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Lia Li: the physicist unlocking navigation technology

When exploring an unfamiliar place, many of us default to a rather modern method of navigation: watching a blue dot move around a map, on a screen, and course-correcting to keep it on the right track. We owe this convenient technology to global navigation satellite systems (GNSSs) – such as the Global Positioning System (GPS), Galileo and BeiDou – which regularly broadcast radio-waves from their orbits around the Earth. Our devices use the time difference between receiving signals from different satellites to calculate our location on the Earth’s surface to an impressive degree of accuracy.

But GNSSs have their pitfalls; as soon as you venture underground or into, say, a forest or a building (like an airport or train station), your blue dot might start jumping around erratically, leaving you a bit lost. This is because obstructions between you and the satellites can weaken the signals or reflect them so that they bounce around before reaching your device, causing a time delay that leads to errors in the calculations.

A form of location tracking that we could count on while hiking through the woods or traversing a labyrinthine underground station would be a welcome improvement. This is a reality that physicist Lia Li (@OptoLia on Twitter) is seeking to create with her start-up Zero Point Motion, a company developing sensors that track your position independently of a GNSS.

As it happens, our devices do already have so-called “inertial sensors” designed to do exactly this. The problem is they are not very reliable. These chips consist of tiny silicon structures that move slightly in response to acceleration or rotation of the device, thereby altering the capacitance of the component, which in turn changes its electrical signal. The device is programmed to register that change as an acceleration and double integrate it over time to calculate the displacement of the object from its starting point. But these capacitive signals are easily overwhelmed by electrical noise in the device’s circuitry, Li explains. “This is why, if you’re on your phone or in your car and you switch off the GPS, your pin might hop to a random nearby street and you think ‘I’m definitely not there’.”

car-navigation system — **Following the dot** When underground, in a forest or inside a building, global navigation satellite systems struggle to pinpoint your exact location because the radio waves bounce off the surrounding obstructions. (Courtesy: iStock/svetikd)

Beyond mere convenience, Li points out that better GNSS-independent motion tracking is essential for safety in some applications. Autonomous vehicles, for example, will need extremely reliable and live information about their trajectory relative to their surroundings to ensure that they don’t swerve or change speed in a dangerous way.

As it happens, Li’s scientific background has set her up well to tackle this problem. She was immersed in an academic environment as a child, with both her parents doing PhDs in mechanical engineering at the University of Bristol, UK. The family lived in student halls for several years and Li often spent time in the university labs, surrounded by huge pieces of equipment testing the strength of metals. She knew from a young age that she liked the freedom that research allows for, as well as being at the cutting edge of a field and having access to state-of-the-art equipment.

Best of both worlds

Li went on to study physics at Imperial College London, but it wasn’t until her final-year project, which involved building a laser from scratch, that she discovered her love of photonics. She enjoyed the work so much that she did a summer placement with the same research group following her final exams and even considered doing a PhD with them. However, Li already had a job lined up at BAE Systems’ photonics department for after graduation. Although she questioned whether this was the right decision for her, she decided to give industry a go after her father pointed out that she’d only known the education system so far. It proved a very valuable experience. “I learned so much from these fantastic scientists who had worked there for 20+ years,” she says. “It was a different way of gaining knowledge, and I loved the focus on application and needing something tangible to come out of it.”

After a while, BAE Systems was keen to promote Li to project management, but she wanted to stay in the labs doing hands-on work. So in 2012, after a year and a half in the job, she left to do a PhD in quantum physics at University College London (UCL).

That time in industry has continued to inform her work, however, as it gave her a glimpse into the huge potential of inertial sensors. Many people have the impression that these kinds of sensors are not worth spending time on, largely because of their low quality in everyday devices like phones. But during her spell in the defence industry, Li became aware of quarter-of-a-million-pound inertial sensors that can track your position accurately for hours without a GNSS. “I know there’s technology out there that gives you exactly what you require to enhance safety in cars or to do indoor navigation for long periods,” she says, “so I felt that there was a definitive need to bring some of that high performance to consumers.”

prototype device — **Sensor revolution** Lia Li’s prototype optomechanical sensor from her UCL research, which uses the same mechanism as LIGO but on a much smaller scale. Now, she is developing chipscale versions that could revolutionize inertial sensor chips in phones. (Courtesy: Lia Li)

The key to developing this technology is to change the sensing mechanism from the noisy capacitive sensor currently found in our devices to one that uses cavity optomechanics – the interactions between light and the motion of physical objects. Instead of placing the tiny moveable silicon structure so that it influences the capacitance, you put it next to a chipscale optical cavity, which traps laser light by reflecting it internally. Li uses ones with curved boundaries, like rings, spheres and doughnut shapes.

Depending on the cavity’s exact parameters, it will have an optical resonance at certain frequencies of light where the reflections combine to form standing waves. But if the sensor is accelerated, the silicon structure will move closer or further away from the cavity in response, temporarily changing the effective refractive index experienced by the cavity, and therefore causing a slight shift in its resonant frequencies. This shift can be used to calculate the acceleration, which can be double integrated over time, as with the capacitive sensors, to find the displacement.

