well, why couldn’t we exploit an extra time dimension?’,” recalls Pettini. Particles could have five-dimensional wavefunctions (three spatial, two temporal), and the collapse could propagate through the extra time dimension – allowing it to appear instantaneous. Pettini says that the problem Penrose foresaw was that this would enable time travel, and the consequent possibility that one could travel back through the “extra time” to kill one’s ancestors or otherwise violate causality. However, Pettini says he “recently found in the literature a paper which has inspired some relatively standard modifications of the metric of an enlarged space–time in which massive particles are confined with respect to the extra time dimension…Since we are made of massive particles, we don’t see it.”

Toy model

Pettini believes it might be possible to test this idea experimentally. In a new paper, he proposes a hypothetical experiment (which he describes as a toy model), in which two sources emit pairs of entangled, polarized photons simultaneously. The photons from one source are collected by recipients Alice and Bob, while the photons from the other source are collected by Eve and Tom using identical detectors. Alice and Eve compare the polarizations of the photons they detect. Alice’s photon must, by fundamental quantum mechanics, be entangled with Bob’s photon, and Eve’s with Tom’s, but otherwise simple quantum mechanics gives no reason to expect any entanglement in the system.

Pettini proposes, however, that Alice and Eve should be placed much closer together, and closer to the photon sources, than to the other observers. In this case, he suggests, the communication of entanglement through the extra time dimension when the wavefunction of Alice’s particle collapses, transmitting this to Bob, or when Eve’s particle is transmitted to Tom would also transmit information between the much closer identical particles received by the other woman. This could affect the interference between Alice’s and Eve’s photons and cause a violation of Bell’s inequality. “[Alice and Eve] would influence each other as if they were entangled,” says Pettini. “This would be the smoking gun.”

Bub, now a distinguished professor emeritus at the University of Maryland, College Park, is not holding his breath. “I’m intrigued by [Pettini] exploiting my old hidden variable paper with Bohm to develop his two-time model of entanglement, but to be frank I can’t see this going anywhere,” he says. “I don’t feel the pull to provide a causal explanation of entanglement, and I don’t any more think of the ‘collapse’ of the wave function as a dynamical process.” He says the central premise of Pettini’s – that adding an extra time dimension could allow the transmission of entanglement between otherwise unrelated photons, is “a big leap”. “Personally, I wouldn’t put any money on it,” he says.

The research is described in Physical Review Research.

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The world’s smallest pacemaker to date is smaller than a single grain of rice, optically controlled and dissolves after it’s no longer needed. According to researchers involved in the work, the pacemaker could work in human hearts of all sizes that need temporary pacing, including those of newborn babies with congenital heart defects.

“Our major motivation was children,” says Igor Efimov, a professor of medicine and biomedical engineering, in a press release from Northwestern University. Efimov co-led the research with Northwestern bioelectronics pioneer John Rogers.

“About 1% of children are born with congenital heart defects – regardless of whether they live in a low-resource or high-resource country,” Efimov explains. “Now, we can place this tiny pacemaker on a child’s heart and stimulate it with a soft, gentle, wearable device. And no additional surgery is necessary to remove it.”

The current clinical standard-of-care involves sewing pacemaker electrodes directly onto a patient’s heart muscle during surgery. Wires from the electrodes protrude from the patient’s chest and connect to an external pacing box. Placing the pacemakers – and removing them later – does not come without risk. Complications include infection, dislodgment, torn or damaged tissues, bleeding and blood clots.

To minimize these risks, the researchers sought to develop a dissolvable pacemaker, which they introduced in Nature Biotechnology in 2021. By varying the composition and thickness of materials in the devices, Rogers’ lab can control how long the pacemaker functions before dissolving. The dissolvable device also eliminates the need for bulky batteries and wires.

“The heart requires a tiny amount of electrical stimulation,” says Rogers in the Northwestern release. “By minimizing the size, we dramatically simplify the implantation procedures, we reduce trauma and risk to the patient, and, with the dissolvable nature of the device, we eliminate any need for secondary surgical extraction procedures.”

The latest iteration of the device – reported in Nature – advances the technology further. The pacemaker is paired with a small, soft, flexible, wireless device that is mounted onto the patient’s chest. The skin-interfaced device continuously captures electrocardiogram (ECG) data. When it detects an irregular heartbeat, it automatically shines a pulse of infrared light to activate the pacemaker and control the pacing.

“The new device is self-powered and optically controlled – totally different than our previous devices in those two essential aspects of engineering design,” says Rogers. “We moved away from wireless power transfer to enable operation, and we replaced RF wireless control strategies – both to eliminate the need for an antenna (the size-limiting component of the system) and to avoid the need for external RF power supply.”

Measurements demonstrated that the pacemaker – which is 1.8 mm wide, 3.5 mm long and 1 mm thick – delivers as much stimulation as a full-sized pacemaker. Initial studies in animals and in the human hearts of organ donors suggest that the device could work in human infants and adults. The devices are also versatile, the researchers say, and could be used across different regions of the heart or the body. They could also be integrated with other implantable devices for applications in nerve and bone healing, treating wounds and blocking pain.

The next steps for the research (supported by the Querrey Simpson Institute for Bioelectronics, the Leducq Foundation and the National Institutes of Health) include further engineering improvements to the device. “From the translational standpoint, we have put together a very early-stage startup company to work individually and/or in partnerships with larger companies to begin the process of designing the device for regulatory approval,” Rogers says.

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Harvard University is suing the Trump administration over its plan to block up to $9bn of government research grants to the institution. The suit, filed in a federal court on 21 April, claims that the administration’s “attempt to coerce and control” Harvard violates the academic freedom protected by the first amendment of the US constitution.

The action comes in the wake of the US administration claiming that Harvard and other universities have not protected Jewish students during pro-Gaza campus demonstrations. Columbia University has already agreed to change its teaching policies and clamp down on demonstrations in the hope of regaining some $400,000 of government grants.

Harvard president Alan Garber also sought negotiations with the administration on ways that it might satisfy its demands. But a letter sent to Garber dated 11 April, signed by three Trump administration officials, asserted that the university had “failed to live up to both the intellectual and civil rights conditions that justify federal investments”.

The letter demanded that Harvard reform and restructure its governance, stop all diversity, equality and inclusion (DEI) programmes and reform how it hires staff and students. It also said Harvard must stop recruiting international students who are “hostile to American values” and provide an audit on “viewpoint diversity” on admissions and hiring.

Some administration sources suggested that the letter, which effectively insists on government oversight of Harvard’s affairs, was an internal draft sent to Harvard by mistake. Nevertheless, Garber decided to end negotiations, leading Harvard to instead sue the government over the blocked funds.

We stand for the values that have made American higher education a beacon for the world

Alan Garber

A letter on 14 April from Harvard’s lawyers states that the university is “committed to fighting antisemitism and other forms of bigotry in its community”. It adds that it is “open to dialogue” about what it has done, and is planning to do, to “improve the experience of every member” of its community but concludes that Harvard “is not prepared to agree to demands that go beyond the lawful authority of this or any other administration”.

Writing in an open letter to the community dated 22 April, Garber says that “we stand for the values that have made American higher education a beacon for the world”. The administration has hit back by threatening to withdraw Harvard’s non-profit status, tax its endowment and jeopardise its ability to enrol overseas students, who currently make up more than 27% of its intake.

