A new experiment has offered the clearest view yet of how gluons behave inside atomic nuclei. Conducted at the Thomas Jefferson National Accelerator Facility in the US, the study focused on a rare process called photoproduction. This involves high-energy photons interacting with protons confined in nuclei to produce J/psi mesons. The research sheds light on how gluons are distributed in nuclear matter and is a crucial step toward understanding the nature of protons within nuclei.
While gluons are responsible for generating most of the visible mass in the universe, their role inside nuclei remains poorly understood. These massless particles mediate the strong nuclear force, which binds quarks as well as protons and neutrons in nuclei. Gluons carry no electric charge and cannot be directly detected.
The theory that describes gluons is called quantum chromodynamics (QCD) and it is notoriously complex and difficult to test – especially in the dense, strongly interacting environment of a nucleus. That makes precision experiments essential for revealing how matter is held together at the deepest level.
Probing gluons with light
The Jefferson Lab experiment focused on photoproduction, a process in which a high-energy photon strikes a particle and creates something new, in this case, a J/psi meson.
The J/psi comprises a charm quark and its antiquark and is especially useful for studying gluons. Charm quarks are much heavier than those found in ordinary matter and are not present in protons or neutrons. Therefore, they must be created entirely during the interaction, making the J/psi a particularly clean and sensitive probe of gluon behaviour inside nuclei.
Earlier studies had observed this process using free protons. This new experiment extends the approach to protons confined in nuclei to see how that environment affects gluon behaviour. The modification of quarks inside nuclei has been known since the 1980s and is called the EMC effect. However, much less is known about how gluons behave under the same conditions.
“Protons and neutrons do behave differently when they are bound inside nuclei than they do on their own,” says Jackson Pybus, now a postdoctoral fellow at Los Alamos National Laboratory and one of the experiment’s collaborators. “The nuclear physics community is still trying to work out the mechanisms behind the EMC effect. Until now, the distribution of high-momentum gluons in nuclei has remained an unexplored area.”
Pybus and colleagues used Jefferson Lab’s Experimental Hall D, which delivers an intense beam of high-energy photons. This setup had previously been used to study simpler systems, but this was the first time it was applied to heavier nuclei.
“This study looked for events where a photon strikes a proton inside the nucleus to knock it out while producing a J/psi,” Pybus explains. “By measuring the knocked-out proton, the produced J/psi, and the energy of the photon, we can reconstruct the reaction and learn how the gluons were behaving inside the nucleus.” This was done using the GlueX spectrometer.
Unexpected signals
Significantly, the experiment was accessing the “threshold” region – where the photon has just enough energy to produce a J/psi meson. Near-threshold interactions are particularly valuable because they are highly sensitive to the gluon structure of the target. Creating a heavy charm-anticharm pair requires a large energy transfer so interactions in this region reveal how gluons behave when little momentum is available. This is a regime where theoretical uncertainties in QCD are especially large.
Even more striking were the observations below this threshold. In so-called “sub-threshold” photoproduction, the incoming photon does not carry enough energy to produce the J/psi on its own, so it must draw additional energy from the internal motion of protons or from the nuclear medium itself. This is a well-understood mechanism in principle, but the rate at which it occurred in the experiment came as a surprise.
“Our study was the first to measure J/psi photoproduction from nuclei in the threshold region,” Pybus said. “The data indicate that the J/psi is produced more commonly than expected from protons that are moving with large momentum inside the nucleus, suggesting that these fast-moving protons could experience significant distortion to their internal gluons.”
The sub-threshold results were even harder to explain. “The number of subthreshold J/psi exceeded expectations,” Pybus added. “That raises questions about how the photon is able to pick up so much energy from the nucleus.”
Towards a deeper theory
The results suggest that gluons may be modified inside nuclei in ways that are not described by existing models – suggesting a new frontier in nuclear physics.
“This study has given us the first look at this sort of rare phenomenon that can teach us about the gluon inside the nucleus – just enough data to point to unexpected behaviours,” said Pybus. “Now that we know this measurement is possible, and that there are signs of interesting and unexplored phenomena, we’d like to perform a dedicated measurement focused on pinning down the sort of exotic effects we’re just now glimpsing.”
Follow-up experiments, including those planned at the future Electron-Ion Collider, are expected to build on these results. For now, this first glimpse at gluons in nuclei reveals that even decades after QCD’s development, the inner workings of nuclear matter remain only partially illuminated.
Modern-day communications rely on both fibre-optic cables and wireless radiofrequency (RF) microwave communications. Reaching higher data transmission capabilities is going to require technologies that can efficiently process and convert both optical and microwave signals in a small and energy-efficient package that’s compatible with existing communication networks.
Microwave photonics (MWP) is one of the frontrunning technologies, as it can perform signal processing tasks within the optical domain. Current MWP approaches, however, are typically power intensive and often require many off-chip devices to achieve the desired device capabilities and functionalities – so are not very scalable. Researchers from Belgium and France have now managed to overcome some of these limitations, reporting their findings in Nature Communications.
“We wanted to demonstrate that photonic chips can be as versatile as electronic chips, and one of the fields where the two overlap is that of microwave photonics,” one of the paper’s lead authors, Wim Bogaerts from Ghent University, tells Physics World.
