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

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

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

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

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

Digging holes

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

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

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

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

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

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

The big questions

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

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

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

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

Bury the problem

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

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

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

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

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

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

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

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

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

A tomb in a mine

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

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

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

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

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

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

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

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

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

A simple solution

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

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

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

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

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

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

Managing one’s mental workload is a tricky balancing act that can affect cognitive performance and decision making abilities. Too little engagement with an ongoing task can lead to boredom and mistakes; too high could cause a person to become overwhelmed.

For those performing safety-critical tasks, such as air traffic controllers or truck drivers for example, monitoring how hard their brain is working is even more important – lapses in focus could have serious consequences. But how can a person’s mental workload be assessed? A team at the University of Texas at Austin proposes the use of temporary face tattoos that can track when a person’s brain is working too hard.

“Technology is developing faster than human evolution. Our brain capacity cannot keep up and can easily get overloaded,” says lead author Nanshu Lu in a press statement. “There is an optimal mental workload for optimal performance, which differs from person to person.”

The traditional approach for monitoring mental workload is electroencephalography (EEG), which analyses the brain’s electrical activity. But EEG devices are wired, bulky and uncomfortable, making them impractical for real-world situations. Measurements of eye movements using electrooculography (EOG) are another option for assessing mental workload.

Lu and colleagues have developed an ultrathin wireless e-tattoo that records high-fidelity EEG and EOG signals from the forehead. The e-tattoo combines a disposable sticker-like electrode layer and a reusable battery-powered flexible printed circuit (FPC) for data acquisition and wireless transmission.

The serpentine-shaped electrodes and interconnects are made from low-cost, conductive graphite-deposited polyurethane, coated with an adhesive polymer composite to reduce contact impedance and improve skin attachment. The e-tattoo stretches and conforms to the skin, providing reliable signal acquisition, even during dynamic activities such as walking and running.

To assess the e-tattoo’s ability to record basic neural activities, the team used it to measure alpha brainwaves as a volunteer opened and closed their eyes. The e-tattoo captured equivalent neural spectra to that recorded by a commercial gel electrode-based EEG system with comparable signal fidelity.

The researchers next tested the e-tattoo on six participants while they performed a visuospatial memory task that gradually increased in difficulty. They analysed the signals collected by the e-tattoo during the tasks, extracting EEG band powers for delta, theta, alpha, beta and gamma brainwaves, plus various EOG features.

As the task got more difficult, the participants showed higher activity in the theta and delta bands, a feature associated with increased cognitive demand. Meanwhile, activity in the alpha and beta bands decreased, indicating mental fatigue.

The researchers built a machine learning model to predict the level of mental workload experienced during the tasks, training it on forehead EEG and EOG features recorded by the e-tattoo. The model could reliably estimate mental workload in each of the six subjects, demonstrating the feasibility of real-time cognitive state decoding.

“Our key innovation lies in the successful decoding of mental workload using a wireless, low-power, low-noise and ultrathin EEG/EOG e-tattoo device,” the researchers write. “It addresses the unique challenges of monitoring forehead EEG and EOG, where wearability, non-obstructiveness and signal stability are critical to assessing mental workload in the real world.”

They suggest that future applications could include real-time cognitive load monitoring in pilots, operators and healthcare professionals. “We’ve long monitored workers’ physical health, tracking injuries and muscle strain,” says co-author Luis Sentis. “Now we have the ability to monitor mental strain, which hasn’t been tracked. This could fundamentally change how organizations ensure the overall well-being of their workforce.”

The e-tattoo is described in Device.

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

Now, it’s not looking so sure.

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

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

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

“A step in the right direction”

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

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

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

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

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

“A very delicate task”

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

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

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

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

The research is published in Nature Astronomy.

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

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

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

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

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

COVID impact

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

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

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

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What skills do you use every day in your job?

I co-founded Mutual Credit Services in 2020 to help small businesses thrive independently of the banking sector. As a financial technology start-up, we’re essentially trying to create a “commons” economy, where power lies in the hands of people, not big institutions, thereby making us more resilient to the climate crisis.

Those goals are probably as insanely ambitious as they sound, which is why my day-to-day work is a mix of complexity economics, monetary theory and economic anthropology. I spend a lot of time thinking hard about how these ideas fit together, before building new tech platforms, apps and services, which requires analytical and design thinking.

There are still many open questions about business, finance and economics that I’d like to do research on, and ultimately develop into new services. I’m constantly learning through trial projects and building a pipeline of ideas for future exploration.

Developing the business involves a lot of decision-making, project management and team-building. In fact, I’m spending more and more of my time on commercialization – working out how to bring new services to market, nurturing partnerships and talking to potential early adopters. It’s vital that I can explain novel financial ideas to small businesses in a way they can understand and have confidence in. So I’m always looking for simpler and more compelling ways to describe what we do.

What do you like best and least about your job?

What I like best is the variety and creativity. I’m a generalist by nature, and love using insights from a variety of disciplines. The practical application of these ideas to create a better economy feels profoundly meaningful, and something that I’d be unlikely to get in any other job. I also love the autonomy of running a business. With a small but hugely talented and enthusiastic team, we’ve so far managed to avoid the company becoming rigid and institutionalized. It’s great to work with people on our team and beyond who are excited by what we’re doing, and want to be involved.

The hardest thing is facing the omnicrisis of climate breakdown and likely societal collapse that makes this work necessary in the first place. As with all start-ups, the risk of failure is huge, no matter how good the ideas are, and it’s frustrating to spend so much time on tasks that just keep things afloat, rather than move the mission forward. I work long hours and the job can be stressful.

What do you know today, that you wish you knew when you were starting out in your career?

I spent a lot of time during my PhD at Liverpool worrying that I’d get trapped in one narrow field, or drift into one of the many default career options. I wish I’d known how many opportunities there are to do original, meaningful and self-directed work – especially if you’re open to unconventional paths, such as the one I’ve followed, and can find the right people to do it with.

It’s also easy to assume that certain skills or fields are out of reach, whereas I’ve found again and again that a mix of curiosity, self-education and carefully-chosen guidance can get you surprisingly far. Many things that once seemed intimidating now feel totally manageable. That said, I’ve also learned that everything takes at least three times longer than expected – especially when you’re building something new. Progress often looks like small compounding steps, rather than a handful of breakthroughs.

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Trips to synchrotron facilities could become a thing of the past for some researchers thanks to a new laboratory-scale three-dimensional X-ray diffraction microscope designed by a team from the University of Michigan, US. The device, which is the first of its kind, uses a liquid-metal-jet electrode to produce high-energy X-rays and can probe almost everything a traditional synchrotron can. It could therefore give a wider community of academic and industrial researchers access to synchrotron-style capabilities.

Synchrotrons are high-energy particle accelerators that produce bright, high-quality beams of coherent electromagnetic radiation at wavelengths ranging from the infrared to soft X-rays. To do this, they use powerful magnets to accelerate electrons in a storage ring, taking advantage of the fact that accelerated electrons emit electromagnetic radiation.

One application for this synchrotron radiation is a technique called three-dimensional X-ray diffraction (3DXRD) microscopy. This powerful technique enables scientists to study the mechanical behaviour of polycrystalline materials, and it works by constructing three-dimensional images of a sample from X-ray images taken at multiple angles, much as a CT scan images the human body. Instead of the imaging device rotating around a patient, however, it is the sample that rotates in the focus of the powerful X-ray beam.

At present, 3DXRD can only be performed at synchrotrons. These are national and international facilities, and scientists must apply for beamtime months or even years in advance. If successful, they receive a block of time lasting six days at the most, during which they must complete all their experiments.

A liquid-metal-jet anode

Previous attempts to make 3DXRD more accessible by downscaling it have largely been unsuccessful. In particular, efforts to produce high-energy X-rays using electrical anodes have foundered because these anodes are traditionally made of solid metal, which cannot withstand the extremely high power of electrons needed to produce X-rays.

The new lab-scale device developed by mechanical engineer Ashley Bucsek and colleagues overcomes this problem thanks to a liquid-metal-jet anode that can absorb more power and therefore produce a greater number of X-ray photons per electrode surface area. The sample volume is illuminated by a monochromatic box or line-focused X-ray beam while diffraction patterns are serially recorded as the sample rotates full circle. “The technique is capable of measuring the volume, position, orientation and strain of thousands of polycrystalline grains simultaneously,” Bucsek says.

When members of the Michigan team tested the device by imaging samples of titanium alloy samples, they found it was as accurate as synchrotron-based 3DXRD, making it a practical alternative. “I conducted my PhD doing 3DXRD experiments at synchrotron user facilities, so having full-time access to a personal 3DXRD microscope was always a dream,” Bucsek says. “My colleagues and I hope that the adaptation of this technology from the synchrotron to the laboratory scale will make it more accessible.”

The design for the device, which is described in Nature Communications, was developed in collaboration with a US-based instrumentation firm, PROTO Manufacturing. Bucsek says she is excited by the possibility that commercialization will make 3DXRD more “turn-key” and thus reduce the need for specialized knowledge in the field.