Budget woes

The Trump administration is also planning swingeing cuts to government science agencies. If its budget request for 2026 is approved by Congress, funding for NASA’s Science Mission Directorate would be almost halved from $7.3bn to $3.9bn. The Nancy Grace Roman Space Telescope, a successor to the Hubble and James Webb space telescopes, would be axed. Two missions to Venus – the DAVINCI atmosphere probe and the VERITAS surface-mapping project – as well as the Mars Sample Return mission would lose their funding too.

“The impacts of these proposed funding cuts would not only be devastating to the astronomical sciences community, but they would also have far-reaching consequences for the nation,” says Dara Norman, president of the American Astronomical Society. “These cuts will derail not only cutting-edge scientific advances, but also the training of the nation’s future STEM workforce.”

The National Oceanic and Atmospheric Administration (NOAA) also stands to lose key programmes, with the budget for its Ocean and Atmospheric Research Office slashed from $485m to just over $170m. Surviving programmes from the office, including research on tornado warning and ocean acidification, would move to the National Weather Service and National Ocean Service.

“This administration’s hostility toward research and rejection of climate science will have the consequence of eviscerating the weather forecasting capabilities that this plan claims to preserve,” says Zoe Lofgren, a senior Democrat who sits on the House of Representatives’ Science, Space, and Technology Committee.

The National Science Foundation (NSF), meanwhile, is unlikely to receive $234m for major building projects this financial year, which could spell the end of the Horizon supercomputer being built at the University of Texas at Austin. The NSF has already halved the number of graduate students in its research fellowship programme, while Science magazine says it is calling back all grant proposals that had been approved but not signed off, apparently to check that awardees conform to Trump’s stance on DEI.

A survey of 292 department chairs at US institutions in early April, carried out by the American Institute of Physics, reveals that almost half of respondents are experiencing or anticipate cuts in federal funding in the coming months. Entitled Impacts of Restrictions on Federal Grant Funding in Physics and Astronomy Graduate Programs, the report also says that the number of first-year graduate students in physics and astronomy is expected to drop by 13% in the next enrolment.

Update: 25/04/2025: Sethuraman Panchanathan has resigned as NSF director five years into his six-year term. Panchanathan took up the position in 2020 during Trump’s first term as US President. “I believe that I have done all I can to advance the mission of the agency and feel that it is time to pass the baton to new leadership,” Panchanathan said in a statement yesterday. “This is a pivotal moment for our nation in terms of global competitiveness. We must not lose our competitive edge.”

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Precise control over the generation of intense, ultrafast changes in magnetic fields called “magnetic steps” has been achieved by researchers in Hamburg, Germany. Using ultrashort laser pulses, Andrea Cavalleri and colleagues at the Max Planck Institute for the Structure and Dynamics of Matter disrupted the currents flowing through a superconducting disc. This alters the superconductor’s local magnetic environment on very short timescales – creating a magnetic step.

Magnetic steps rise to their peak intensity in just a few picoseconds, before decaying more slowly in several nanoseconds. They are useful to scientists because they rise and fall on timescales far shorter than the time it takes for materials to respond to external magnetic fields. As a result, magnetic steps could provide fundamental insights into the non-equilibrium properties of magnetic materials, and could also have practical applications in areas such as magnetic memory storage.

So far, however, progress in this field has been held back by technical difficulties in generating and controlling magnetic steps on ultrashort timescales. Previous strategies  have employed technologies including microcoils, specialized antennas, and circularly polarized light pulses. However, each of these schemes offer a limited degree of control over the properties of the magnetic steps they generated.

Quenching supercurrents

Now, Cavalleri’s team has developed a new technique that involves the quenching of currents in a superconductor. Normally, these “supercurrents” will flow indefinitely without losing energy, and will act to expel any external magnetic fields from the superconductor’s interior. However, if these currents are temporarily disrupted on ultrashort timescales, a sudden change will be triggered in the magnetic field close to the superconductor – which could be used to create a magnetic step.

To create this process, Cavalleri and colleagues applied ultrashort laser pulses to a thin, superconducting disc of yttrium barium copper oxide (YBCO), while also exposing the disc to an external magnetic field.

To detect whether magnetic steps had been generated, they placed a crystal of the semiconductor gallium phosphide in the superconductor’s vicinity. This material exhibits an extremely rapid Faraday response. This involves the rotation of the polarization of light passing through the semiconductor in response to changes in the local magnetic field. Crucially, this rotation can occur on sub-picosecond timescales.

In their experiments, researchers monitored changes to the polarization of an ultrashort “probe” laser pulse passing through the semiconductor shortly after they quenched supercurrents in their YBCO disc using a separate ultrashort “pump” laser pulse.

“By abruptly disrupting the material’s supercurrents using ultrashort laser pulses, we could generate ultrafast magnetic field steps with rise times of approximately one picosecond – or one trillionth of a second,” explains team member Gregor Jotzu.

Broadband step

This was used to generate an extremely broadband magnetic step, which contains frequencies ranging from sub-gigahertz to terahertz. In principle, this should make the technique suitable for studying magnetization in a diverse variety of materials.

To demonstrate practical applications, the team used these magnetic steps to control the magnetization of a ferrimagnet. Such a magnet has opposing magnetic moments, but has a non-zero spontaneous magnetization in zero magnetic field.

When they placed a ferrimagnet on top of their superconductor and created a magnetic step, the step field caused the ferrimagnet’s magnetization to rotate.

For now, the magnetic steps generated through this approach do not have the speed or amplitude needed to switch materials like a ferrimagnet between stable states. Yet through further tweaks to the geometry of their setup, the researchers are confident that this ability may not be far out of reach.

“Our goal is to create a universal, ultrafast stimulus that can switch any magnetic sample between stable magnetic states,” Cavalleri says. “With suitable improvements, we envision applications ranging from phase transition control to complete switching of magnetic order parameters.”

The research is described in Nature Photonics.

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Until now, researchers have had to choose between thermal and visible imaging: One reveals heat signatures while the other provides structural detail. Recording both and trying to align them manually — or harder still, synchronizing them temporally — can be inconsistent and time-consuming. The result is data that is close but never quite complete. The new FLIR MIX is a game changer, capturing and synchronizing high-speed thermal and visible imagery at up to 1000 fps. Visible and high-performance infrared cameras with FLIR Research Studio software work together to deliver one data set with perfect spatial and temporal alignment — no missed details or second guessing, just a complete picture of fast-moving events.

Jerry Beeney is a seasoned global business development leader with a proven track record of driving product growth and sales performance in the Teledyne FLIR Science and Automation verticals. With more than 20 years at Teledyne FLIR, he has played a pivotal role in launching new thermal imaging solutions, working closely with technical experts, product managers, and customers to align products with market demands and customer needs. Before assuming his current role, Beeney held a variety of technical and sales positions, including senior scientific segment engineer. In these roles, he managed strategic accounts and delivered training and product demonstrations for clients across diverse R&D and scientific research fields. Beeney’s dedication to achieving meaningful results and cultivating lasting client relationships remains a cornerstone of his professional approach.

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Researchers at the University of Victoria in Canada are developing a low-cost radiotherapy system for use in low- and middle-income countries and geographically remote rural regions. Initial performance characterization of the proof-of-concept device produced encouraging results, and the design team is now refining the system with the goal of clinical commercialization.