Unlike capacitive sensors, this mechanism is not affected much by noise. “Because there’s a resonance condition, you get this extra boost in signal-to-noise that makes it super sensitive,” Li explains. “So a tiny amount of motion results in a huge amount of measurable shifting.”

Indeed, a similar mechanism is at the heart of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has detected some of the subtlest vibrations scientists have ever looked for. By measuring the shift in resonant frequencies between mirrors that are placed at great distances, LIGO is able to sense gravitational waves as they squeeze and stretch space–time.

LIGO’s historic success in detecting gravitational waves – which won three of its scientists the 2017 Nobel Prize for Physics – strengthened Li’s faith in the revolutionary power of the technology. “LIGO is a giant version of a cavity optomechanics experiment,” she says. “So to me it was really obvious that if it can transform fundamental physics, it would absolutely do the same for normal sensing devices. Lots of academic groups have demonstrated similar effects, but no one had done it at a commercial product level yet. With the power of photonic integrated circuits, which can mass produce these cavities the same way computer chips are made, it feels like the market is ready.”

Finding your feet

Li made her first foray into entrepreneurship straight after finishing her PhD in 2016, when she participated in the Nature/Entrepreneur First Innovation Forum. This was a three-month Dragons’ Den-type scheme where participants were trained in assessing business concepts and pitched their own ideas to a panel of academics and industry experts. She enjoyed being around other entrepreneurs as well as veteran businesspeople who were looking to mentor new founders. “I thought ‘Wow, I really want to be a CEO’,” she says.

Another reason Li wanted to start a company was that she found it difficult to support herself in academia, let alone give others opportunities – a cycle of trying to stay afloat but not having the resources to seek help or grow a team. Her motivation was often dampened by experiencing or witnessing gender and racial inequity in science, but her desire to help younger scientists remained alive due to the support she’s received from strong women in the field.

While in academia, Li was involved with the UCL Women in Physics group, which she led for three years, as well as Tigers in STEMM, a UK-wide grassroots group promoting diversity and inclusion. As part of her work in this area, Li wrote a paper on race equity in science funding with Erinma Ochu of Manchester Metropolitan University, and Hope Bretscher and Rachel Oliver from the University of Cambridge for the journal of the Parliamentary and Scientific Committee of the British Houses of Parliament. By analysing science funding data, the team showed that, on average, ethnic minority scientists write grants for six months longer than white scientists before grant success but then receive nearly £200,000 less.

This can have a particularly negative impact in the already harsh academic environment of short-term contracts and job insecurity. Li experienced the difficult system of constant grant applications when she returned to academia as a postdoctoral researcher at UCL, after finishing the Nature/Entrepreneur First Innovation Forum. She continued to think about commercializing her research, but encountered various dead ends, often because she would burn out trying to do that and her academic work simultaneously.

Lia with her puppy and papers from setting up her company — **Success in lockdown** While furloughed from her academic position in 2020 because of the UK’s COVID lockdown, Lia Li used the time to write the business plan for Zero Point Motion. (Courtesy: Joe Burgess)

Then, in 2019, Li won an innovation grant of £50,000 from the UCL Quantum Science and Technology Institute. The award had no strings attached; the only condition was that she had to officially set up a business and have a business bank account. Li found that going through that formal process felt surprisingly significant, and made the idea of Zero Point Motion seem much more real.

When COVID hit the following year, she was furloughed from her academic work, but this gave her the time and freedom she needed to focus on the company. “I wrote the entire business plan during that furlough period,” she recalls. “I actually ended up inventing the two foundation patents for the company during that time.”

The final piece of the puzzle fell into place when Gordon Aspin, a scientist and engineer who has developed four companies relating to smartphone chips in the past, joined as executive chairman. “As a physicist, I have no connection to mass-producing smartphone chips, which he has,” says Li, “so he’s the exact kind of mentor that I needed.”

Last year Li won the Institute of Physics (IOP) Clifford Paterson Medal and Prize, which recognizes “exceptional early-career contributions to the application of physics in an industrial or commercial context”. The IOP cited her pioneering research into commercializing optomechanical sensors, as well as her drive to build a better and more supportive research community. In February this year, she was able to become full-time chief executive of Zero Point Motion.

Silicon vision

Although Li is looking at enhanced navigation as the primary application of her optomechanical sensors, they have potential for many other uses too. The technology is sensitive enough to pick up someone’s heartbeat from underneath a mattress, so it could be used for health monitoring. Since structures like bridges also vibrate in specific ways that change with wear and tear, the sensors could be used to monitor these changes over time and implement predictive maintenance too.

Now, after raising a £2.58m seed round from Foresight Williams Technology, Verve Ventures and u-blox, the next steps for Li are to expand the team and work through early prototype iterations. The aim is to have prototypes that can be tested and demonstrated next year, and to receive feedback from potential end users. “I think I’m slowly learning to be more ambitious,” she says. “It’s hard when you come from a really competitive research area where you feel that you don’t want to dream too big. But in business, you can’t afford to do that because so much of it is how you inspire employees and bring people to the company; hiring is what we’re focussed on right now. So if any readers want to work on this technology and own part of the company, come to Bristol and join us!”