The Michigan researchers now hope to use their instrument to perform experiments that must be carried out over long periods of time. “Conducting such prolonged experiments at synchrotron user facilities would be difficult, if not impossible, due to the high demand, so, lab-3DXRD can fill a critical capability gap in this respect,” Bucsek tells Physics World.

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

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

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

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

 Controlling disorder

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

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

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

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

Zero energy

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

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

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

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

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

 The research is published in Nature.

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The Leinweber Foundation has awarded five US institutions $90m to create their own theoretical research institutes. The investment, which the foundation says is the largest ever for theoretical physics research, will be used to fund graduate students and postdocs at each institute as well as several Leinweber Physics Fellows.

The Leinweber Foundation was founded in 2015 by the software entrepreneur Larry Leinweber. In 1982 Leinweber founded the software company New World Systems Corporation, which provided software to the emergency services. In 2015 he sold the company to Tyler Technologies for $670m.

Based in Michigan, Leinweber Foundation supports research, education and community endeavours where it has provided Leinweber Software Scholarships to undergraduates at Michigan’s universities.

A Leinweber Institute for Theoretical Physics (LITP) will now be created at the universities of California, Berkeley, Chicago and Michigan as well as at the Massachusetts Institute of Technology (MIT) and at Princeton’s Institute for Advanced Study (IAS), where the institute will instead be named the Leinweber Forum for Theoretical and Quantum Physics.

The MIT LIPT, initially led by Washington Taylor before physicist Tracy Slatyer takes over later this year, will receive $20m from the foundation and will provide support for six postdocs, six graduate students as well as visitors, seminars and “other scholarly activities”.

“This landmark endowment from the Leinweber Foundation will enable us to support the best graduate students and postdoctoral researchers to develop their own independent research programmes and to connect with other researchers in the Leinweber Institute network,” says Taylor.

Spearing innovation

UC Berkeley, meanwhile, will receive $14.4m from the foundation in which the existing Berkeley Center for Theoretical Physics (BITP) will be renamed LITP at Berkeley and led by physicist Yasunori Nomura.

The money will be used for four postdoc positions to join the existing 15 at the BITP as well as to support graduate students and visitors. “This is transformative,” notes Nomura. “The gift will really have a huge impact on a wide range of research at Berkeley, including particle physics, quantum gravity, quantum information, condensed matter physics and cosmology.”

Chicago will receive $18.4m where the existing Kadanoff Center for Theoretical Physics will be merged into a new LITP at the University of Chicago and led by physicist Dam Thanh Son.

The remaining $37.2m will be split between the Leinweber Forum for Theoretical and Quantum Physics at the IAS and at Michigan, in which the existing Leinweber Center for Theoretical Physics will expand and become an institute.

“Theoretical physics may seem abstract to many, but it is the tip of the spear for innovation. It fuels our understanding of how the world works and opens the door to new technologies that can shape society for generations,” says Leinweber in a statement. “As someone who has had a lifelong fascination with theoretical physics, I hope this investment not only strengthens U.S. leadership in basic science, but also inspires curiosity, creativity, and groundbreaking discoveries for generations to come.”

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China has launched its first mission to retrieve samples from an asteroid. The Tianwen-2 mission launched at 01:31 a.m. local time on 28 May from the Xichang satellite launch center, southwest China, aboard a Long March B rocket.

Tianwen-2’s target is a small near-Earth asteroid called 469219 Kamoʻoalewa, which is between 15-39 million km away and is known as a “quasi-satellite” of Earth.

The mission is set to reach the body, which is between 40-100 m wide, in July 2026 where it will first study it up close using a suite of 11 instruments including cameras, spectrometers and radar, before aiming to collect about 100 g of material.

This will be achieved via three possible methods. One is via hovering close to the asteroid , the other is using a robotic arm to collect samples from the body while a third is dubbed “touch and go”, which involves gently landing on the asteroid and using drills at the end of each leg to retrieve material.

The collected samples will then be stored in a module that is released and returned to Earth in November 2027. If successful, it will make China the third nation to retrieve asteroid material behind the US and Japan.

Next steps

The second part of the 10-year mission involves using Earth for a gravitational swing-by to spend six year travelling to another target – 311P/PanSTARRS. The body lies in the main asteroid belt between Mars and Jupiter and at its closest distance is about 140 million km away from Earth.

The 480 m-wide object, which was discovered in 2013, has six dust tails and has characteristics of both asteroids and comets. Tianwen-2 will not land on 311P/PanSTARRS but instead use its instruments to study the “active asteroid” from a distance.

Tianwen-2’s predecessor, Tianwen-1, was China’s first mission to Mars, successfully landing on Utopia Planitia – a largely flat impact basin but scientifically interesting with potential water-ice underneath – following a six-month journey.

China’s third interplanetary mission, Tianwen-3, will aim to retrieve sample from Mars and could launch as soon as 2028. If successful, it would make China the first country to achieve the feat.

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Researchers in China have adapted the interlocking structure of mortise-and-tenon joints – as used by woodworkers around the world since ancient times – to the design of nanoscale devices known as memristors. The new devices are far more uniform than previous such structures, and the researchers say they could be ideal for scientific computing applications.

The memory-resistor, or “memristor”, was described theoretically at the beginning of the 1970s, but the first practical version was not built until 2008. Unlike standard resistors, the resistance of a memristor changes depending on the current previously applied to it, hence the “memory” in its name. This means that a desired resistance can be programmed into the device and subsequently stored. Importantly, the remembered value of the resistive state persists even when the power is switched off.

Thanks to numerous technical advances since 2008, memristors can now be integrated onto chips in large numbers. They are also capable of processing large amounts of data in parallel, meaning they could be ideal for emerging “in-memory” computing technologies that require calculations known as large-scale matrix-vector multiplications (MVMs). Many such calculations involve solving partial differential equations (PDEs), which are used to model complex behaviour in fields such as weather forecasting, fluid dynamics and astrophysics, to name but a few.

One remaining hurdle, however, is that it is hard to make memristors with uniform characteristics. The electronic properties of devices containing multiple memristors can therefore vary considerably, which adversely affects the computational accuracy of large-scale arrays.

Inspiration from an ancient technique

Physicists co-led by Shi-Jun Liang and Feng Miao of Nanjing University’s School of Physics say they have now overcome this problem by designing a memristor that uses a mortise-tenon-shaped (MTS) architecture. Humans have been using these strong and stable structures in wooden furniture for thousands of years, with one of the earliest examples dating back to the Hemudu culture in China 7 000 years ago.

Liang, Miao and colleagues created the mortise part of their structure by using plasma etching to create a hole within a nanosized-layer of hexagonal boron nitride (h-BN). They then constructed a tenon in a top electrode made of tantalum (Ta) that precisely matches the mortise. This ensures that this electrode directly contacts the device’s switching layer (which is made from HfO₂) only in the designated region. A bottom electrode completes the device.

The new architecture ensures highly uniform switching within the designated mortise-and-tenon region, resulting in a localized path for electronic conduction. “The result is a memristor with exceptional fundamental properties across three key metrics,” Miao tells Physics World. “These are: high endurance (over more than 10⁹ cycles); long-term and stable memory retention (of over 10⁴ s), and a fast switching speed of around 4.2 ns.”

The cycle-to-cycle variation of the low-resistance state (LRS) can also be reduced from 30.3% for a traditional memristor to 2.5% for the MTS architecture and the high-resistance state (HRS) from 62.4 to 27.2%.

To test their device, the researchers built a PDE solver with it. They found that their new MTS memristor could solve the Poisson equation five times faster than a conventional memristor based on HfO₂ without h-BN.

The new technique, which is detailed in Science Advances, is a promising strategy for developing high-uniformity memristors, and could pave the way for high-accuracy, energy-efficient scientific computing platforms, Liang claims. “We are now looking to develop large-scale integration of our MTS device and make a prototype system,” he says.

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A new contact lens enables humans to see near-infrared light without night vision goggles or other bulky equipment. The lens, which incorporates metallic nanoparticles that “upconvert” normally-invisible wavelengths into visible ones, could have applications for rescue workers and others who would benefit from enhanced vision in conditions with poor visibility.

The infrared (IR) part of the electromagnetic spectrum encompasses light with wavelengths between 700 nm and 1 mm. Human eyes cannot normally detect these wavelengths because opsins, the light-sensitive protein molecules that allow us to see, do not have the required thermodynamic properties. This means we see only a small fraction of the electromagnetic spectrum, typically between 400‒700 nm.

While devices such as night vision goggles and infrared-visible converters can extend this range, they require external power sources. They also cannot distinguish between different wavelengths of IR light.

Photoreceptor-binding nanoparticles

In a previous work, researchers led by neuroscientist Tian Xue of the University of Science and Technology of China (USTC) injected photoreceptor-binding nanoparticles into the retinas of mice. While this technique was effective, it is too invasive and risky for human volunteers. In the new study, therefore, Xue and colleagues integrated the nanoparticles into biocompatible polymeric materials similar to those used in standard soft contact lenses.