This could be good news for people living in low-resource settings, where access to cancer treatment is an urgent global health concern. The WHO’s International Agency for Research on Cancer estimates that there are at least 20 million new cases of cancer diagnosed annually and 9.7 million annual cancer-related deaths, based on 2022 data. By 2030, approximately 75% of cancer deaths are expected to occur in low- and middle-income countries, due to rising populations, healthcare and financial disparities, and a general lack of personnel and equipment resources compared with high-income countries.

The team’s orthovoltage radiotherapy system, known as KOALA (kilovoltage optimized alternative for adaptive therapy), is designed to create, optimize and deliver radiation treatments in a single session. The device, described in Biomedical Physics & Engineering Express, consists of a dual-robot system with a 225 kVp X-ray tube mounted onto one robotic arm and a flat-panel detector mounted on the other.

The same X-ray tube can be used to acquire cone-beam CT (CBCT) images, as well as to deliver treatment, with a peak tube voltage of 225 kVp and a maximum tube current of 2.65 mA for a 1.2 mm focal spot. Due to its maximum reach of 2.05 m and collision restrictions, the KOALA system has a limited range of motion, achieving 190° arcs for both CBCT acquisition and treatments.

Device testing

To characterize the KOALA system, lead author Olivia Masella and colleagues measured X-ray spectra for tube voltages of 120, 180 and 225 kVp. At 120 and 180 kVp, they observed good agreement with spectra from SpekPy (a Python software toolkit for modelling X-ray tube spectra). For the 225 kVp spectrum, they found a notable overestimation in the higher energies.

The researchers performed dosimetric tests by measuring percent depth dose (PDD) curves for a 120 kVp imaging beam and a 225 kVp therapy beam, using solid water phantom blocks with a Farmer ionization chamber at various depths. They used an open beam with 40° divergence and a source-to-surface distance of 30 cm. They also measured 2D dose profiles with radiochromic film at various depths in the phantom for a collimated 225 kVp therapy beam and a dose of approximately 175 mGy at the surface.

The PDD curves showed excellent agreement between experiment and simulations at both 120 and 225 kVp, with dose errors of less than 2%. The 2D profile results were less than optimal. The team aims to correct this by using a more optimal source-to-collimator distance (100 mm) and a custom-built motorized collimator.

Geometrical evaluation conducted using a coplanar star-shot test showed that the system demonstrated excellent geometrical accuracy, generating a wobble circle with a diameter of just 0.3 mm.

Low costs and clinical practicality

Principal investigator Magdalena Bazalova-Carter describes the rationale behind the KOALA’s development. “I began the computer simulations of this project about 15 years ago, but the idea originated from Michael Weil, a radiation oncologist in Northern California,” she tells Physics World. “He and our industrial partner, Tai-Nang Huang, the president of Linden Technologies, are overseeing the progress of the project. Our university team is diversified, working in medical physics, computer science, and electrical and mechanical engineering. Orimtech, a medical device manufacturer and collaborator, developed the CBCT acquisition and reconstruction software and built the imaging prototype.”

Masella says that the team is keeping costs low is various ways. “Megavoltage X-rays are most commonly used in conventional radiotherapy, but KOALA’s design utilizes low-energy kilovoltage X-rays for treatment. By using a 225 kVp X-ray tube, the X-ray generation alone is significantly cheaper compared to a conventional linac, at a cost of USD $150,000 compared to $3 million,” she explains. “By operating in the kilovoltage instead of megavoltage range, only about 4 mm of lead shielding is required, instead of 6 to 7 feet of high-density concrete, bringing the shielding cost down from $2 million to $50,000. We also have incorporated components that are much lower cost than [those in] a conventional radiotherapy system.”

“Our novel iris collimator leaves are only 1-mm thick due to the lower treatment X-ray beam energy, and its 12 leaves are driven by a single motor,” adds Bazalova-Carter. “Although multileaf collimators with 120 leaves utilized with megavoltage X-ray radiotherapy are able to create complex fields, they are about 8-cm thick and are controlled by 120 separate motors. Given the high cost and mechanical vulnerability of multileaf collimators, our single motor design offers a more robust and reliable alternative.”

The team is currently developing a new motorized collimator, an improved treatment couch and a treatment planning system. They plan to improve CBCT imaging quality with hardware modifications, develop a CBCT-to-synthetic CT machine learning algorithm, refine the auto-contouring tool and integrate all of the software to smooth the workflow.

The researchers are planning to work with veterinarians to test the KOALA system with dogs diagnosed with cancer. They will also develop quality assurance protocols specific to the KOALA device using a dog-head phantom.

“We hope to demonstrate the capabilities of our system by treating beloved pets for whom available cancer treatment might be cost-prohibitive. And while our system could become clinically adopted in veterinary medicine, our hope is that it will be used to treat people in regions where conventional radiotherapy treatment is insufficient to meet demand,” they say.

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Physicists working on the ATLAS experiment on the Large Hadron Collider (LHC) are the first to report the production of top quark–antiquark pairs in collisions involving heavy nuclei. By colliding lead ions, CERN’s LHC creates a fleeting state of matter called the quark–gluon plasma. This is an extremely hot and dense soup of subatomic particles that includes deconfined quarks and gluons. This plasma is believed to have filled the early universe microseconds after the Big Bang.

“Heavy-ion collisions at the LHC recreate the quark–gluon plasma in a laboratory setting,” Anthony Badea, a postdoctoral researcher at the University of Chicago and one of the lead authors of a paper describing the research. As well as boosting our understanding of the early universe, studying the quark–gluon plasma at the LHC could also provide insights into quantum chromodynamics (QCD), which is the theory of how quarks and gluons interact.

Although the quark–gluon plasma at the LHC vanishes after about 10^-23 s, scientists can study it by analysing how other particles produced in collisions move through it. The top quark is the heaviest known elementary particle and its short lifetime and distinct decay pattern offer a unique way to explore the quark–gluon plasma. This because the top quark decays before the quark–gluon plasma dissipates.

“The top quark decays into lighter particles that subsequently further decay,” explains Stefano Forte at the University of Milan, who was not involved in the research. “The time lag between these subsequent decays is modified if they happen within the quark–gluon plasma, and thus studying them has been suggested as a way to probe [quark–gluon plasma’s] structure. In order for this to be possible, the very first step is to know how many top quarks are produced in the first place, and determining this experimentally is what is done in this [ATLAS] study.”

First observations

The ATLAS team analysed data from lead–lead collisions and searched for events in which a top quark and its antimatter counterpart were produced. These particles can then decay in several different ways and the researchers focused on a less frequent but more easily identifiable mode known as the di-lepton channel. In this scenario, each top quark decays into a bottom quark and a W boson, which is a weak force-carrying particle that then transforms into a detectable lepton and an invisible neutrino.

The results not only confirmed that top quarks are created in this complex environment but also showed that their production rate matches predictions based on our current understanding of the strong nuclear force.

“This is a very important study,” says Juan Rojo, a theoretical physicist at the Free University of Amsterdam who did not take part in the research. “We have studied the production of top quarks, the heaviest known elementary particle, in the relatively simple proton–proton collisions for decades. This work represents the first time that we observe the production of these very heavy particles in a much more complex environment, with two lead nuclei colliding among them.”