Li’s goal is for Zero Point Motion to become a world-leading high-volume inertial-sensor chip company, but her vision goes beyond her own business. She wants to see the UK grow its semiconductor presence, pointing out that it isn’t currently very strong, with Arm Holdings being the major breakthrough UK-based semiconductor company.

Something needs to galvanize the industry to move towards mass volume, and to embody the Silicon Valley mentality
Lia Li

“There are amazing scientists and engineers in the UK. At BAE, I used to work with people who invented the first silicon gyroscopes inside Segways,” she says. “But I think something needs to galvanize the industry to move towards mass volume, and to embody the Silicon Valley mentality and the ambition as well. Hitting 100 million units per year would be ground-breaking for the UK.”

Glassy globules found on the Moon

The Moon’s highlands may be scattered with transparent globules of a glass-like substance, say scientists involved in China’s Chang’e-4 mission. Initial observations from the mission’s Yutu-2 rover suggest that these structures are widespread, and their presence could help us better understand the early history of lunar impacts as well as providing sampling targets for future expeditions to the lunar surface.

The Chang’e-4 mission is exploring the South Pole-Aitken basin on the far side of the Moon. As part of this mission, Yutu-2 has travelled more than 1000 m across the Von Kármán crater since its deployment in early 2019. Reflectance spectra sent back by orbiters and Yutu-2 had previously revealed that the lunar soil, or regolith, in the basin is mainly iron-poor, with substantial amounts of an intrusive igneous rock known as anorthosite as well as coarse-grained rocks. Now, thanks to its panoramic camera, Yutu-2 has obtained images of transparent globules of a type never seen on the Moon before.

Dramatically different

Upon examining the images, a team led by Zhiyong Xiao of the Planetary Environmental and Astrobiological Research Laboratory at Sun Yat-sen University found that the globules ranged from light grey to brown in colour and were either spherical or dumbbell-shaped, measuring 1.5 to 2.5 cm in diameter. Although the Apollo astronauts also collected glassy globules from the lunar surface, their samples were mostly dark and opaque. The Apollo samples were also much bigger and contained a lot of clasts, which are fragments of older rock broken up and embedded in a younger one.

Xiao and colleagues say that the newly-discovered globules are very different from the blocky fragments that surround them. They speculate that they could have been created by high-energy impacts causing iron-poor materials such as pure anorthosites to melt. If this is indeed the case, the glassy deposits are likely to be widespread on the Moon’s far side, since pure anorthosites are a common component of the regolith in the vast lunar highlands and high-energy melts formed by impact events are abundant.

Globules might be extremely young

The shape and surface structure of the globules suggests that they may have formed (or been exposed) relatively recently, since surface materials on the Moon are continually being abraded and impacted. Indeed, growth rates of lunar regolith imply that the top 2-3 cm is completely overturned in less than 100 000 years. However, Xiao explains that the globules’ formation age cannot be calculated exactly based solely on their shape or surface structures.

The discovery of these globules on the Moon suggest that impacts on other planetary bodies could also form such “tektite-like” glasses. What is more, Xiao suggests that analysing their composition and isotope ages should yield valuable insights into the Moon’s impact history, since such impacts should have been common during the early years of Earth’s satellite. At this stage, however, the researchers are not able to confirm whether these structures are similar to tektites found here on Earth, where they were produced in craters large than 10 km across formed following impacts with water-rich targets.

Valuable sampling targets

Xiao’s team, which reports its work in Science Bulletin, say that the translucent globules on the Moon could provide valuable sampling targets for future lunar surface explorations, as they should be easy to recognize. They add that the globules could also be excellent raw materials from which to manufacture glass in situ, as they have good light-admitting quality.

Tracking China’s rapid rise in lunar exploration

The researchers are now analysing additional data returned by the Chang’e-4 mission, and hope to confirm the presence of other types of globules. “We are currently investigating the 500 mg of regolith samples returned by the Chang’e-5 mission (allocated to my group), in which hundreds of glass spherules (<500 𝜇m) have been discovered,” Xiao tells Physics World. “From the perspective of comparative planetology, we are combining our study of lunar glass spherules with our ongoing studies of tektites collected in southern China that belong to the vast Australasian strewn field. Our aim is to better understand the mechanisms by which various impact glasses form.”

Quantum batteries harvest energy from light

Your night-mode photos might get a lot crisper thanks to a new device that exploits quantum mechanics to absorb photons more efficiently. Known as a quantum battery, the device stores the energy of the absorbed photons and can be charged simply by shining light on it – great news not just for low-light iPhone photography, but also for solar panels, which could use similar technology to capture the Sun’s energy a lot faster.

In a normal medium for storing energy, like a car battery, the time to charge the battery increases in proportion to the battery’s size. Quantum mechanical effects, however, make it possible to create systems with an energy absorption capacity that increases drastically as they get bigger. This is known as superabsorption, and it enables researchers to create a battery that, somewhat counter-intuitively, charges faster as its size increases.