The nanoparticles in the lenses are made from Au/NaGdF4: Yb³⁺, Er³⁺ and have a diameter of approximately 45 nm each. They work by capturing photons with lower energies (longer wavelengths) and re-emitting them as photons with higher energies (shorter wavelengths). This process is known as upconversion and the emitted light is said to be anti-Stokes shifted.

When the researchers tested the new upconverting contact lenses (UCLs) on mice, the rodents’ behaviour suggested they could sense IR wavelengths. For example, when given a choice between a dark box and an IR-illuminated one, the lens-wearing mice scurried into the dark box. In contrast, a control group of mice not wearing lenses showed no preference for one box over the other. The pupils of the lens-wearing mice also constricted when exposed to IR light, and brain imaging revealed that processing centres in their visual cortex were activated.

Flickering seen even with eyes closed

The team then moved on to human volunteers. “In humans, the near-infrared UCLs enabled participants to accurately detect flashing Morse code-like signals and perceive the incoming direction of near-infrared (NIR) light,” Xue says, referring to light at wavelengths between 800‒1600 nm. Counterintuitively, the flashing images appeared even clearer when the volunteers closed their eyes – probably because IR light is better than visible light at penetrating biological tissue such as eyelids. Importantly, Xue notes that wearing the lenses did not affect participants’ normal vision.

The team also developed a wearable system with built-in flat UCLs. This system allowed volunteers to distinguish between patterns such as horizontal and vertical lines; S and O shapes; and triangles and squares.

But Xue and colleagues did not stop there. By replacing the upconverting nanoparticles with trichromatic orthogonal ones, they succeeded in converting NIR light into three different spectral bands. For example, they converted infrared wavelengths of 808, 980 nm and 1532 nm into 540, 450, and 650 nm respectively – wavelengths that humans perceive as green, blue and red.

“As well as allowing wearers to garner more detail within the infrared spectrum, this technology could also help colour-blind individuals see wavelengths they would otherwise be unable to detect by appropriately adjusting the absorption spectrum,” Xue tells Physics World.

According to the USTC researchers, who report their work in Cell, the devices could have several other applications. Apart from providing humans with night vision and offering an adaptation for colour blindness, the lenses could also give wearers better vision in foggy or dusty conditions.

At present, the devices only work with relatively bright IR emissions (the study used LEDs). However, the researchers hope to increase the photosensitivity of the nanoparticles so that lower levels of light can trigger the upconversion process.

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

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

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

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

Giant flares

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

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

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

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

Mysterious mechanism

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

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

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

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

Gamma-ray burst

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

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

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

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

Lots of magnetars

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

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

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

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

The research is described in Astrophysical Journal Letters.

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Quantum science is enjoying a renaissance as nascent quantum computers emerge from the lab and quantum sensors are being used for practical applications.

As the technologies we use become more quantum in nature, it follows that everyone should have a basic understanding of quantum physics. To explore how quantum physics can be taught to the masses, I am joined by Arjan Dhawan, Aleks Kissinger and Bob Coecke – who are all based in the UK.

Coecke is chief scientist at Quantinuum – which develops quantum computing hardware and software. Kissinger is associate professor of quantum computing at the University of Oxford; and Dhawan is studying mathematics at the University of Durham.

Kissinger and Coecke have developed a way of teaching quantum physics using diagrams. In 2023, Oxford and Quantinuum joined forces to use the method in a pilot summer programme for 15 to 17 year-olds. Dhawan was one of their students.

• The books mentioned in the podcast are Picturing Quantum Processes: A First Course in Quantum Theory and Diagrammatic Reasoning by Bob Coecke and Aleks Kissinger; and Quantum in Pictures: A New Way to Understand the Quantum World by Bob Coecke and Stefano Gogioso.

 

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Adaptive radiotherapy, an advanced cancer treatment in which each fraction is tailored to the patient’s daily anatomy, offers the potential to maximize target conformality and minimize dose to surrounding healthy tissue. Based on daily scans – such as MR images recorded by an MR-Linac, for example – treatment plans are adjusted each day to account for anatomical changes in the tumour and surrounding healthy tissue.

Creating a new plan for every treatment fraction, however, increase the potential for errors, making fast and effective quality assurance (QA) procedures more important than ever. To meet this need, the physics team at Hospital Almater in Mexicali, Mexico, is using Elekta ONE | QA, powered by ThinkQA Secondary Dose Check^* (ThinkQA SDC) software to ensure that each adaptive plan is safe and accurate before it is delivered to the patient.

Radiotherapy requires a series of QA checks prior to treatment delivery, starting with patient-specific QA, where the dose calculated by the treatment planning system is delivered to a phantom. This procedure ensures that the delivered dose distribution matches the prescribed plan. Alongside, secondary dose checks can be performed, in which an independent algorithm verifies that the calculated dose distribution corresponds with that delivered to the actual patient anatomy.

“The secondary dose check is an independent dose calculation that uses a different algorithm to the one in the treatment planning system,” explains Alexis Cabrera Santiago, a medical physicist at Hospital Almater. “ThinkQA SDC software calculates the dose based on the patient anatomy, which is actually more realistic than using a rigid phantom, so we can compare both results and catch any differences before treatment.”

For adaptive radiotherapy in particular, this second check is invaluable. Performing phantom-based QA following each daily imaging session is often impractical. Instead, in many cases, it’s possible to use ThinkQA SDC instead.

“Secondary dose calculation is necessary in adaptive treatments, for example using the MR-Linac, because you are changing the treatment plan for each session,” says José Alejandro Rojas‑López, who commissioned and validated ThinkQA SDC at Hospital Almater. “You are not able to shift the patient to realise patient-specific QA, so this secondary dose check is needed to analyse each treatment session.”

ThinkQA SDC’s ability to achieve patient-specific QA without shifting the patient is extremely valuable, allowing time savings while upholding the highest level of QA safety. “The AAPM TG 219 report recognises secondary dose verification as a validated alternative to patient-specific QA, especially when there is no time for traditional phantom checks in adaptive fractions,” adds Cabrera Santiago.

The optimal choice

At Hospital Almater, all external-beam radiation treatments are performed using an Elekta Unity MR-Linac (with brachytherapy employed for gynaecological cancers). This enables the hospital to offer adaptive radiotherapy for all cases, including head-and-neck, breast, prostate, rectal and lung cancers.

To ensure efficient workflow and high-quality treatments, the team turned to the ThinkQA SDC software. ThinkQA SDC received FDA 510(k) clearance in early 2024 for use with both the Unity MR-Linac and conventional Elekta linacs.

Rojas‑López (who now works at Hospital Angeles Puebla) says that the team chose ThinkQA SDC because of its user-friendly interface, ease of integration into the clinical workflow and common integrated QA platform for both CT and MR-Linac systems. The software also offers the ability to perform 3D evaluation of the entire planning treatment volume (PTV) and the organs-at-risk, making the gamma evaluation more robust.

Commissioning of ThinkQA SDC was fast and straightforward, Rojas‑López notes, requiring minimal data input into the software. For absolute dose calibration, the only data needed are the cryostat dose attenuation response, the output dose geometry and the CT calibration.

“This makes a difference compared with other commercial solutions where you have to introduce more information, such as MLC [multileaf collimator] leakage and MLC dosimetric leaf gap, for example,” he explains. “If you have to introduce more data for commissioning, this delays the clinical introduction of the software.”

Cabrera Santiago is now using ThinkQA SDC to provide secondary dose calculations for all radiotherapy treatments at Hospital Almater. The team has established a protocol with a 3%/2 mm gamma criterion, a tolerance limit of 95% and an action limit of 90%. He emphasizes that the software has proved robust and flexible, and provides confidence in the delivered treatment.

“ThinkQA SDC lets us work with more confidence, reduces risk and saves time without losing control over the patient’s safety,” he says. “It checks that the plan is correct, catches issues before treatment and helps us find any problems like set-up errors, contouring mistakes and planning issues.”

The software integrates smoothly into the Elekta ONE adaptive workflow, providing reliable results without slowing down the clinical workflow. “In our institution, we set up ThinkQA SDC so that it automatically receives the new plan, runs the check, compares it with the original plan and creates a report – all in around two minutes,” says Cabrera Santiago. “This saves us a lot of time and removes the need to do everything manually.”

A case in point

As an example of ThinkQA SDC’s power to ease the treatment workflow, Rojas‑López describes a paediatric brain tumour case at Hospital Almater. The young patient needed sedation during their treatment, requiring the physics team to optimize the treatment time for the entire adaptive radiotherapy workflow. “ThinkQA SDC served to analyse, in a fast mode, the treatment plan QA for each session. The measurements were reliable, enabling us to deliver all of the treatment sessions without any delay,” he explains.

Indeed, the ability to use secondary dose checks for each treatment fraction provides time advantages for the entire clinical workflow over phantom-based pre-treatment QA. “Time in the bunker is very expensive,” Rojas‑López points out. “If you reduce the time required for QA, you can use the bunker for patient treatments instead and treat more patients during the clinical time. Secondary dose check can optimize the workflow in the entire department.”

Importantly, in a recent study comparing patient-specific QA measurements using Sun Nuclear’s ArcCheck with ThinkQA SDC calculations, Rojas‑López and colleagues confirmed that the two techniques provided comparable results, with very similar gamma passing rates. As such, they are working to reduce phantom measurements and, in most cases, replace them with a secondary dose check using ThinkQA SDC.