As well as confirming QCD’s prediction of heavy-quark production in heavy-nuclei collisions, Rojo explains that “we have a novel probe to resolve the structure of the quark–gluon plasma”. He also says that future studies will enable us “to understand novel phenomena in the strong interactions such as how much gluons in a heavy nucleus differ from gluons within the proton”.

Crucial first step

“This is a first step – a crucial one – but further studies will require larger samples of top quark events to explore more subtle effects,” adds Rojo.

The number of top quarks created in the ATLAS lead–lead collisions agrees with theoretical expectations. In the future, more detailed measurements could help refine our understanding of how quarks and gluons behave inside nuclei. Eventually, physicists hope to use top quarks not just to confirm existing models, but to reveal entirely new features of the quark–gluon plasma.

Rojo says we could, “learn about the time structure of the quark–gluon plasma, measurements which are ‘finer’ would be better, but for this we need to wait until more data is collected, in particular during the upcoming high-luminosity run of the LHC”.

Badea agrees that ATLAS’s observation opens the door to deeper explorations. “As we collect more nuclei collision data and improve our understanding of top-quark processes in proton collisions, the future will open up exciting prospects”.

The research is described in Physical Review Letters.

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In the early days of quantum mechanics, physicists found its radical nature difficult to accept – even though the theory had successes. In particular Werner Heisenberg developed the first comprehensive formulation of quantum mechanics in 1925, while the following year Erwin Schrödinger was able to predict the spectrum of light emitted by hydrogen using his eponymous equation. Satisfying though these achievements were, there was trouble in store.

Long accustomed to Isaac Newton’s mechanical view of the universe, physicists had assumed that identical systems always evolve with time in exactly the same way, that is to say “deterministically”. But Heisenberg’s uncertainty principle and the probabilistic nature of Schrödinger’s wave function suggested worrying flaws in this notion. Those doubts were famously expressed by Albert Einstein, Boris Podolsky and Nathan Rosen in their “EPR” paper of 1935 (Phys. Rev. 47 777) and in debates between Einstein and Niels Bohr.

But the issues at stake went deeper than just a disagreement among physicists. They also touched on long-standing philosophical questions about whether we inhabit a deterministic universe, the related question of human free will, and the centrality of cause and effect. One person who rigorously addressed the questions raised by quantum theory was the German mathematician and philosopher Grete Hermann (1901–1984).

Hermann stands out in an era when it was rare for women to contribute to physics or philosophy, let alone to both. Writing in The Oxford Handbook of the History of Quantum Interpretations, published in 2022, the City University of New York philosopher of science Elise Crull has called Hermann’s work “one of the first, and finest, philosophical treatments of quantum mechanics”.

Grete Hermann upended the famous ‘proof’, developed by the Hungarian-American mathematician and physicist John von Neumann, that ‘hidden variables’ are impossible in quantum mechanics

What’s more, Hermann upended the famous “proof”, developed by the Hungarian-American mathematician and physicist John von Neumann, that “hidden variables” are impossible in quantum mechanics. But why have Hermann’s successes in studying the roots and meanings of quantum physics been so often overlooked? With 2025 being the International Year of Quantum Science and Technology, it’s time to find out.

Free thinker

Hermann was born on 2 March 1901 in the north German port city of Bremen. One of seven children, her mother was deeply religious, while her father was a merchant, a sailor and later an itinerant preacher. According to the 2016 book Grete Hermann: Between Physics and Philosophy by Crull and Guido Bacciagaluppi, she was raised according to her father’s maxim: “I train my children in freedom!” Essentially, he enabled Hermann to develop a wide range of interests and benefit from the best that the educational system could offer a woman at the time.

She was eventually admitted as one of a handful of girls at the Neue Gymnasium – a grammar school in Bremen – where she took a rigorous and broad programme of subjects. In 1921 Hermann earned a certificate to teach high-school pupils – an interest in education that reappeared in her later life – and began studying mathematics, physics and philosophy at the University of Göttingen.

In just four years, Hermann earned a PhD under the exceptional Göttingen mathematician Emmy Noether (1882–1935), famous for her groundbreaking theorem linking symmetry to physical conservation laws. Hermann’s final oral exam in 1925 featured not just mathematics, which was the subject of her PhD, but physics and philosophy too. She had specifically requested to be examined in the latter by the Göttingen philosopher Leonard Nelson, whose “logical sharpness” in lectures had impressed her.

By this time, Hermann’s interest in philosophy was starting to dominate her commitment to mathematics. Although Noether had found a mathematics position for her at the University of Freiburg, Hermann instead decided to become Nelson’s assistant, editing his books on philosophy. “She studies mathematics for four years,” Noether declared, “and suddenly she discovers her philosophical heart!”

Hermann found Nelson to be demanding and sometimes overbearing but benefitted from the challenges he set. “I gradually learnt to eke out, step by step,” she later declared, “the courage for truth that is necessary if one is to utterly place one’s trust, also within one’s own thinking, in a method of thought recognized as cogent.” Hermann, it appeared, was searching for a path to the internal discovery of truth, rather like Einstein’s Gedankenexperimente.

After Nelson died in 1927 aged just 45, Hermann stayed in Göttingen, where she continued editing and expanding his philosophical work and related political ideas. Espousing a form of socialism based on ethical reasoning to produce a just society, Nelson had co-founded a political action group and set up the associated Philosophical-Political Academy (PPA) to teach his ideas. Hermann contributed to both and also wrote for the PPA’s anti-Nazi newspaper.

Hermann’s involvement in the organizations Nelson had founded later saw her move to other locations in Germany, including Berlin. But after Hitler came to power in 1933, the Nazis banned the PPA, and Hermann and her socialist associates drew up plans to leave Germany. Initially, she lived at a PPA “school-in-exile” in neighbouring Denmark. As the Nazis began to arrest socialists, Hermann feared that Germany might occupy Denmark (as it indeed later did) and so moved again, first to Paris and then London.

Amid all these disruptions, Hermann continued to bring her dual philosophical and mathematical perspectives to physics, and especially to quantum mechanics

Arriving in Britain in early 1938, Hermann became acquainted with Edward Henry, another socialist, whom she later married. It was, however, merely a marriage of convenience that gave Hermann British citizenship and – when the Second World War started in 1939 – stopped her from being interned as an enemy alien. (The couple divorced after the war.) Amid all these disruptions, Hermann continued to bring her dual philosophical and mathematical perspectives to physics, and especially to quantum mechanics.

Mixing philosophy and physics

A major stimulus for Hermann’s work came from discussions she had in 1934 with Heisenberg and Carl Friedrich von Weizsäcker, who was then his research assistant at the Institute for Theoretical Physics in Leipzig. The previous year Hermann had written an essay entitled “Determinism and quantum mechanics”, which analysed whether the indeterminate nature of quantum mechanics – central to the “Copenhagen interpretation” of quantum behaviour – challenged the concept of causality.

Much cherished by physicists, causality says that every event has a cause, and that a given cause always produces a single specific event. Causality was also a tenet of the 18th-century German philosopher Immanuel Kant, best known for his famous 1781 treatise Critique of Pure Reason. He believed that causality is fundamental for how humans organize their experiences and make sense of the world.