Capturing light between mirrors

In the latest study, which is described in Science Advances, James Quach and colleagues at the University of Adelaide, Australia; the Politecnico di Milano and the National Research Center (CNR), Italy; and the Universities of St Andrews, Sheffield and Heriot-Watt in the UK created a quantum battery from molecules of an organic dye, trademarked Lumogen-F Orange. Dye molecules of this type can be modelled as an effective two-level system in which the molecule is most likely to be in one of two states: the ground state with minimal energy, or an excited state with higher energy. If laser light is fired at it at the right wavelength, it may absorb a photon and jump to the excited state.

To ensure that these dye molecules absorb photons efficiently, the researchers suspended them in a matrix of a non-reactive polymer and placed them in a cavity consisting of two mirrors. Because everyday mirrors (which are simply sheets of glass coated with reflective metal) are not reflective enough to keep the photons trapped in the cavity, the team used alternating layers of dielectric materials to create a device known as a distributed Bragg reflector.

When a laser beam shines into this highly reflective cavity, the dye molecules absorb photons and jump to the excited state. The amount of energy they absorb can be estimated by sending in a probe beam and monitoring how much of it gets reflected: once excited, molecules cannot absorb any more photons, so they reflect. Therefore, measuring the intensity of reflected light tells you how many dye molecules were excited, and how much energy was absorbed by the dye-battery.

The right concentration

To measure the battery’s charging rates, the group had to develop ultrafast measurement techniques to charge and probe the cavity in rapid succession, at time scales as short as 10 ^–14seconds. The researchers also measured the absorption properties of different concentrations of dye while keeping the charging power constant. Remarkably, the time required to charge the cavity went down as the dye concentration went up, meaning that it took less time to charge N molecules in one microcavity than it would to charge N microcavities containing one molecule each. This phenomenon – a signature of superabsorption – occurs because the shared quantum entanglement between dye molecules lets them trap photons better than molecules on their own.

Given this result, one might wonder whether it would be possible to build batteries that charge instantaneously by increasing dye concentrations to some arbitrarily high level. Unfortunately, the way the molecules interact with each other and with light changes at very high concentrations, pushing the battery away from superabsorption. A further drawback is that because the dye molecules absorb energy faster, they also release it faster. While fast discharge may be desirable for some applications, like charging an electric vehicle, it is not good for a battery which should, ideally, store energy for a long time without dissipating it.

In this particular case, noise comes to the rescue. If the noise in the device is just right, the absorption-dissipation cycle can be disrupted as the excited dye molecules steer to something called a “dark mode” where photon emission is much suppressed.

Cavities in cameras

Although the dye molecules in this experiment are good at absorbing energy, and you can store a little bit of energy in a cavity, a useful battery would need to be able to extract the energy from the cavity before it could power, for instance, a mobile phone or a smart watch, or transport energy to a location where it can be stored long-term. To do this, the team would need to add additional components to the cavity that can transport the dye excitations outside, in the form of an electric current. “At the moment, the proof-of-principle devices we made are still quite small, and the charging occurs with light, so immediate applications would need to work with those constraints,” says Quach. Since superabsorption is a general quantum mechanical phenomenon, he adds, it may manifest in other systems as well.

One near-term application of cavity-based quantum batteries would be to improve low-light energy capture in photovoltaic cells used in solar cells and cameras. However, substantial work remains to be done before we can reliably use superabsorption outside of a laboratory. For example, today’s solar cells and cameras can store energy of a wide range of wavelengths, but the quantum battery demonstrated in this experiment can only absorb light at a particular frequency. Quach says he and his colleagues are optimistic that they can scale up the system, and they are looking into ways of storing and transferring energy, with the goal of producing a device that can be easily integrated into existing technologies.

Brain implant allows person with complete paralysis to communicate

Scientists have successfully used an implanted brain–computer interface (BCI) to enable a person with complete paralysis to communicate – suggesting that verbal communication with such devices may one day be possible for patients in a completely locked-in state.

Electrode arrays

For the trial, a team from the Wyss Center for Bio and Neuroengineering, ALS Voice and the University of Tübingen implanted a type of BCI called an auditory neurofeedback system into the brain of a 34-year-old male patient with amyotrophic lateral sclerosis (ALS) – also known as motor neurone disease. The patient was in a completely locked-in state and had no voluntary muscular control. After implantation, he could form words and phrases and communicate at an average rate of about one character per minute.

Reporting their findings in Nature Communications, the researchers describe how they inserted two tiny 3.2 x 3.2 mm microelectrode arrays, known as Utah arrays, into the surface of the participant’s motor cortex – the part of the brain responsible for movement.

Each array contains 64 needle-like electrodes that record neural signals, before transferring them to a computer via a wired connection. Software then decodes the data and runs an auditory feedback speller that prompts the user to select letters to form words and sentences. The participant learned how to alter his own brain activity according to the audio feedback received, enabling him to select letters and ultimately spell words and sentences.

As joint lead author Jonas Zimmermann, senior neuroscientist at the Wyss Center, explains, the participant, who lives at home with his family, expressed his wish to take part in the clinical case study before he became completely locked-in.

“A member of the research team was usually present at the patient’s home for experimental sessions,” explains Zimmerman. “Every day the system was in use, there was first a calibration and training session to adapt the software to changes in brain activity. When we were convinced that there was reliable enough control, we ran the speller.”