The team at Hospital Almater concur that ThinkQA SDC provides a reliable tool to evaluate radiation treatments, including the first fraction and all of the adaptive sessions, says Rojas‑López. “You can use it for all anatomical sites, with reliable and confident results,” he notes. “And you can reduce the need for measurements using another patient-specific QA tool.”

“I think that any centre doing adaptive radiotherapy should seriously consider using a tool like ThinkQA SDC,” adds Cabrera Santiago.

*ThinkQA is manufactured by DOSIsoft S.A. and distributed by Elekta.

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The first high-resolution images of Bolivia’s Uturuncu volcano have yielded unprecedented insights into whether this volcanic “zombie” is likely to erupt in the near future. The images were taken using a technique that combines seismology, rock physics and petrological analyses, and the scientists who developed it say it could apply to other volcanoes, too.

Volcanic eruptions occur when bubbles of gases such as SO₂ and CO₂ rise to the Earth’s surface through dikes and sills in the planet’s crust, bringing hot, molten rock known as magma with them. To evaluate the chances of this happening, researchers need to understand how much gas and melted rock have accumulated in the volcano’s shallow upper crust, or crater. This is not easy, however, as the structures that convey gas and magma to the surface are complex and mapping them is challenging with current technologies.

A zombie volcano

In the new work, a team led by Mike Kendall of the University of Oxford, UK and Haijiang Zhang from the University of Science and Technology of China (USTC) employed a combination of seismological and petrophysical analyses to create such a map for Uturuncu. Located in the Central Andes, this volcano formed in the Pleistocene era (around 2.58 million to 11,700 years ago) as the oceanic Nazca plate was forced beneath the South American continental plate. It is made up of around 50 km³ of homogeneous, porphyritic dacite lava flows that are between 62% and 67% silicon dioxide (SiO₂) by weight, and it sits atop the Altiplano–Puna magma body, which is the world’s largest body of partially-melted silicic rock.

Although Uturuncu has not erupted for nearly 250,000 years, it is not extinct. It regularly emits plumes of gas, and earthquakes are a frequent occurrence in the shallow crust beneath and around it. Previous geodetic studies also detected a 150-km-wide deformed region of rock centred around 3 km south-west of its summit. These signs of activity, coupled with Uturuncu’s lack of a geologically recent eruption, have led some scientists to describe it as a “zombie”.

Movement of liquid and gas explains Uturuncu’s unrest

To tease out the reasons for Uturuncu’s semi-alive behaviour, the team turned to seismic tomography – a technique Kendall compares to medical imaging of a human body. The idea is to detect the seismic waves produced by earthquakes travelling through the Earth’s crust, analyse their arrival times, and use this information to create three-dimensional images of what lies beneath the surface of the structure being studied.

Writing in PNAS, Kendall and colleagues explain that they used seismic tomography to analyse signals from more than 1700 earthquakes in the region around Uturuncu. They performed this analysis in two ways. First, they assumed that seismic waves travel through the crust at the same speed regardless of their direction of propagation. This isotropic form of tomography gave them a first image of the region’s structure. In their second analysis, they took the directional dependence of the seismic waves’ speed into account. This anisotropic tomography gave them complementary information about the structure.

The researchers then combined their tomographic measurements with previous geophysical imaging results to construct rock physics models. These models contain information about the paths that hot migrating fluids and gases take as they migrate to the surface. In Uturuncu’s case, the models showed fluids and gases accumulating in shallow magma reservoirs directly below the volcano’s crater and down to a depth of around 5 km. This movement of liquid and gas explains Uturuncu’s unrest, the team say, but the good news is that it has a low probability of producing eruptions any time soon.

According to Kendall, the team’s methods should be applicable to more than 1400 other potentially active volcanoes around the world. “It could also be applied to identifying potential geothermal energy sites and for critical metal recovery in volcanic fluids,” he tells Physics World.

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

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

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

Why are you fascinated by 2D materials?

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

What kinds of 2D materials do you study?

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

Why do you find transition metal dichalcogenides interesting?

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

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

What can such sources of single photons be used for?

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

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

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

So how do you study the samples?

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

What sort of experiments do you do in your lab?

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

Is it difficult adding defects to a sample?

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

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

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

How do your PicoQuant instruments help in that regard?

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

Did you buy the equipment off-the-shelf?

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

So how good are your samples?

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

What are your next steps?

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

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

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The 2025 Shaw Prize in Astronomy has been awarded to Richard Bond and George Efstathiou “for their pioneering research in cosmology, in particular for their studies of fluctuations in the cosmic microwave background”. The prize citation continues, “Their predictions have been verified by an armada of ground-, balloon- and space-based instruments, leading to precise determinations of the age, geometry, and mass–energy content of the universe”.

Efstathiou is professor of astrophysics at the University of Cambridge in the UK. Bond is a professor at the Canadian Institute for Theoretical Astrophysics (CITA) and university professor at the University of Toronto in Canada. They share the $1.2m prize money equally.

The annual award is given by the Shaw Prize Foundation, which was founded in 2002 by the Hong Kong-based filmmaker, television executive and philanthropist Run Run Shaw (1907–2014). It will be presented at a ceremony in Hong Kong on 21 October. There are also Shaw Prizes for life sciences and medicine; and mathematical sciences.

Bond studied mathematics and physics at Toronto. In 1979 he completed a PhD in theoretical physics at the California Institute of Technology (Caltech). He directed CITA in 1996-2006.

Efstathiou studied physics at Oxford before completing a PhD in astronomy at the UK’s Durham University in 1979. He is currently director of the Institute of Astronomy in Cambridge.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• 2024 Particular Books £25.00hb 196pp

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The 20th of May is World Metrology Day, and this year it was extra special because it was also the 150th anniversary of the treaty that established the metric system as the preferred international measurement standard. Known as the Metre Convention, the treaty was signed in 1875 in Paris, France by representatives of all 17 nations that belonged to the Bureau International des Poids et Mesures (BIPM) at the time, making it one of the first truly international agreements. Though nations might come and go, the hope was that this treaty would endure “for all times and all peoples”.

To celebrate the treaty’s first century and a half, the BIPM and the United Nations Educational, Scientific and Cultural Organisation (UNESCO) held a joint symposium at the UNESCO headquarters in Paris. The event focused on the achievements of BIPM as well as the international scientific collaborations the Metre Convention enabled. It included talks from the Nobel prize-winning physicist William Phillips of the US National Institute of Standards and Technology (NIST) and the BIPM director Martin Milton, as well as panel discussions on the future of metrology featuring representatives of other national metrology institutes (NMIs) and metrology professionals from around the globe.

A long and revolutionary tradition

The history of metrology dates back to ancient times. As UNESCO’s Hu Shaofeng noted in his opening remarks, the Egyptians recognized the importance of precision measurements as long ago as the 21st century BCE.  Like other early schemes, the Egyptians’ system of measurement used parts of the human body as references, with units such as the fathom (the length of a pair of outstretched arms) and the foot. This was far from ideal since, as Phillips pointed out in his keynote address, people come in various shapes and sizes. These variations led to a profusion of units. By some estimates, pre-revolutionary France had a whopping 250,000 different measures, with differences arising not only between towns but also between professions.

The French Revolutionaries were determined to put an end to this mess. In 1795, just six years after the Revolution, the law of 18 Geminal An III (according to the new calendar of the French Republic) created a preliminary version of the world’s first metric system. The new system tied length and mass to natural standards (the metre was originally one-forty-millionth of the Paris meridian, while the kilogram is the mass of a cubic decimetre of water), and it became the standard for all of France in 1799. That same year, the system also became more practical, with units becoming linked, for the first time, to physical artefacts: a platinum metre and kilogram deposited in the French National Archives.

When the Metre Convention adopted this standard internationally 80 years later, it kick-started the construction of new length and mass standards. The new International Prototype of the Metre and International Prototype of the Kilogram were manufactured in 1879 and officially adopted as replacements for the Revolutionaries’ metre and kilogram in 1889, though they continued to be calibrated against the old prototypes held in the National Archives.

A short history of the BIPM

The BIPM itself was originally conceived as a means of reconciling France and Germany after the 1870–1871 Franco–Prussian War. At first, its primary roles were to care for the kilogram and metre prototypes and to calibrate the standards of its member states. In the opening decades of the 20th century, however, it extended its activities to cover other kinds of measurements, including those related to electricity, light and radiation. Then, from the 1960s onwards, it became increasingly interested in improving the definition of length, thanks to new interferometer technology that made it possible to measure distance at a precision rivalling that of the physical metre prototype.

It was around this time that the BIPM decided to replace its expanded metric system with a framework encompassing the entire field of metrology. This new framework consisted of six basic units – the metre, kilogram, second, ampere, degree Kelvin (later simply the kelvin), candela and mole – plus a set of “derived” units (the Newton, Hertz, Joule and Watt) built from the six basic ones. Thus was born the International System of Units, or SI after the French initials for Système International d’unités.