Hermann, like Nelson, was a “neo-Kantian” who believed that Kant’s ideas should be treated with scientific rigour. In her 1933 essay, Hermann examined how the Copenhagen interpretation undermines Kant’s principle of causality. Although the article was not published at the time, she sent copies to Heisenberg, von Weizsäcker, Bohr and also Paul Dirac, who was then at the University of Cambridge in the UK.

In fact, we only know of the essay’s existence because Crull and Bacciagaluppi discovered a copy in Dirac’s archives at Churchill College, Cambridge. They also found a 1933 letter to Hermann from Gustav Heckmann, a physicist who said that Heisenberg, von Weizsäcker and Bohr had all read her essay and took it “absolutely and completely seriously”. Heisenberg added that Hermann was a “fabulously clever woman”.

Heckmann then advised Hermann to discuss her ideas more fully with Heisenberg, who he felt would be more open than Bohr to new ideas from an unexpected source. In 1934 Hermann visited Heisenberg and von Weizsäcker in Leipzig, with Heisenberg later describing their interaction in his 1971 memoir Physics and Beyond: Encounters and Conversations.

In that book, Heisenberg relates how rigorously Hermann wanted to treat philosophical questions. “[She] believed she could prove that the causal law – in the form Kant had given it – was unshakable,” Heisenberg recalled. “Now the new quantum mechanics seemed to be challenging the Kantian conception, and she had accordingly decided to fight the matter out with us.”

Their interaction was no fight, but a spirited discussion, with some sharp questioning from Hermann. When Heisenberg suggested, for instance, that a particular radium atom emitting an electron is an example of an unpredictable random event that has no cause, Hermann countered by saying that just because no cause has been found, it didn’t mean no such cause exists.

Significantly, this was a reference to what we now call “hidden variables” – the idea that quantum mechanics is being steered by additional parameters that we possibly don’t know anything about. Heisenberg then argued that even with such causes, knowing them would lead to complications in other experiments because of the wave nature of electrons.

Suppose, using a hidden variable, we could predict exactly which direction an electron would move. The electron wave wouldn’t then be able to split and interfere with itself, resulting in an extinction of the electron. But such electron interference effects are experimentally observed, which Heisenberg took as evidence that no additional hidden variables are needed to make quantum mechanics complete. Once again, Hermann pointed out a discrepancy in Heisenberg’s argument.

In the end, neither side fully convinced the other, but inroads were made, with Heisenberg concluding in his 1971 book that “we had all learned a good deal about the relationship between Kant’s philosophy and modern science”. Hermann herself paid tribute to Heisenberg in a 1935 paper “Natural-philosophical foundations of quantum mechanics”, which appeared in a relatively obscure philosophy journal called Abhandlungen der Fries’schen Schule (6 69). In it, she thanked Heisenberg “above all for his willingness to discuss the foundations of quantum mechanics, which was crucial in helping the present investigations”.

Quantum indeterminacy versus causality

In her 1933 paper, Hermann aimed to understand if the indeterminacy of quantum mechanics threatens causality. Her overall finding was that wherever indeterminacy is invoked in quantum mechanics, it is not logically essential to the theory. So without claiming that quantum theory actually supports causality, she left the possibility open that it might.

To illustrate her point, Hermann considered Heisenberg’s uncertainty principle, which says that there’s a limit to the accuracy with which complementary variables, such as position, q, and momentum, p, can be measured, namely ΔqΔp ≥ h where h is Planck’s constant. Does this principle, she wondered, truly indicate quantum indeterminism?

Hermann asserted that this relation can mean only one of two possible things. One is that measuring one variable leaves the value of the other undetermined. Alternatively, the result of measuring the other variable can’t be precisely predicted. Hermann dismissed the first option because its very statement implies that exact values exist, and so it cannot be logically used to argue against determinism. The second choice could be valid, but that does not exclude the possibility of finding new properties – hidden variables – that give an exact prediction.

Hermann used her mathematical training to point out a flaw in von Neumann’s famous 1932 proof, which said that no hidden-variable theory can ever reproduce the features of quantum mechanics

In making her argument about hidden variables, Hermann used her mathematical training to point out a flaw in von Neumann’s famous 1932 proof, which said that no hidden-variable theory can ever reproduce the features of quantum mechanics. Quantum mechanics, according to von Neumann, is complete and no extra deterministic features need to be added.

For decades, his result was cited as “proof” that any deterministic addition to quantum mechanics must be wrong. Indeed, von Neumann had such a well-deserved reputation as a brilliant mathematician that few people had ever bothered to scrutinize his analysis. But in 1964 the Northern Irish theorist John Bell famously showed that a valid hidden-variable theory could indeed exist, though only if it’s “non-local” (Physics Physique Fizika 1 195).

Non-locality says that things can happen at different parts of the universe simultaneously without needing faster-than-light communication. Despite being a notion that Einstein never liked, non-locality has been widely confirmed experimentally. In fact, non-locality is a defining feature of quantum physics and one that’s eminently useful in quantum technology.

Then, in 1966 Bell examined von Neumann’s reasoning and found an error that decisively refuted the proof (Rev. Mod, Phys. 38 447). Bell, in other words, showed that quantum mechanics could permit hidden variables after all – a finding that opened the door to alternative interpretations of quantum mechanics. However, Hermann had reported the very same error in her 1933 paper, and again in her 1935 essay, with an especially lucid exposition that almost exactly foresees Bell’s objection.

She had got there first, more than three decades earlier (see box).

A new view of causality

After rebutting von Neumann’s proof in her 1935 essay, Hermann didn’t actually turn to hidden variables. Instead, Hermann went in a different and surprising direction, probably as a result of her discussions with Heisenberg. She accepted that quantum mechanics is a complete theory that makes only statistical predictions, but proposed an alternative view of causality within this interpretation.

We cannot foresee precise causal links in a quantum mechanics that is statistical, she wrote. But once a measurement has been made with a known result, we can work backwards to get a cause that led to that result. In fact, Hermann showed exactly how to do this with various examples. In this way, she maintains, quantum mechanics does not refute the general Kantian category of causality.

Not all philosophers have been satisfied by the idea of retroactive causality. But writing in The Oxford Handbook of the History of Quantum Interpretations, Crull says that Hermann “provides the contours of a neo-Kantian interpretation of quantum mechanics”. “With one foot squarely on Kant’s turf and the other squarely on Bohr’s and Heisenberg’s,” Crull concludes, “[Hermann’s] interpretation truly stands on unique ground.”

Grete Hermann’s 1935 paper shows a deep and subtle grasp of elements of the Copenhagen interpretation.

But Hermann’s 1935 paper did more than just upset von Neumann’s proof. In the article, she shows a deep and subtle grasp of elements of the Copenhagen interpretation such as its correspondence principle, which says that – in the limit of large quantum numbers – answers derived from quantum physics must approach those from classical physics.

The paper also shows that Hermann was fully aware – and indeed extended the meaning – of the implications of Heisenberg’s thought experiment that he used to illustrate the uncertainty principle. Heisenberg envisaged a photon colliding with an electron, but after that contact, she writes, the wave function of the physical system is a linear combination of terms, each being “the product of one wave function describing the electron and one describing the light quantum”.