Based on previous work by the Wyss Center and other groups, Zimmerman says the team had a strong belief that, in this case too, a locked-in patient could communicate using electrodes implanted directly into the brain.

“The patient changes the activity of single neurones. He hears the activity as a sequence of lower pitch or higher pitch tones, depending on how much the neurones are active. We can then ask him to hold the tone high to say ‘yes’ and hold it low to say ‘no’. This enables him to select letters and ultimately spell words and sentences,” he explains.

Wireless device

According to Zimmerman, the study answers a long-standing question about whether people with complete locked-in syndrome (CLIS) – who have lost all voluntary muscle control, including movement of the eyes or mouth – also lose the ability of their brain to generate commands for communication. Successful communication has previously been demonstrated with BCIs in individuals with paralysis but, to Zimmerman’s knowledge, this study is the first to achieve communication by someone who has no remaining voluntary movement and hence for whom the BCI is now the sole means of communication.

“For our participant there are no alternative approaches for communication. He has amyotrophic lateral sclerosis (ALS) – a progressive neurodegenerative disease in which people lose all ability to talk and to move. He cannot move his eyes nor voluntarily move any muscles, so the BCI is his only means of communicating with his family and medical caregivers,” notes Zimmerman.

At present, the BCI system used in the study is for clinical investigations only and is not available outside a research setting. Further demonstrations of its longevity, applicability in other patients, safety and efficacy are needed before it is suitable for widespread clinical use.

“At the Wyss Center, we are currently developing ABILITY, a wireless implantable BCI device for clinical use in people with CLIS,” Zimmerman adds. “Use of a wireless device avoids the potential infection risk associated with the percutaneous cable that connects the electrodes to the computer. We also plan to use this system for speech decoding. The next step is validating the new ABILITY device in pre-clinical and clinical trials.”

Gamma-ray detector helps archaeologists, honey sweetens neuromorphic computing

Archaeologists have adapted several techniques from physicists and the next one could be the detection of gamma rays in the field. For the first time, a gamma-ray spectrometer has been used in an archaeological setting. The instrument is normally used to identify radioactive contamination on nuclear sites, but now researchers at the University of Reading excavation in Roman Silchester in southern England have used it to find buried buildings and other objects. This was done by detecting gamma radiation emitted during the natural decay of elements in these materials – which can be measured up to one metre under the ground.

The detector was able to find a buried wall, for example, because the wall was made of materials that had a lower level of radioactivity than the surrounding soil. This suggests the wall was made from materials imported from a different geographical area. The technique is also particularly good at identifying materials that have been altered by industrial processes such firing in a kiln – which can change the amount of radioactive elements that an object contains.

The spectrometer is called Groundhog and it is made by Nuvia. You can read more about the study in this paper: “Portable gamma ray spectrometry for archaeological prospection: A preliminary investigation at Silchester Roman Town”.

Remembering charge

Our brains have a remarkable capacity for storing and processing information in very efficient ways. That is why researchers are keen on developing neuromorphic computers that mimic how the brain works. The memristor is a component that is used in many neuromorphic architectures. It “remembers” the amount of charge that has flowed through it, with the information being stored in terms of the device’s resistance. Or at least it does this in principle, because researchers have not yet been able to create a device that operates as an ideal memristor.

Now, Feng Zhao and Brandon Sueoka at Washington State University have created a memristor that takes advantage of the material properties of honey. They sandwiched a layer of honey between a copper electrode and a copper oxide electron to create their memristor. They then looked at how the memristor performs in a circuit that mimics how information is passed between two neurons.

They found that the device’s performance was on par with other comparable memristors. They also showed that the device could be dissolved in water. This means that the device is partly biodegradable and that it would be easy to recycle.

They report their results in this paper: “Memristive synaptic device based on a natural organic material—honey for spiking neural network in biodegradable neuromorphic systems”.

Loopy currents appear in a kagome superlattice

Exotic materials known as kagome superconductors can play host to a rare state of matter in which electric currents form “loops” around unit cells in the material’s crystalline lattice. This discovery, made by researchers at Switzerland’s Paul Scherrer Institute (PSI) together with international collaborators, could reveal new information about how superconductivity emerges in materials where complex effects such as frustrated magnetism and intertwined orders play a major role.

Kagome metals are named after a traditional Japanese basket-weaving technique that produces a lattice of interlaced, corner-sharing symmetrical triangles. When the atoms of a metal or other conductor are arranged in this so-called kagome pattern, their electrons behave in unusual ways, giving rise to interaction-driven electronic phases of matter that can be identified by studying symmetries of the material.

In one such electronic phase, electrons organize themselves into a looped pattern along the kagome bonds between regions of the lattice with high and low concentrations of electric charge. Researchers have long debated whether such unusual loop currents could be a precursor to important condensed-matter phenomena such as high-temperature superconductivity (the ability to conduct electricity without resistance below critical temperatures of 77 K or higher) or the quantum anomalous Hall effect, in which a material exhibits a quantized voltage drop in a direction transverse to the flow of an electric current even in the absence of an external magnetic field.