The next major step – a “brilliant choice”, in Phillips’ words – came in 1983, when the BIPM decided to redefine the metre in terms of the speed of light. In the future, the Bureau decreed that the metre would officially be the length travelled by light in vacuum during a time interval of 1/299,792,458 seconds.

This decision set the stage for defining the rest of the seven base units in terms of natural fundamental constants. The most recent unit to join the club was the kilogram, which was defined in terms of the Planck constant, h, in 2019. In fact, the only base unit currently not defined in terms of a fundamental constant is the second, which is instead determined by the transition between the two hyperfine levels of the ground state of caesium-133. The international metrology community is, however, working to remedy this, with meetings being held on the subject in Versailles this month.

Measurement affects every aspect of our daily lives, and as the speakers at last week’s celebrations repeatedly reminded the audience, a unified system of measurement has long acted as a means of building trust across international and disciplinary borders. The Metre Convention’s survival for 150 years is proof that peaceful collaboration can triumph, and it has allowed humankind to advance in ways that would not have been possible without such unity. A lesson indeed for today’s troubled world.

The post The evolution of the metre: How a product of the French Revolution became a mainstay of worldwide scientific collaboration appeared first on Physics World.

What do pulsars, nuclear politics and hypothetical love particles have in common? They’ve all inspired songs by Lynda Williams – physicist, performer and self-styled “Physics Chanteuse”.

In this month’s Physics World Stories podcast, host Andrew Glester is in conversation with Williams, whose unique approach to science communication blends physics with cabaret and satire. You’ll be treated to a selection of her songs, including a toe-tapping tribute to Jocelyn Bell Burnell, the Northern Irish physicist who discovered pulsars.

Williams discusses her writing process, which includes a full-blooded commitment to getting the science right. She describes how her shows evolve throughout the course of a tour, how she balances life on the road with other life commitments, and how Kip Thorne once arranged for her to perform at a birthday celebration for Stephen Hawking. (Yes, really.)

Her latest show, Atomic Cabaret, dives into the existential risks of the nuclear age, marking 80 years since Hiroshima and Nagasaki. The one-woman musical kicks off in Belfast on 18 June and heads to the Edinburgh Festival in August.

The post The Physics Chanteuse: when science hits a high note appeared first on Physics World.

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

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

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

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

A double twist on the double slit

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

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

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

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

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

Rewriting the past – or not?

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

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

Eraser vs Bell: a battle of the bizarre

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

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

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

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

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

The eraser as a brainteaser

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

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

Information, observers and quantum computers

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

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

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

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

The post The quantum eraser doesn’t rewrite the past – it rewrites observers appeared first on Physics World.

Bismuth has puzzled scientists for nearly 20 years. Notably, the question of whether it is topological – that is, whether electrons behave differently on its surface than they do inside it – gets different answers depending on whether you ask a theorist or an experimentalist. Researchers in Japan now say they have found a way to resolve this conflict. A mechanism called surface relaxation, they report, may have masked or “blocked” bismuth’s true topological nature.

The classic way of describing topology is to compare objects that have a hole, such as a doughnut or a coffee mug, with objects that don’t, such as a muffin. Although we usually think of doughnuts as having more in common with muffins than with mugs – you can’t eat a mug – the fact that they have the same number of holes means the mug and doughnut share topological features that the muffin does not.

While no-one has ever wondered whether they can eat an electron, scientists have long been curious about whether materials conduct electricity. As it turns out, topology is one way of answering that question.

“Previously, people classified materials as metallic or insulating,” says Yuki Fuseya, a quantum solid state physicist at Kobe University. Beginning in the 2000s, however, Fuseya says scientists started focusing more on the topology of the electrons’ complex wavefunctions. This enriched our understanding of how materials behave, because wavefunctions with apparently different shapes can share important topological features.

For example, if the topology of certain wavefunctions on a material’s surface corresponds to that of apparently different wavefunctions within its bulk, the material may be insulating in its bulk, yet still able to conduct electricity on its surface. Materials with this property are known as topological insulators, and they have garnered a huge amount of interest due to the possibility of exploiting them in quantum computing, spintronics and magnetic devices.

Topological or not topological

While it’s not possible to measure the topology of wavefunctions directly, it is generally possible to detect whether a material supports certain surface states. This information can then be used to infer something about its bulk using the so-called bulk-edge state correspondence.

In bismuth, the existence of these surface states ought to indicate that the bulk material is topologically trivial. However, experiments have delivered conflicting information.

Fuseya was intrigued. “If you look at the history of solid-state physics, many physical phenomena were found firstly in bismuth,” he tells Physics World. Examples include diamagnetism, the Seebeck effect and the Shubnikov-de Haas effect, as well as phenomena related to the giant spin Hall effect and the potential for Turing patterns that Fuseya discovered himself. “That’s one of the reasons why I am so interested in bismuth,” he says.

Fuseya’s interest attracted colleagues with different specialisms. Using density functional theory, Rikako Yaguchi of the University of Electro-Communications in Tokyo calculated that layers of bismuth’s crystal lattice expand, or relax, by 3-6% towards the surface. According to Fuseya, this might not have seemed noteworthy. However, since the team was already looking at bismuth’s topological properties, another colleague, Kazuki Koie, went ahead and calculated how this lattice expansion changed the material’s surface wavefunction.

These calculations showed that the expansion is, in fact, significant. This is because bismuth is close to the topological transition point, where a change in parameters can flip the shape of the wavefunction and give topological properties to a material that was once topologically trivial. Consequently, the reason it is not possible to observe surface states indicating that bulk bismuth is topologically trivial is that the material is effectively different – and topologically non-trivial – on its surface.

Topological blocking

Although “very surprised” at first, Fuseya says that after examining the physics in more detail, they found the result “quite reasonable”. They are now looking for evidence of similar “topological blocking” in other materials near the transition point, such as lead telluride and tin telluride.

“It is remarkable that there are still big puzzles when trying to match data to the theoretical predictions,” says Titus Mangham Neupert, a theoretical physicist at the University of Zurich, Switzerland, who was not directly involved in the research. Since “so many compounds that made the headlines in topological physics” contain bismuth, Neupert says it will be interesting to re-evaluate existing experiments and conceive new ones. “In particular, the implication for higher-order topology could be tested,” he says.

Fuseya’s team is already studying how lattice relaxation might affect hinges where two surfaces come together. In doing so, they hope to understand why angle resolved photoemission spectroscopy (ARPES), which probes surfaces, yields results that contradict those from scanning tunnelling microscopy experiments, which probe hinges. “Maybe we can find a way to explain every experiment consistently,” Fuseya says. The insights they gain, he adds, might also be useful for topological engineering: by bending a material, scientists could alter its lattice constants, and thereby tailor its topological properties.

This aspect also interests Zeila Zanolli and Matthieu Verstraete of Utrecht University in the Netherlands. Though not involved in the current study, they had previously shown that free-standing two-dimensional bismuth (bismuthene) can take on several geometric structures in-plane – not all of which are topological – depending on the material’s strain, bonding coordination and directionality. The new work, they say, “opens the way to (computational) design of topological materials, playing with symmetries, strain and the substrate interface”.

The research is published in Physical Review B.

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

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

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

Progression towards dynamic proton arc therapy

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

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

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

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

The first clinical treatment

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

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

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

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

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

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

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

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

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

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

New detection paradigm

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

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

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

Tested at Fermilab

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

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

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

The research is described in the Journal of Instrumentation.

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

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

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

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

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

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

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

About this journal

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

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

 

The post What is meant by neuromorphic computing – a webinar debate appeared first on Physics World.

In this episode of the Physics World Weekly podcast I look at what’s new in the world of physics with the help of my colleagues Margaret Harris and Matin Durrani.

We begin on Mars, where NASA’s Perseverance Rover has made the first observation of an aurora from the surface of the Red Planet. Next, we look deep into the future of the universe and ponder the physics that will govern how the last stars will fade away.

Then, we run time in reverse and go back to the German island of Helgoland, where in 1925 Werner Heisenberg laid the foundations of modern quantum mechanics. The island will soon host an event celebrating the centenary and Physics World will be there.

Finally, we explore how neutrons are being used to differentiate between real and fake antique coins and chat about the Physics World Quantum Briefing 2025.

The post A Martian aurora, how the universe fades away, Heisenberg on holiday, physics of fake coins appeared first on Physics World.

When implanted medical devices like urinary stents and catheters get clogged with biofilms, the usual solution is to take them out and replace them with new ones. Now, however, researchers at the University of Bern and ETH Zurich, Switzerland have developed an alternative. By incorporating ultrasound-activated moving structures into their prototype “stent-on-a-chip” device, they showed it is possible to remove biofilms without removing the device itself. If translated into clinical practice, the technology could increase the safe lifespan of implants, saving money and avoiding operations that are uncomfortable and sometimes hazardous for patients.

Biofilms are communities of bacterial cells that adhere to natural surfaces in the body as well as artificial structures such as catheters, stents and other implants. Because they are encapsulated by a protective, self-produced extracellular matrix made from polymeric substances, they are mechanically robust and resistant to standard antibacterial measures. If not removed, they can cause infections, obstructions and other complications.