As she went on to say, “The light quantum and the electron are thus not described each by itself, but only in their relation to each other. Each state of the one is associated with one of the other.” Remarkably, this amounts to an early perception of quantum entanglement, which Schrödinger described and named later in 1935. There is no evidence, however, that Schrödinger knew of Hermann’s insights.

Hermann’s legacy                             

On the centenary of the birth of a full theory of quantum mechanics, how should we remember Hermann? According to Crull, the early founders of quantum mechanics were “asking philosophical questions about the implications of their theory [but] none of these men were trained in both physics and philosophy”. Hermann, however, was an expert in the two. “[She] composed a brilliant philosophical analysis of quantum mechanics, as only one with her training and insight could have done,” Crull says.

Had Hermann’s 1935 paper been more widely known, it could have altered the early development of quantum mechanics

Sadly for Hermann, few physicists at the time were aware of her 1935 paper even though she had sent copies to some of them. Had it been more widely known, her paper could have altered the early development of quantum mechanics. Reading it today shows how Hermann’s style of incisive logical examination can bring new understanding.

Hermann leaves other legacies too. As the Second World War drew to a close, she started writing about the ethics of science, especially the way in which it was carried out under the Nazis. After the war, she returned to Germany, where she devoted herself to pedagogy and teacher training. She disseminated Nelson’s views as well as her own through the reconstituted PPA, and took on governmental positions where she worked to rebuild the German educational system, apparently to good effect according to contemporary testimony.

Hermann also became active in politics as an adviser to the Social Democratic Party. She continued to have an interest in quantum mechanics, but it is not clear how seriously she pursued it in later life, which saw her move back to Bremen to care for an ill comrade from her early socialist days.

Hermann’s achievements first came to light in 1974 when the physicist and historian Max Jammer revealed her 1935 critique of von Neumann’s proof in his book The Philosophy of Quantum Mechanics. Following Hermann’s death in Bremen on 15 April 1984, interest slowly grew, culminating in Crull and Bacciagaluppi’s 2016 landmark study Grete Hermann: Between Physics and Philosophy.

The life of this deep thinker, who also worked to educate others and to achieve worthy societal goals, remains an inspiration for any scientist or philosopher today.

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A few years ago, I wrote in Physics World about various bizarre structures I’d built from tennis balls, the most peculiar of which I termed “tennis-ball towers”. They consisted of a series of three-ball layers topped by a single ball (“the locker”) that keeps the whole tower intact. Each tower had (3n + 1) balls, where n is the number of triangular layers. The tallest tower I made was a seven-storey, 19-ball structure (n = 6). Shortly afterwards, I made an even bigger, nine-storey, 25-ball structure (n = 8).

Now, in the latest exciting development, I have built a new, record-breaking tower with 34 balls (n = 11), in which all 30 balls from the second to the eleventh layer are kept in equilibrium by the locker on the top (see photo a). The three balls in the bottom layer aren’t influenced by the locker as they stay in place by virtue of being on the horizontal surface of a table.

I tried going even higher but failed to build a structure that would stay intact without supporting “scaffolds”. Now in case you think I’ve just glued the balls together, watch the video below to see how the incredible 34-ball structure collapses spontaneously, probably due to a slight vibration as I walked around the table.

Even more unexpectedly, I have been able to make tennis-ball towers consisting of layers of four balls (4n + 1) and five balls too (5n + 1). Their equilibria are more delicate and, in the case of four-ball structures, so far I have only managed to build (photo b) a 21-ball, six-storey tower (n = 5). You can also see the tower in the video below.

The (5n + 1) towers are even trickier to make and (photo c) I have only got up to a three-storey structure with 11 balls (n = 2): two lots of five balls with a sixth single ball on top. In case you’re wondering, towers with six balls in each layer are physically impossible to build because they form a regular hexagon. You can’t just use another ball as a locker because it would simply sit between the other six (photo d).

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Researchers from the Karlsruhe Tritium Neutrino experiment (KATRIN) have announced the most precise upper limit yet on the neutrino’s mass. Thanks to new data and upgraded techniques, the new limit – 0.45 electron volts (eV) at 90% confidence – is half that of the previous tightest constraint, and marks a step toward answering one of particle physics’ longest-standing questions.

Neutrinos are ghostlike particles that barely interact with matter, slipping through the universe almost unnoticed. They come in three types, or flavours: electron, muon, and tau. For decades, physicists assumed all three were massless, but that changed in the late 1990s when experiments revealed that neutrinos can oscillate between flavours as they travel. This flavour-shifting behaviour is only possible if neutrinos have mass.

Although neutrino oscillation experiments confirmed that neutrinos have mass, and showed that the masses of the three flavours are different, they did not divulge the actual scale of these masses. Doing so requires an entirely different approach.

Looking for clues in electrons

In KATRIN’s case, that means focusing on a process called tritium beta decay, where a tritium nucleus (a proton and two neutrons) decays into a helium-3 nucleus (two protons and one neutron) by releasing an electron and an electron antineutrino. Due to energy conservation, the total energy from the decay is shared between the electron and the antineutrino. The neutrino’s mass determines the balance of the split.

“If the neutrino has even a tiny mass, it slightly lowers the energy that the electron can carry away,” explains Christoph Wiesinger, a physicist at the Technical University of Munich, Germany and a member of the KATRIN collaboration. “By measuring that [electron] spectrum with extreme precision, we can infer how heavy the neutrino is.”

Because the subtle effects of neutrino mass are most visible in decays where the antineutrino carries away very little energy (most of it bound up in mass), KATRIN concentrates on measuring electrons that have taken the lion’s share. From these measurements, physicists can calculate neutrino mass without having to detect these notoriously weakly-interacting particles directly.

Improvements over previous results

The new neutrino mass limit is based on data taken between 2019 and 2021, with 259 days of operations yielding over 36 million electron measurements. “That’s six times more than the previous result,” Wiesinger says.

Other improvements include better temperature control in the tritium source and a new calibration method using a monoenergetic krypton source. “We were able to reduce background noise rates by a factor of two, which really helped the precision,” he adds.

At 0.45 eV, the new limit means the neutrino is at least a million times lighter than the electron. “This is a fundamental number,” Wiesinger says. “It tells us that neutrinos are the lightest known massive particles in the universe, and maybe that their mass has origins beyond the Standard Model.”

Despite the new tighter limit, however, definitive answers about the neutrino’s mass are still some ways off. “Neutrino oscillation experiments tell us that the lower bound on the neutrino mass is about 0.05 eV,” says Patrick Huber, a theoretical physicist at Virginia Tech, US, who was not involved in the experiment. “That’s still about 10 times smaller than the new KATRIN limit… For now, this result fits comfortably within what we expect from a Standard Model that includes neutrino mass.”

Model independence

Though Huber emphasizes that there are “no surprises” in the latest measurement, KATRIN has a key advantage over its rivals. Unlike cosmological methods, which infer neutrino mass based on how it affects the structure and evolution of the universe, KATRIN’s direct measurement is model-independent, relying only on energy and momentum conservation. “That makes it very powerful,” Wiesinger argues. “If another experiment sees a measurement in the future, it will be interesting to check if the observation matches something as clean as ours.”

KATRIN’s own measurements are ongoing, with the collaboration aiming for 1000 days of operations by the end of 2025 and a final sensitivity approaching 0.3 eV. Beyond that, the plan is to repurpose the instrument to search for sterile neutrinos – hypothetical heavier particles that don’t interact via the weak force and could be candidates for dark matter.