A rare state of matter

A multinational team led by scientists at the PSI’s Laboratory for Muon Spin Spectroscopy has now identified a signature of such a phase in a kagome superlattice material with the chemical formula KV₃Sb₅, which becomes a superconductor at a critical temperature of 1.2 K. After spotting the weak internal magnetic fields that indicate the presence of charge ordering, the researchers also found evidence that these magnetic fields break “time-reversal symmetry”, which requires the laws of physics to look the same irrespective of whether time moves forwards or backwards.

To detect the tell-tale signs of time-reversal-symmetry breaking, the physicists used a technique called muon spin rotation/relaxation spectroscopy (uSR). Muons are elementary particles that are similar to electrons but have more than 200 times the electron’s mass. They have a finite lifetime, and at the end of it, they decay into lighter particles such as positrons, which are the antiparticles of electrons.

In the current study, which is described in Nature, the physicists implanted muons into their sample of KV₃Sb₅. The muons interact with their local environment via their internal angular momentum, which is sensitive to magnetic fields produced by electronic current loops. This means that as the muons decay into positrons, the way the polarization of the angular momentum evolves with time changes in a manner that signifies the presence of such loops.

Quantum anomalous Hall effect could come from orbital currents

The team observed a systematic shift in the magnetic signal and concluded that time-reversal symmetry breaks in KV₃Sb₅ at 80 K, the temperature at which the current loops form. Interestingly, the material also exhibits a giant quantum anomalous Hall effect at this temperature. This effect was previously unexplained, but the PSI researchers say their result provides the best evidence yet that it stems from orbital currents. This hypothesis, long debated for Kagome superlattices, could also apply to other unconventional superconductors that exhibit a large quantum anomalous Hall effect, such as graphene.

“Normally, these loop currents are hard to detect since their signals are usually too weak,” explains team leader Zurab Guguchia. “We overcame this problem by applying a high magnetic field of up to 9.5 Tesla to amplify the electronic response.”

The researchers say that their discovery of time-reversal symmetry-breaking fields – which implies both orbital currents and the peculiar charge ordering that gives rise to them – opens doors to “exotic avenues of physics and next-generation device research”. They add that the concept of orbital currents also forms the basis of “orbitronics”, in which the orbital degree of freedom is used as an information carrier in solid-state devices.

The researchers acknowledge that there is as yet no consensus on the superconducting gap structure in kagome materials with the formula AV₃Sb₅(where A can be K, Rb or Cs). They attribute this lack of consensus to “the challenges of performing spectroscopic studies under extreme conditions including ultra-low temperatures and large pressures”. They plan to tackle these barriers by performing zero-field, high-field and high-pressure muon spin relaxation experiments to directly probe the interplay between chiral charge order and superconductivity across the temperature-pressure phase diagram of AV₃Sb₅. “This will allow us to assess not only the time-reversal symmetry-breaking nature of these two states, but also the evolution of the low-energy superconducting excitations as charge order is suppressed under pressure,” Guguchia tells Physics World.

W boson mass measurement surprises physicists

The most precise measurement to date of the mass of the W boson has yielded a result seven standard deviations away from that predicted by the Standard Model of particle physics. The stunning result was obtained by a painstaking analysis of data taken at the Fermilab Tevatron collider in the US before it closed in 2011. The particle physics community must now study the results carefully to work out whether it is an incredible statistical fluke, an unknown experimental error, a flaw in the Standard Model or a genuine indication of physics beyond the Standard Model.

The W boson is one of the most intriguing particles described by the Standard Model. Together with the neutral Z boson, the charged W boson mediates the weak interaction, which causes beta decay and several other important processes in particle physics. The weak interaction has long intrigued scientists searching for physics beyond the Standard Model, partly because it is the only force known to violate charge-parity symmetry. If particles in a process are exchanged for their antiparticles and the spatial co-ordinates are inverted, the weak interaction in this mirror image process are not always identical. This puzzle is not explained in the Standard Model.

The weak interaction is also unique in that, whereas the photons that mediate electromagnetism and the gluons that mediate the strong interaction are massless, the W and Z bosons have mass. The Standard Model does not predict the absolute mass of the W boson directly, but it does predict the ratio between the masses of the W and Z bosons. To make absolute predictions, however, physicists needed to know the mass of the Higgs boson, which was not known when Tevatron switched off in 2011. The very next year, however, CERN’s new Large Hadron Collider famously discovered the missing boson at a mass of 125.35 GeV.

Encouraging measurements

“The Z boson mass was measured already much more precisely than the W,” explains Ashutosh Kotwal of Duke University in the US, who is part of the Collider Detector at Fermilab (CDF) collaboration, which was one of two experiments at the Tevatron. “That’s one of the things that’s been driving us: if you know the Z boson mass precisely, that allows you to calculate the W boson mass precisely, because it’s the connection that the theory tells you.” The Higgs boson mass allowed physicists to infer that the mass of the W boson would be 80,357±6 MeV. This encouraged physicists to work to either confirm or refute this prediction.

Measurements of the W boson’s mass date back to 1983, and many of the most recent have been based on data from CDF and the Tevatron’s other detector D0. Until now, the most precise value – from the ATLAS collaboration on the LHC – stood at 80,370±19 MeV – just within agreement with the theoretical prediction.