Intense, steady flows push away impurities

The new technology, which was co-developed by Cornel Dillinger, Pedro Amado and other members of Francesco Clavica and Daniel Ahmed’s research teams, takes advantage of recent advances in the fields of robotics and microfluidics. Its main feature is a coating made from microscopic hair-like structures known as cilia. Under the influence of an acoustic field, which is applied externally via a piezoelectric transducer, these cilia begin to move. This movement produces intense, steady fluid flows with velocities of up to 10 mm/s – enough to break apart encrusted deposits (made from calcium carbonate, for example) and flush away biofilms from the inner and outer surfaces of implanted urological devices.

“This is a major advance compared to existing stents and catheters, which require regular replacements to avoid obstruction and infections,” Clavica says.

The technology is also an improvement on previous efforts to clear implants by mechanical means, Ahmed adds. “Our polymeric cilia in fact amplify the effects of ultrasound by allowing for an effect known as acoustic streaming at frequencies of 20 to 100 kHz,” he explains. “This frequency is lower than that possible with previous microresonator devices developed to work in a similar way that had to operate in the MHz-frequency range.”

The lower frequency achieves the desired therapeutic effects while prioritizing patient safety and minimizing the risk of tissue damage, he adds.

Wider applications

In creating their technology, the researchers were inspired by biological cilia, which are a natural feature of physiological systems such as the reproductive and respiratory tracts and the central nervous system. Future versions, they say, could apply the ultrasound probe directly to a patient’s skin, much as handheld probes of ultrasound scanners are currently used for imaging. “This technology has potential applications beyond urology, including fields like visceral surgery and veterinary medicine, where keeping implanted medical devices clean is also essential,” Clavica says.

The researchers now plan to test new coatings that would reduce contact reactions (such as inflammation) in the body. They will also explore ways of improving the device’s responsiveness to ultrasound – for example by depositing thin metal layers. “These modifications could not only improve acoustic streaming performance but could also provide additional antibacterial benefits,” Clavica tells Physics World.

In the longer term, the team hope to translate their technology into clinical applications. Initial tests that used a custom-built ultrasonic probe coupled to artificial tissue have already demonstrated promising results in generating cilia-induced acoustic streaming, Clavica notes. “In vivo animal studies will then be critical to validate safety and efficacy prior to clinical adoption,” he says.

The present study is detailed in PNAS.

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Cyril Hilsum, a former president of the Institute of Physics (IOP), celebrated his 100th birthday last week at a special event held at the Royal Society of Chemistry.

Born on 17 May 1925, Hilsum completed a degree in physics at University College London in 1945. During his career he worked at the Services Electronics Research Laboratory and the Royal Radar Establishment and in 1983 was appointed chief scientist of GEC Hirst Research Centre, where he later became research director before retiring aged 70.

Hilsum helped develop commercial applications for the semiconductor gallium arsenide and is responsible for creating the UK’s first semiconductor laser as well as developments that led to modern liquid crystal display technologies.

Between 1988 and 1990 he was president of the IOP, which publishes Physics World, and in 1990 was appointed a Commander of the Order of the British Empire (CBE) for “services to the electrical and electronics industry”.

Hilsum was honoured by many prizes during his career including IOP awards such as the Max Born Prize in 1987, the Faraday Medal in 1988 as well as the Richard Glazebrook Medal and Prize in 1998. In 2007 he was awarded the Royal Society’s Royal Medal “for his many outstanding contributions and for continuing to use his prodigious talents on behalf of industry, government and academe to this day”.

Despite now being a centenarian, Hilsum still works part-time as chief science officer for Infi-tex Ltd, which produces force sensors for use in textiles.

“My birthday event was an amazing opportunity for me to greet old colleagues and friends,” Hilsum told Physics World. “Many had not seen each other since they had worked together in the distant past. It gave me a rare opportunity to acknowledge the immense contributions they had made to my career.”

Hilsum says that while the IOP gives much support to applied physics, there is still a great need for physicists “to give critical contributions to the lives of society as a whole”.

“As scientists, we may welcome progress in the subject, but all can get pleasure in seeing the results in their home, on their iPhone, or especially in their hospital!” he adds.

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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.

The post Bacteria-killing paint could dramatically improve hospital hygiene appeared first on Physics World.

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.

The post Why I stopped submitting my work to for-profit publishers appeared first on Physics World.

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.”

The post Visual assistance system helps blind people navigate appeared first on Physics World.

The universe’s maximum lifespan may be considerably shorter than was previously thought, but don’t worry: there’s still plenty of time to finish streaming your favourite TV series.

According to new calculations by black hole expert Heino Falcke, quantum physicist Michael Wondrak, and mathematician Walter van Suijlekom of Radboud University in the Netherlands, the most persistent stellar objects in the universe – white dwarf stars – will decay away to nothingness in around 10⁷⁸ years. This, Falcke admits, is “a very long time”, but it’s a far cry from previous predictions, which suggested that white dwarfs could persist for at least 10¹¹⁰⁰ years. “The ultimate end of the universe comes much sooner than expected,” he says.

Writing in the Journal of Cosmology and Astroparticle Physics, Falcke and colleagues explain that the discrepancy stems from different assumptions about how white dwarfs decay. Previous calculations of their lifetime assumed that, in the absence of proton decay (which has never been observed experimentally), their main decay process would be something called pyconuclear fusion. This form of fusion occurs when nuclei in a crystalline lattice essentially vibrate their way into becoming fused with their nearest neighbours.

If that sounds a little unlikely, that’s because it is. However, in the dense, cold cores of white dwarf stars, and over stupendously long time periods, pyconuclear fusion happens often enough to gradually (very, very gradually) turn the white dwarf’s carbon into nickel, which then transmutes into iron by emitting a positron. The resulting iron-cored stars are known as black dwarfs, and some theories predict that they will eventually (very, very eventually) collapse into black holes. Depending on how massive they were to start with, the whole process takes between 10¹¹⁰⁰‒10^{32 000} years.

An alternative mechanism

Those estimates, however, do not take into account an alternative decay mechanism known as Hawking radiation. First proposed in the early 1970s by Stephen Hawking and Jacob Bekenstein, Hawking radiation arises from fluctuations in the vacuum of spacetime. These fluctuations allow particle-antiparticle pairs to pop into existence by essentially “borrowing” energy from the vacuum for brief periods before the pairs recombine and annihilate.

If this pair production happens in the vicinity of a black hole, one particle in the pair may stray over the black hole’s event horizon before it can recombine. This leaves its partner free to carry away some of the “borrowed” energy as Hawking radiation. After an exceptionally long time – but, crucially, not as long as the time required to disappear a white dwarf via pyconuclear fusion – Hawking radiation will therefore cause black holes to dissipate.

The fate of life, the universe and everything?

But what about objects other than black holes? Well, in a previous work published in 2023, Falcke, Wondrak and van Suijlekom showed that a similar process can occur for any object that curves spacetime with its gravitational field, not just objects that have an event horizon. This means that white dwarfs, neutron stars, the Moon and even human beings can, in principle, evaporate away into nothingness via Hawking radiation – assuming that what the trio delicately call “other astrophysical evolution and decay channels” don’t get there first.

Based on this tongue-in-cheek assumption, the trio calculated that white dwarfs will dissipate in around 10⁷⁸ years, while denser objects such as black holes and neutron stars will vanish in no more than 10⁶⁷ years. Less dense objects such as humans, meanwhile, could persist for as long as 10⁹⁰ years – albeit only in a vast, near-featureless spacetime devoid of anything that would make life worth living, or indeed possible.

While that might sound unrealistic as well as morbid, the trio’s calculations do have a somewhat practical goal. “By asking these kinds of questions and looking at extreme cases, we want to better understand the theory,” van Suijlekom says. “Perhaps one day, we [will] unravel the mystery of Hawking radiation.”

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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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Proton therapy is a highly effective and conformal cancer treatment. Proton beams deposit most of their energy at a specific depth – the Bragg peak – and then stop, enabling proton treatments to destroy tumour cells while sparing surrounding normal tissue. To further optimize the clinical treatment planning process, there’s recently been increased interest in considering the radiation quality, quantified by the proton linear energy transfer (LET).

LET – defined as the mean energy deposited by a charged particle over a given distance – increases towards the end of the proton range. Incorporating LET as an optimization parameter could better exploit the radiobiological properties of protons, by reducing LET in healthy tissue, while maintaining or increasing it within the target volume. This approach, however, requires a method for experimental verification of proton LET distributions and patient-specific quality assurance in terms of proton LET.

To meet this need, researchers at the Institute of Nuclear Physics, Polish Academy of Sciences have used the miniaturized semiconductor pixel detector Timepix3 to perform LET characterization of intensity-modulated proton therapy (IMPT) plans in homogeneous and heterogeneous phantoms. They report their findings in Physics in Medicine & Biology.

Experimental validation

First author Paulina Stasica-Dudek and colleagues performed a series of experiments in a gantry treatment room at the Cyclotron Centre Bronowice (CCB), a proton therapy facility equipped with a proton cyclotron accelerator and pencil-beam scanning system that provides IMPT for up to 50 cancer patients per day.