“We’re testing things like atomic tritium sources and ultra-precise energy detectors,” Wiesinger says. “There are exciting ideas, but it’s not yet clear what the next-generation experiment after KATRIN will look like.”

The research appears in Science.

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Rapid technical innovation in quantum computing is expected to yield an array of hardware platforms that can run increasingly sophisticated algorithms. In the real world, however, such technical advances will remain little more than a curiosity if they are not adopted by businesses and the public sector to drive positive change. As a result, one key priority for the UK’s National Quantum Computing Centre (NQCC) has been to help companies and other organizations to gain an early understanding of the value that quantum computing can offer for improving performance and enhancing outcomes.

To meet that objective the NQCC has supported several feasibility studies that enable commercial organizations in the UK to work alongside quantum specialists to investigate specific use cases where quantum computing could have a significant impact within their industry. One prime example is a project involving the high-street bank HSBC, which has been exploring the potential of quantum technologies for spotting the signs of fraud in financial transactions. Such fraudulent activity, which affects millions of people every year, now accounts for about 40% of all criminal offences in the UK and in 2023 generated total losses of more than £2.3 bn across all sectors of the economy.

Banks like HSBC currently exploit classical machine learning to detect fraudulent transactions, but these techniques require a large computational overhead to train the models and deliver accurate results. Quantum specialists at the bank have therefore been working with the NQCC, along with hardware provider Rigetti and the Quantum Software Lab at the University of Edinburgh, to investigate the capabilities of quantum machine learning (QML) for identifying the tell-tale indicators of fraud.

“HSBC’s involvement in this project has brought transactional fraud detection into the realm of cutting-edge technology, demonstrating our commitment to pushing the boundaries of quantum-inspired solutions for near-term benefit,” comments Philip Intallura, Group Head of Quantum Technologies at HSBC. “Our philosophy is to innovate today while preparing for the quantum advantage of tomorrow.”

Another study focused on a key problem in the aviation industry that has a direct impact on fuel consumption and the amount of carbon emissions produced during a flight. In this logistical challenge, the aim was to find the optimal way to load cargo containers onto a commercial aircraft. One motivation was to maximize the amount of cargo that can be carried, the other was to balance the weight of the cargo to reduce drag and improve fuel efficiency.

“Even a small shift in the centre of gravity can have a big effect,” explains Salvatore Sinno of technology solutions company Unisys, who worked on the project along with applications engineers at the NQCC and mathematicians at the University of Newcastle. “On a Boeing 747 a displacement of just 75 cm can increase the carbon emissions on a flight of 10,000 miles by four tonnes, and also increases the fuel costs for the airline company.”

With such a large number of possible loading combinations, classical computers cannot produce an exact solution for the optimal arrangement of cargo containers. In their project the team improved the precision of the solution by combining quantum annealing with high-performance computing, a hybrid approach that Unisys believes can offer immediate value for complex optimization problems. “We have reached the limit of what we can achieve with classical computing, and with this work we have shown the benefit of incorporating an element of quantum processing into our solution,” explains Sinno.

The HSBC project team also found that a hybrid quantum–classical solution could provide an immediate performance boost for detecting anomalous transactions. In this case, a quantum simulator running on a classical computer was used to run quantum algorithms for machine learning. “These simulators allow us to execute simple QML programmes, even though they can’t be run to the same level of complexity as we could achieve with a physical quantum processor,” explains Marco Paini, the project lead for Rigetti. “These simulations show the potential of these low-depth QML programmes for fraud detection in the near term.”

The team also simulated more complex QML approaches using a similar but smaller-scale problem, demonstrating a further improvement in performance. This outcome suggests that running deeper QML algorithms on a physical quantum processor could deliver an advantage for detecting anomalies in larger datasets, even though the hardware does not yet provide the performance needed to achieve reliable results. “This initiative not only showcases the near-term applicability of advanced fraud models, but it also equips us with the expertise to leverage QML methods as quantum computing scales,” comments Intellura.

Indeed, the results obtained so far have enabled the project partners to develop a roadmap that will guide their ongoing development work as the hardware matures. One key insight, for example, is that even a fault-tolerant quantum computer would struggle to process the huge financial datasets produced by a bank like HSBC, since a finite amount of time is needed to run the quantum calculation for each data point. “From the simulations we found that the hybrid quantum–classical solution produces more false positives than classical methods,” says Paini. “One approach we can explore would be to use the simulations to flag suspicious transactions and then run the deeper algorithms on a quantum processor to analyse the filtered results.”

This particular project also highlighted the need for agreed protocols to navigate the strict rules on data security within the banking sector. For this project the HSBC team was able to run the QML simulations on its existing computing infrastructure, avoiding the need to share sensitive financial data with external partners. In the longer term, however, banks will need reassurance that their customer information can be protected when processed using a quantum computer. Anticipating this need, the NQCC has already started to work with regulators such as the Financial Conduct Authority, which is exploring some of the key considerations around privacy and data security, with that initial work feeding into international initiatives that are starting to consider the regulatory frameworks for using quantum computing within the financial sector.

For the cargo-loading project, meanwhile, Sinno says that an important learning point has been the need to formulate the problem in a way that can be tackled by the current generation of quantum computers. In practical terms that means defining constraints that reduce the complexity of the problem, but that still reflect the requirements of the real-world scenario. “Working with the applications engineers at the NQCC has helped us to understand what is possible with today’s quantum hardware, and how to make the quantum algorithms more viable for our particular problem,” he says. “Participating in these studies is a great way to learn and has allowed us to start using these emerging quantum technologies without taking a huge risk.”

Indeed, one key feature of these feasibility studies is the opportunity they offer for different project partners to learn from each other. Each project includes an end-user organization with a deep knowledge of the problem, quantum specialists who understand the capabilities and limitations of present-day solutions, and academic experts who offer an insight into emerging theoretical approaches as well as methodologies for benchmarking the results. The domain knowledge provided by the end users is particularly important, says Paini, to guide ongoing development work within the quantum sector. “If we only focused on the hardware for the next few years, we might come up with a better technical solution but it might not address the right problem,” he says. “We need to know where quantum computing will be useful, and to find that convergence we need to develop the applications alongside the algorithms and the hardware.”

Another major outcome from these projects has been the ability to make new connections and identify opportunities for future collaborations. As a national facility NQCC has played an important role in providing networking opportunities that bring diverse stakeholders together, creating a community of end users and technology providers, and supporting project partners with an expert and independent view of emerging quantum technologies. The NQCC has also helped the project teams to share their results more widely, generating positive feedback from the wider community that has already sparked new ideas and interactions.

“We have been able to network with start-up companies and larger enterprise firms, and with the NQCC we are already working with them to develop some proof-of-concept projects,” says Sinno. “Having access to that wider network will be really important as we continue to develop our expertise and capability in quantum computing.”

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Through new experiments, researchers in Switzerland have tested models of how microwaves affect low-temperature chemical reactions between ions and molecules. Through their innovative setup, Valentina Zhelyazkova and colleagues at ETH Zurich showed for the first time how the application of microwave pulses can slow down reaction rates via nonthermal mechanisms.