According to Kotwal, the Tevatron offers two advantages over the LHC for measuring the W boson mass. First, its lower beam luminosity means that events of interest are less likely to be obscured by other events at the detector. More fundamentally, whereas the LHC collides protons with other protons, Tevatron collided protons with antiprotons: this makes quark-antiquark annihilation a simpler process with fewer errors to consider.

Four million collisions

The CDF collaboration’s new analysis is based on four million collisions in which a W boson was created by quark-antiquark annihilation, before decaying to produce a lepton or muon and a neutrino. The researchers had previously published results based on a subsets of the data and these were statistically consistent with the predicted mass value. For the new work, they incorporated a dataset four times larger, as well as improved calibration of many experimental parameters. Their result is a stunning 80,433±9MeV, seven standard deviations away from the theoretical prediction.

“At this point, there really isn’t much we can think to improve further without input from the outside,” says Kotwal, “We really need to share what we have done with the community, and it’s their feedback that will help us think more.”

If the result does hold up to scrutiny and if similar results appear in experiments at other detectors, Kotwal says that “from the theory perspective it is way out of our field of expertise”. However, he notes that theorists are forced to make approximations and assumptions to make calculations tractable. “It has happened historically every once in a while that something that was left out of calculations because, from experience, people assumed it would be a small effect turned out when actually calculated not to be such a small effect after all.”

“It’s both significant and surprising, and people will scratch their chins for some years to come, I suspect,” says Paul Grannis of Stony Brook University, who is part of the D0 and ATLAS Collaborations. “I know the researchers; I know how careful they are; I know how talented they are, so I am inclined to believe that they have been as thorough as they possibly can be. However, as their result is so far away from the Standard Model prediction, everybody is going to be looking at their result as hard as they possibly can.” He says that, compared to the theoretical uncertainties within the Standard Model, the uncertainties measured are “in the direction you would get from most models of beyond the Standard Model physics”. “However, most of those departures from the Standard Model have been exhaustively searched for unsuccessfully at the LHC,” he says. “So I suspect there is no cheap way to find an explanation in terms of beyond the Standard Model physics. I haven’t heard any exhaustive study of that yet and people will surely do that, but it’s going to be be tough.”

The research is described in Science.

Kirigami-inspired electrocardiogram sensors can be worn all day

The Japanese art form of kirigami has inspired a new heart sensor that can monitor electrical signals from the surface of the skin while being worn for long periods of time. Borrowing from the principles of kirigami, Kuniharu Takei and colleagues at Osaka Prefecture University cut a pattern of holes in an electrode-coated PET film, which allowed the material to conform to the skin’s surface more easily.

The latest advances in wearable technology include devices that can monitor aspects of a wearer’s health in real time. In the future, wearable technologies could identify diseases in their early stages or even predict the likelihood of future medical conditions.

Electrocardiography (ECG) is an important technique for monitoring the health of the heart. The heartbeat causes subtle electrical changes to the skin’s surface, and these changes are captured by placing pairs of electrodes on specific parts of the body. Information provided by an EKG can be used to diagnose heart conditions such as arrhythmias.

Comfort and stability

Most ECG monitoring is done in a matter of minutes, with most of the time devoted to placing and removing the electrodes. However, being able to monitor the heart continuously could benefit some people. An important challenge of doing continuous monitoring is creating electrodes that can remain on the skin for long periods of time without causing discomfort to the wearer. Such devices must be able to conform to the intricate and changing contours of the body, while remaining comfortable and stable, even during exercise.

Previous attempts at developing electrodes have focussed on making electrodes as thin and flexible as possible. So far, however, there has been less focus on making sensors smaller and more breathable. Reducing sensor size is a particular challenge because smaller devices tend to have higher levels of noise.

To address size and breathability, Takei’s team turned to kirigami, an art form whereby paper is cut before being folded to create 3D objects. To create their sensor, they first printed sheets of silver electrode on both sides of a plastic (PET) film. A second film of PET is then deposited on one of the silver sides. The remaining silver side of the device contacts the skin, while the PET protects the device.

Millimetre-sized holes

A laser was then used to cut a kirigami pattern of millimetre-sized holes and slits into this multi-layered structure. This allowed the relatively stiff PET material to stretch and bend far more easily, while also allowing the wearer’s sweat to readily pass through. To perform ECG’s, the electrodes were connected to a battery-powered sensor, which uses Bluetooth to transmit detected signals to an app on the wearer’s smartphone.

Takei and colleagues found that an optimal balance between comfort and noise could be reached through an electrode size of roughly 200 mm² with the sensors placed 1.5 cm apart. This resulted in accurate, reliable heart data when the device was tested on people carrying out a range of everyday activities, including sitting and walking.

Through further improvements, the researchers now hope to integrate more sensors into their system. This could allow it collect multiple types of data from the skin surface and approve its ability to help doctors make non-invasive diagnoses.

The research is described in Applied Physics Reviews.