The MiniPIX Timepix3 is a radiation imaging pixel detector based on the Timepix3 chip developed at CERN within the Medipix collaboration (provided commercially by Advacam). It provides quasi-continuous single particle tracking, allowing particle type recognition and spectral information in a wide range of radiation environments.

For this study, the team used a Timepix3 detector with a 300 µm-thick silicon sensor operated as a miniaturized online radiation camera. To overcome the problem of detector saturation in the relatively high clinical beam currents, the team developed a pencil-beam scanning method with the beam current reduced to the picoampere (pA) level.

The researchers used Timepix3 to measure the deposited energy and LET spectra for spread-out Bragg peak (SOBP) and IMPT plans delivered to a homogeneous water-equivalent slab phantom, with each plan energy layer irradiated and measured separately. They also performed measurements on an IMPT plan delivered to a heterogeneous head phantom. For each scenario, they used a Monte Carlo (MC) code to simulate the corresponding spectra of deposited energy and LET for comparison.

The team first performed a series of experiments using a homogeneous phantom irradiated with various fields, mimicking patient-specific quality assurance procedures. The measured and simulated dose-averaged LET (LET_d) and LET spectra agreed to within a few percent, demonstrating proper calibration of the measurement methodology.

The researchers also performed an end-to-end test in a heterogeneous CIRS head phantom, delivering a single field of an IMPT plan to a central 4 cm-diameter target volume in 13 energy layers (96.57–140.31 MeV) and 315 spots.

For head phantom measurements, the peak positions for deposited energy and LET spectra obtained based on experiment and simulation agreed within the error bars, with LET_d values of about 1.47 and 1.46 keV/µm, respectively. The mean LET_d values derived from MC simulation and measurement differed on average by 5.1% for individual energy layers.

Clinical translation

The researchers report that implementing the proposed LET measurement scheme using Timepix3 in a clinical setting requires irradiating IMPT plans with a reduced beam current (at the pA level). While they successfully conducted LET measurements at low beam currents in the accelerator’s research mode, pencil-beam scanning at pA-level currents is not currently available in the commercial clinical or quality assurance modes. Therefore, they note that translating the proposed approach into clinical practice would require vendors to upgrade the beam delivery system to enable beam monitoring at low beam currents.

“The presented results demonstrate the feasibility of the Timepix3 detector to validate LET computations in IMPT fields and perform patient-specific quality assurance in terms of LET. This will support the implementation of LET in treatment planning, which will ultimately increase the effectiveness of the treatment,” Stasica-Dudek and colleagues write. “Given the compact design and commercial availability of the Timepix3 detector, it holds promise for broad application across proton therapy centres.”

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Physicists at CERN have completed a “test run” for taking antimatter out of the laboratory and transporting it across the site of the European particle-physics facility. Although the test was carried out with ordinary protons, the team that performed it says that antiprotons could soon get the same treatment. The goal, they add, is to study antimatter in places other than the labs that create it, as this would enable more precise measurements of the differences between matter and antimatter. It could even help solve one of the biggest mysteries in physics: why does our universe appear to be made up almost entirely of matter, with only tiny amounts of antimatter?

According to the Standard Model of particle physics, each of the matter particles we see around us – from baryons like protons to leptons such as electrons – should have a corresponding antiparticle that is identical in every way apart from its charge and magnetic properties (which are reversed). This might sound straightforward, but it leads to a peculiar prediction. Under the Standard Model, the Big Bang that formed our universe nearly 14 billion years ago should have generated equal amounts of antimatter and matter. But if that were the case, there shouldn’t be any matter left, because whenever pairs of antimatter and matter particles collide, they annihilate each other in a burst of energy.

Physicists therefore suspect that there are other, more subtle differences between matter particles and their antimatter counterparts – differences that could explain why the former prevailed while the latter all but disappeared. By searching for these differences, they hope to shed more light on antimatter-matter asymmetry – and perhaps even reveal physics beyond the Standard Model.

Extremely precise measurements

At CERN’s Baryon-Antibaryon Symmetry Experiment (BASE) experiment, the search for matter-antimatter differences focuses on measuring the magnetic moment (or charge-to-mass ratio) of protons and antiprotons. These measurements need to be extremely precise, but this is difficult at CERN’s “Antimatter Factory” (AMF), which manufactures the necessary low-energy antiprotons in profusion. This is because essential nearby equipment – including the Antiproton Decelerator and ELENA, which reduce the energy of incoming antiprotons from GeV to MeV – produces magnetic field fluctuations that blur the signal.

To carry out more precise measurements, the team therefore needs a way of transporting the antiprotons to other, better-shielded, laboratories. This is easier said than done, because antimatter needs to be carefully isolated from its environment to prevent it from annihilating with the walls of its container or with ambient gas molecules.

The BASE team’s solution was to develop a device that can transport trapped antiprotons on a truck for substantial distances. It is this device, known as BASE-STEP (for Symmetry Tests in Experiments with Portable Antiprotons), that has now been field-tested for the first time.

Protons on the go

During the test, the team successfully transported a cloud of about 105 trapped protons out of the AMF and across CERN’s Meyrin campus over a period of four hours. Although protons are not the same as antiprotons, BASE-STEP team leader Christian Smorra says they are just as sensitive to disturbances in their environment caused by, say, driving them around. “They are therefore ideal stand-ins for initial tests, because if we can transport protons, we should also be able to transport antiprotons,” he says.

The BASE-STEP device is mounted on an aluminium frame and measures 1.95 m x 0.85 m x 1.65 m. At 850‒900 kg, it is light enough to be transported using standard forklifts and cranes.

Like BASE, it traps particles in a Penning trap composed of gold-plated cylindrical electrode stacks made from oxygen-free copper. To further confine the protons and prevent them from colliding with the trap’s walls, this trap is surrounded by a superconducting magnet bore operated at cryogenic temperatures. The second electrode stack is also kept at ultralow pressures of 10^-19 bar, which Smorra says is low enough to keep antiparticles from annihilating with residual gas molecules. To transport antiprotons instead of protons, Smorra adds, they would just need to switch the polarity of the electrodes.

The transportable trap system, which is detailed in Nature, is designed to remain operational on the road. It uses a carbon-steel vacuum chamber to shield the particles from stray magnetic fields, and its frame can handle accelerations of up to 1g (9.81 m/s²) in all directions over and above the usual (vertical) force of gravity. This means it can travel up and down slopes with a gradient of up to 10%, or approximately 6°.

Once the BASE-STEP device is re-configured to transport antiprotons, the first destination on the team’s list is a new Penning-trap system currently being constructed at the Heinrich Heine University in Düsseldorf, Germany. Here, physicists hope to search for charge-parity-time (CPT) violations in protons and antiprotons with a precision at least 100 times higher than is possible at CERN’s AMF.

“At BASE, we are currently performing measurements with a precision of 16 parts in a trillion,” explains BASE spokesperson Stefan Ulmer, an experimental physicist at Heinrich Heine and a researcher at CERN and Japan’s RIKEN laboratory. “These experiments are the most precise tests of matter/antimatter symmetry in the baryon sector to date, but to make these experiments better, we have no choice but to transport the particles out of CERN’s antimatter factory,” he tells Physics World.

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Many creative industries rely on cutting-edge digital technologies, so it is not surprising that this sector could easily become an early adopter of quantum computing.

In this episode of the Physics World Weekly podcast I am in conversation with James Wootton, who is chief scientific officer at Moth Quantum. Based in the UK and Switzerland, the company is developing quantum-software tools for the creative industries – focusing on artists, musicians and game developers.

Wootton joined Moth Quantum in September 2024 after working on quantum error correction at IBM. He also has long-standing interest in quantum gaming and creating tools that make quantum computing more accessible. If you enjoyed this interview with Wootton, check out this article that he wrote for Physics World in 2018: “Playing games with quantum computers“.

 

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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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The Mars rover Perseverance has captured the first image of an aurora as seen from the surface of another planet. The visible-light image, which was taken during a solar storm on 18 March 2024, is not as detailed or as colourful as the high-resolution photos of green swirls, blue shadows and pink whorls familiar to aurora aficionados on Earth. Nevertheless, it shows the Martian sky with a distinctly greenish tinge, and the scientists who obtained it say that similar aurorae would likely be visible to future human explorers.

“Kind of like with aurora here on Earth, we need a good solar storm to induce a bright green colour, otherwise our eyes mostly pick up on a faint grey-ish light,” explains Elise Wright Knutsen, a postdoctoral researcher in the Centre for Space Sensors and Systems at the University of Oslo, Norway. The storm Knutsen and her colleagues captured was, she adds, “rather moderate”, and the aurora it produced was probably too faint to see with the naked eye. “But with a camera, or if the event had been more intense, the aurora will appear as a soft green glow covering more or less the whole sky.”