Physicists have been studying chemical reactions between ions and neutral molecules for some time. At close to room temperature, classical models can closely predict how the electric fields emanating from ions will induce dipoles in nearby neutral molecules, allowing researchers to calculate these reaction rates with impressive accuracy. Yet as temperatures drop close to absolute zero, a wide array of more complex effects come into play, which have gradually been incorporated into the latest theoretical models.

“At low temperatures, models of reactivity must include the effects of the permanent electric dipoles and quadrupole moments of the molecules, the effect of their vibrational and rotational motion,” Zhelyazkova explains. “At extremely low temperatures, even the quantum-mechanical wave nature of the reactants must be considered.”

Rigorous experiments

Although these low-temperature models have steadily improved in recent years, the ability to put them to the test through rigorous experiments has so far been hampered by external factors.

In particular, stray electric fields in the surrounding environment can heat the ions and molecules, so that any important quantum effects are quickly drowned out by noise. “Consequently, it is only in the past few years that experiments have provided information on the rates of ion–molecule reactions at very low temperatures,” Zhelyazkova explains.

In their study, Zhelyazkova’s team improved on these past experiments through an innovative approach to cooling the internal motions of the molecules being heated by stray electric fields. Their experiment involved a reaction between positively-charged helium ions and neutral molecules of carbon monoxide (CO). This creates neutral atoms of helium and oxygen, and a positively-charged carbon atom.

To initiate the reaction, the researchers created separate but parallel supersonic beams of helium and CO that were combined in a reaction cell. “In order to overcome the problem of heating the ions by stray electric fields, we study the reactions within the distant orbit of a highly excited electron, which makes the overall system electrically neutral without affecting the ion–molecule reaction taking place within the electron orbit,” explains ETH’s Frédéric Merkt.

Giant atoms

In such a “Rydberg atom”, the highly excited electron is some distance from the helium nucleus and its other electron. As a result, a Rydberg helium atom can be considered an ion with a “spectator” electron, which has little influence over how the reaction unfolds. To ensure the best possible accuracy, “we use a printed circuit board device with carefully designed surface electrodes to deflect one of the two beams,” explains ETH’s, Fernanda Martins. “We then merged this beam with the other, and controlled the relative velocity of the two beams.”

Altogether, this approach enabled the researchers to cool the molecules internally to temperatures below 10 K – where their quantum effects can dominate over externally induced noise. With this setup, Zhelyazkova, Merkt, Martins, and their colleagues could finally put the latest theoretical models to the test.

According to the latest low-temperature models, the rate of the CO–helium ion reaction should be determined by the quantized rotational states of the CO molecule – whose energies lie within the microwave range. In this case, the team used microwave pulses to put the CO into different rotational states, allowing them to directly probe their influence on the overall reaction rate.

Three important findings

Altogether, their experiment yielded three important findings. It confirmed that the reaction rate can vary, depending on the rotational state of the CO molecule; it showed that this reactivity can be modified by using a short microwave pulse to excite the CO molecule from its ground state to its first excited state – with the first excited state being less reactive than the ground state.

The third and most counterintuitive finding is that microwaves can slow down the reaction rate, via mechanisms unrelated to the heat they impart on the molecules absorbing them. “In most applications of microwaves in chemical synthesis, the microwaves are used as a way to thermally heat the molecules up, which always makes them more reactive,” Zhelyazkova says.

Building on the success of their experimental approach, the team now hopes to investigate these nonthermal mechanisms in more detail – with the aim to shed new light on how microwaves can influence chemical reactions via effects other than heating. In turn, their results could ultimately pave the way for advanced new techniques for fine-tuning the rate of reactions between ions and neutral molecules.

The research is described in Physical Review Letters.

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Superpositions of quantum states known as Schrödinger cat states can be created in “hot” environments with temperatures up to 1.8 K, say researchers in Austria and Spain. By reducing the restrictions involved in obtaining ultracold temperatures, the work could benefit fields such as quantum computing and quantum sensing.

In 1935, Erwin Schrödinger used a thought experiment now known as “Schrödinger’s cat” to emphasize what he saw as a problem with some interpretations of quantum theory. His gedankenexperiment involved placing a quantum system (a cat in a box with a radioactive sample and a flask of poison) in a state that is a superposition of two states (“alive cat” if the sample has not decayed and “dead cat” if it has). These superposition states are now known as Schrödinger cat states (or simply cat states) and are useful in many fields, including quantum computing, quantum networks and quantum sensing.

Creating a cat state, however, requires quantum particles to be in their ground state. This, in turn, means cooling them to extremely low temperatures. Even marginally higher temperatures were thought to destroy the fragile nature of these states, rendering them useless for applications. But the need for ultracold temperatures comes with its own challenges, as it restricts the range of possible applications and hinders the development of large-scale systems such as powerful quantum computers.

Cat on a hot tin…microwave cavity?

The new work, which was carried out by researchers at the University of Innsbruck and IQOQI in Austria together with colleagues at the ICFO in Spain, challenges the idea that ultralow temperatures are a must for generating cat states. Instead of starting from the ground state, they used thermally excited states to show that quantum superpositions can exist at temperatures of up to 1.8 K – an environment that might as well be an oven in the quantum world.

Team leader Gerhard Kirchmair, a physicist at the University of Innsbruck and the IQOQI, says the study evolved from one of those “happy accidents” that characterize work in a collaborative environment. During a coffee break with a colleague, he realized he was well-equipped to prove the hypothesis of another colleague, Oriol Romero-Isart, who had shown theoretically that cat states can be generated out of a thermal state.

The experiment involved creating cat states inside a microwave cavity that acts as a quantum harmonic oscillator. This cavity is coupled to a superconducting transmon qubit that behaves as a two-level system where the superposition is generated. While the overall setup is cooled to 30 mK, the cavity mode itself is heated by equilibrating it with amplified Johnson-Nyquist noise from a resistor, making it 60 times hotter than its environment.

To establish the existence of quantum correlations at this higher temperature, the team directly measured the Wigner functions of the states. Doing so revealed the characteristic interference patterns of Schrödinger cat states.

Benefits for quantum sensing and error correction

According to Kirchmair, being able to realize cat states without ground-state cooling could bring benefits for quantum sensing. The mechanical oscillator systems used to sense acceleration or force, for example, are normally cooled to the ground state to achieve the necessary high sensitivity, but such extreme cooling may not be necessary. He adds that quantum error correction schemes could also benefit, as they rely on being able to create cat states reliably; the team’s work shows that a residual thermal population places fewer limitations on this than previously thought.

“For next steps we will use the system for what it was originally designed, i.e. to mediate interactions between multiple qubits for novel quantum gates,” he tells Physics World.

Yiwen Chu, a quantum physicist from ETH Zürich in Switzerland who was not involved in this research, praises the “creativeness of the idea”. She describes the results as interesting and surprising because they seem to counter the common view that lack of purity in a quantum state degrades quantum features. She also agrees that the work could be important for quantum sensing, adding that many systems – including some more suited for sensing – are difficult to prepare in the ground state.

However, Chu notes that, for reasons stemming from the system’s parameters and the protocols the team used to generate the cat states, it should be possible to cool this particular system very efficiently to the ground state. This, she says, somewhat diminishes the argument that the method will be useful for systems where this isn’t the case. “However, these parameters and the protocols they showed might not be the only way to prepare such states, so on a fundamental level it is still very interesting,” she concludes.

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