Laser plasma accelerators unlock potential for radiobiology studies with protons

Laser plasma accelerators (LPAs) propel particles to high energies over short distances using intense, ultrashort pulses of laser light. These accelerators can supply high-quality particle beams for radiobiological studies that will help scientists better understand how radiation leads to DNA damage, and ultimately, help optimize particle-based cancer treatments.

Proton beams produced by laser plasma acceleration have a broad energy distribution, an exponentially decaying energy spectrum, and are bunched into particle pulses containing up to one trillion protons. This unique combination of beam properties makes proton LPAs suitable for ultrahigh-dose-rate radiobiology studies. Before researchers can conduct these studies, though, they must demonstrate that they can control LPA beam parameters.

To that end, Florian Kroll, a physicist at the Institute of Radiation Physics of Helmholtz-Zentrum Dresden-Rossendorf, his colleagues, and experts from OncoRay Dresden spent the last few years testing a sophisticated proton LPA platform. The platform has reliably displayed accelerator stability for two years, delivers model-compliant dose distributions, and in a pilot study in mice, demonstrated precise dose delivery and dosimetry.

A new laser plasma accelerator platform

Though strides in laser plasma acceleration have been made over the years, one of its primary challenges has persisted: laser plasma accelerators, and the particles that they accelerate, are difficult to stabilize and control.

The LPA method that Kroll and his colleagues implemented to improve this reliability relies on a high-power laser, tightly focused onto a thin (220 nm) plastic target.

“The intensity of the laser is so enormous that, when it impinges on the target, it immediately ionizes the target material, turning it into a plasma,” explains Kroll, first author on the resulting Nature Physics study.

Stripped from their atomic cores, the target’s electrons are pushed through the plasma by the laser. Some of the electrons, unable to escape the now positively charged target, form a “sheath” on the back side of the target and create a quasi-static electric field that “pulls” on the target ions. This field accelerates the ions into the mega-electronvolt (MeV) range.

Dose delivery: The Draco PW laser impinges on the target and drives a broadband LPA proton source. The protons are transported through an energy selection aperture (ESA), and traverse beam monitors and dosimeters installed downstream, before delivering dose to the planning target volume (PTV) in the mouse. (Courtesy: CC BY 4.0/*Nat. Phys.* 10.1038/s41567-022-01520-3)

The researchers optimized the shape of the resulting proton beam with several techniques, including a pulsed two-solenoid beamline that contains apertures and scatterers. To fine tune proton acceleration, they tailored the laser pulse shape temporally (the findings of a previous study demonstrated that the laser pulse’s “temporal shape” influences proton acceleration performance).

The LPA platform generates protons with a cut-off energy of up to 70 MeV and demonstrates stable accelerator performance, as well as daily delivery of prescribed volumetric dose distributions with percent-level margins.

In their accompanying radiobiological pilot experiments, the researchers irradiated human tumours on mouse ears using multi-shot LPA proton irradiation, clinical proton source irradiation, or standard 200 kV X-ray irradiation, comparing dosimetry and tumour control measures (the study design also included control and sham groups).

Although radiation-induced effects on tumour growth were observed, the focus of the pilot study was to demonstrate the feasibility of animal studies and to test the limits of dose delivery.

“We don’t want to speculate about the clinical applicability of laser-driven proton beams,” says Kroll. “In the early days of laser acceleration, many claims with respect to revolutionary, compact and cheap laser-driven therapy machines were made. In the end, everything turned out to be more complex than expected. Nevertheless, LPA machines have always been and will always be an interesting complementary accelerator technique to cyclotrons, synchrotrons and more.”

The future of LPA: FLASH and more

Some researchers note that the Nature Physics study is a breakthrough for the proton LPA community and that laser plasma accelerators are now ready for translational research. The study sets new standards for proton LPA sources, these researchers say, and paves the way for subsequent studies, such as those on the FLASH effect.

The FLASH effect occurs when a therapeutic radiation dose is delivered in a fraction of a second at an ultrahigh dose rate. In clinical settings, FLASH radiotherapy may reduce radiation damage in healthy tissues while remaining effective on tumours. Kroll notes that while the researchers were not trying to induce the FLASH effect in this study, they did find that single-shot sample irradiation “at full power” resulted in parameters that are expected to trigger the FLASH effect. As a result, future applications of their proton LPA platform may include verifying the FLASH effect with protons and investigating radiochemistry in the context of FLASH.

“[This study] was an amazing display of teamwork between technicians, engineers, physicists and biologists, all pulling in the same direction,” says Kroll. “We will continue to try answering radiobiological questions and look closer into the FLASH effect and its mechanisms. In parallel, we constantly strive for higher proton energies, particle numbers and source stability.”

Quantum computing meets machine learning, how motorsport could save the planet

This episode of the Physics World Weekly podcast features an interview with the physicist Maria Schuld, who is a senior researcher and software developer at Xanadu – a Toronto-based quantum technology company. She talks about the challenges and rewards of implementing machine-learning systems on quantum computers.

Also on hand is the author Kit Chapman, who chats about his latest book Racing Green: How Motorsport Science Can Change the World. He explains how the myriad technologies developed to make racing cars faster and safer have already benefitted society – and how they could help us combat climate change.

You can read Maria Schuld’s latest work on quantum machine learning on the arXiv preprint server.

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