The role of planetary magnetic fields

Aurorae happen when charged particles from the Sun – the solar wind – interact with the magnetic field around a planet. On Earth, this magnetic field is the product of an internal, planetary-scale magnetic dynamo. Mars, however, lost its dynamo (and, with it, its oceans and its thick protective atmosphere) around four billion years ago, so its magnetic field is much weaker. Nevertheless, it retains some residual magnetization in its southern highlands, and its conductive ionosphere affects the shape of the nearby interplanetary magnetic field. Together, these two phenomena give Mars a hybrid magnetosphere too feeble to protect its surface from cosmic rays, but strong enough to generate an aurora.

Scientists had previously identified various types of aurorae on Mars (and every other planet with an atmosphere in our solar system) in data from orbiting spacecraft. However, no Mars rover had ever observed an aurora before, and all the orbital aurora observations, from Mars and elsewhere, were at ultraviolet wavelengths.

How to spot an aurora on Mars

According to Knutsen, the lack of visible-light, surface-based aurora observations has several causes. First, the visible-wavelength instruments on Mars rovers are generally designed to observe the planet’s bright “dayside”, not to detect faint emissions on its nightside. Second, rover missions focus primarily on geology, not astronomy. Finally, aurorae are fleeting, and there is too much demand for Perseverance’s instruments to leave them pointing at the sky just in case something interesting happens up there.

“We’ve spent a significant amount of time and effort improving our aurora forecasting abilities,” Knutsen says.

Getting the timing of observations right was the most challenging part, she adds. The clock started whenever solar satellites detected events called coronal mass ejections (CMEs) that create unusually strong pulses of solar wind. Next, researchers at the NASA Community Coordinated Modeling Center simulated how these pulses would propagate through the solar system. Once they posted the simulation results online, Knutsen and her colleagues – an international consortium of scientists in Belgium, France, Germany, the Netherlands, Spain, the UK and the US as well as Norway – had a decision to make. Was this CME likely to trigger an aurora bright enough for Perseverance to detect?

If the answer was “yes”, their next step was to request observation time on Perseverance’s SuperCam and Mastcam-Z instruments. Then they had to wait, knowing that although CMEs typically take three days to reach Mars, the simulations are only accurate to within a few hours and the forecast could change at any moment. Even if they got the timing right, the CME might be too weak to trigger an aurora.

“We have to pick the exact time to observe, the whole observation only lasts a few minutes, and we only get one chance to get it right per solar storm,” Knutsen says. “It took three unsuccessful attempts before we got everything right, but when we did, it appeared exactly as we had imagined it: as a diffuse green haze, uniform in all directions.”

Future observations

Writing in Science Advances, Knutsen and colleagues say it should now be possible to investigate how Martian aurorae vary in time and space – information which, they note, is “not easily obtained from orbit with current instrumentation”. They also point out that the visible-light instruments they used tend to be simpler and cheaper than UV ones.

“This discovery will open up new avenues for studying processes of particle transport and magnetosphere dynamics,” Knutsen tells Physics World. “So far we have only reported our very first detection of this green emission, but observations of aurora can tell us a lot about how the Sun’s particles are interacting with Mars’s magnetosphere and upper atmosphere.”

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Late on Friday 18 April, the provost of Stony Brook University, where I teach, received a standard letter from the National Science Foundation (NSF), the body that funds much academic research in the US. “Termination of certain awards is necessary,” the e-mail ran, “because they are not in alignment with current NSF priorities”. The e-mail mentioned “NSF Award Id 2318247”. Mine.

The termination notice, forwarded to me a few minutes later, was the same one that 400 other researchers all over the US received the same day, in which the agency, following a directive from the Trump administration, grabbed back $233m in grant money. According to the NSF website, projects terminated were “including but not limited to those on diversity, equity, and inclusion (DEI) and misinformation/disinformation”.

Losing grant money is disastrous for research and for the faculty, postdocs, graduate students and support staff who depend on that support. A friend of mine tried to console me by saying that I had earned a badge of honour for being among the 400 people who threatened the Trump Administration so much that it set out to stop their work. Still, I was baffled. Did I really deserve the axe?

My award, entitled “Social and political dynamics of lab-community relations”, was small potatoes. As the sole principal investigator, I’d hired no postdocs or grad students. I’d also finished most of the research and been given a “no-cost extension” to write it up that was due to expire in a few months. In fact, I’d spent all but $21,432 of the $263,266 of cash.

That may sound like a lot for a humanities researcher, but it barely covered a year of my salary and included indirect costs (to which my grant was subject like any other), along with travel and so on. What’s more, my project’s stated aim was to “enhance the effectiveness of national scientific facilities”, which was clearly within the NSF’s mission.

Such facilities, I had pointed out in my official proposal, are vital if the US is to fulfil its national scientific, technological, medical and educational goals. But friction between a facility and the surrounding community can hamper its work, particularly if the lab’s research is seen as threatening – for example, involving chemical, radiological or biological hazards. Some labs, in fact, have had important, yet perfectly safe, facilities permanently closed out of such fear.

“In an age of Big Science,” I argued, “understanding the dynamics of lab-community interaction is crucial to advancing national, scientific, and public interests.” What’s so contentious about that?

“New bad words”

Maybe I had been careless. After all, Ted Cruz, who chairs the Senate’s commerce committee, had claimed in February that 3400 NSF awards worth over $2 billion made during the Biden–Harris administration had promoted DEI and advanced “neo-Marxist class warfare propaganda”. I wondered if I might have inadvertently used some trigger word that outed me as an enemy of the state.

I knew, for instance, that the Trump Administration had marked for deletion photos of the Enola Gay aircraft, which had dropped an atomic bomb on Hiroshima, in a Defense Department database because officials had not realized that “Gay” was part of the name of the pilot’s mother. Administration officials had made similar misinterpretations in scientific proposals that included the words “biodiversity” and “transgenic”.

Had I used one of those “new bad words”? I ran a search on my proposal. Did it mention “equity”? No. “Inclusion”? Also no. The word “diversity” appeared only once, in the subtitle of an article in the bibliography about radiation fallout. “Neo-Marxist”? Again, no. Sure, I’d read Marx’s original texts during my graduate training in philosophy, but my NSF documents hadn’t tapped him or his followers as essential to my project.

Then I remembered a sentence in my proposal. “Well-established scientific findings,” I wrote, “have been rejected by activists and politicians, distorted by lurid headlines, and fuelled partisan agendas.” These lead in turn to “conspiracy theories, fake facts, science denial and charges of corruption”.

Was that it, I wondered? Had the NSF officials thought that I had meant to refer to the administration’s attacks on climate change science, vaccines, green energy and other issues? If so, that was outrageous! There was not a shred of truth to it – no truth at all!

Ructions and retractions

On 23 April – five days after the NSF termination notice – two researchers at Harvard University put together an online “Terminated NSF grant tracker”, which contained information based on what they found in the NSF database. Curious, I scrolled down to SUNY at Stony Brook and found mine: “Social and political dynamics of lab-community relations”.

I was shocked to discover that almost everything about it in the NSF database was wrong, including the abstract

I was shocked to discover that almost everything about it in the NSF database was wrong, including the abstract. The abstract given for my grant was apparently that of another NSF award, for a study that touched on DEI themes – a legitimate and useful thing to study under any normal regime, but not this one. At last, I had the reason for my grant termination: an NSF error.

The next day, 24 April, I managed to speak to the beleaguered NSF programme director, who was kind and understanding and said there’d been a mistake in the database. When I asked her if it could be fixed she said, “I don’t know”. When I asked her if the termination can be reversed, she said, “I don’t know”. I alerted Stony Brook’s grants-management office, which began to press the NSF to reverse its decision. A few hours later I learned that NSF director Sethuraman Panchanathan had resigned.

I briefly wondered if Panchanathan had been fired because my grant had been bungled. No such luck; he was probably disgusted with the administration’s treatment of the agency. But while the mistake over my abstract evidently wasn’t deliberate, the malice behind my grant’s termination certainly was. Further, doesn’t one routinely double-check before taking such an unprecedented and monumental step as terminating a grant by a major scientific agency?

I then felt guilty about my anger; who was I to complain? After all, some US agencies have been shockingly incompetent lately

I then felt guilty about my anger; who was I to complain? After all, some US agencies have been shockingly incompetent lately. A man was mistakenly sent by the Department of Homeland Security to a dangerous prison in El Salvador and they couldn’t (or wouldn’t) get him back. The Department of Health and Human Services has downplayed the value of vaccines, fuelling a measles epidemic in Texas, while defence secretary Pete Hegseth used the Signal messaging app to release classified military secrets regarding a war in progress to a journalist.

How narcissistic of me to become livid only when personally affected by termination of an award that’s almost over anyway.

A few days later, on 28 April, Stony Brook’s provost received another e-mail about my grant from the NSF. Forwarded to me, it said: “the termination notice is retracted; NSF terminated this project in error”. Since then, the online documents at the NSF, and the information about my grant in the tracker, have thankfully been corrected.

The critical point

In a few years’ time, I’ll put together another proposal to study the difference between the way that US government handles science and the needs of its citizens. I’ll certainly have a lot more material to draw on. Meanwhile, I’ll reluctantly wear my badge of honour. For I deserve it – though not, as I initially thought, because I had threatened the Trump Administration enough that they tried to halt my research.

I got it simply because I’m yet another victim of the Trump Administration’s incompetence.

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