A photonic engine
The researchers have created a photonic engine that processes microwave and optical signals and can convert the signals between the two domains. It is a silicon chip that can generate and detect optical and analogue electrical signals. The chip uses a combination of tuneable lasers (created by using an optical amplifier with on-chip filter circuits), electro-optic modulators and photodetectors, low-loss waveguides and passive components, and a programmable optical filter – which enables the chip to filter signals in both domains.
“We managed to integrate all key functionalities for manipulation of microwave signals and optical signals together on a single silicon chip and use that chip as a programmable engine in different experimental demonstrations,” says Bogaerts.
This setup allowed the researchers to operate the chip as a black-box microwave photonics processor, where the user can process high-frequency RF signals, without being exposed to the internal optical operations (they are hidden).
Optical signals from an external optical fibre are coupled to the chip using a grating coupler and high-speed RF signals are fed into the chip using electro-optic modulators. The RF signal is imprinted into an optical carrier wavelength – which is generated by the on-chip laser – and the signal is then processed on the chip using an optical filter bank. The signal then gets converted back into an RF signal using photodetectors.
All of the signals travelling into and out of the chip can be confined to the RF domain, so the chip doesn’t require any external optical components, unlike many other MWP devices. Moreover, the signals are locally programmed and tuned using thermo-optic phase shifters, enabling users to select any combination of microwave and optical inputs and outputs across the chip.
Extensive applications
The researchers used the photonic engine to create multiple systems that showcase its different optical and RF signal processing capabilities and demonstrate a potential pathway towards smaller MWP systems for high-speed wireless communication networks and microwave sensing applications.
As well as being used for simple light-tuning applications, the chip can also perform optical-to-electrical signal conversion, electrical-to-optical signal conversion, microwave frequency doubling, and microwave/optical filtering and equalization. These functions allow it to be used as a transmitter, receiver, optical/microwave filter, frequency converter or a tuneable opto-electronic oscillator.
When asked about the future of the chip, Bogaerts states that “we plan combine this functionality with more general purpose photonic circuits to enable even more functions and applications to help product developers roll out new photonic products as easily as new electronics products”.
Some other potential applications for the chip that have been touted – but not physically tested in this study – include RF instantaneous frequency measuring, radio-over-fibre links, RF phase tuning, optical and RF switching, optical sensing and signal temporal computing. With so many possibilities, this small-scale and low-power chip could become increasingly important as technologies such as communications advance further.
The development of advance quantum materials and devices often involves making measurements at very low temperatures. This is crucial when developing single-photon emitters and detectors for quantum technologies. And even if a device or material will not be used at cryogenic temperatures, researchers will sometimes make measurements at low temperatures in order to reduce thermal noise.
This R&D will often involve optical techniques such as spectroscopy and imaging, which use lasers and free-space optics. These optical systems must remain in alignment to ensure the quality and repeatability of the measurements. Furthermore, the vibration of optical components must be kept to an absolute minimum because motion will degrade the performance of instrumentation.
Minimizing vibration is usually achieved by doing experiments on optical tables, which are very large, heavy and rigid in order to dampen motion. Therefore, when a cryogenic cooler (cryocooler) is deployed on an optical table it is crucial that it does not introduce unwanted vibrations.
Closed-cycle cryocoolers offer an efficient way to cool samples to temperatures as low as ~2 K to 4 K (−272 °C to −269 °C). Much like a domestic refrigerator or air conditioner, these cryocoolers involve the cyclic compression and expansion of a gas – which is helium in cryogenic systems.
In 2010 Montana Instruments founder Luke Mauritsen, a mechanical engineer and entrepreneur, recognized that the future development of quantum materials and devices would rely on optical cryostats that allow researchers to make optical measurements at very low temperatures and at very low levels of vibration. To make that possible, Mauritsen founded Montana Instruments, which in 2010 launched its first low-vibration cryostats. Based in Bozeman, Montana, the company was acquired by Sweden’s Atlas Copco in 2022 and it continues to develop cryogenic technologies for cutting-edge quantum science and other demanding applications.
Until recently, all of Montana’s low-vibration optical cryostats used Gifford–McMahon (GM) cryocoolers. While these systems provide low temperatures and low vibrations, they are limited in terms of the cooling power that they can deliver. This is because operating GM cryocoolers at higher powers results in greater vibrations.
To create a low-vibration cryostat with more cooling power, Montana has developed the Cryostation 200 PT, which is the first Montana system to use a pulse-tube cryocooler. Pulse tubes offer similar cooling powers to GM cryocoolers but at much lower vibration levels. As a result, the Cryostation 200 PT delivers much higher cooling power, while maintaining very low vibrations on par with Montana’s other cryostats.
Montana’s R&D manager Josh Doherty explains, “One major reason that a pulse tube has lower vibrations is that its valve motor can be ‘remote’, located a short distance from coldhead of the cryostat. This allows us to position the valve motor, which generates vibrations, on a cart next to the optical table so its energy can be shunted to the ground, away from the experimental space on the optical table.”
However, isolating the coldhead from the valve motor is not enough to achieve the new cryostat’s very low levels of vibration. During operation, helium gas moves back and forth in the pulse tube and this causes tiny vibrations that are very difficult to mitigate. Using its extensive experience, Montana has minimized the vibrations at the sample/device mount and has also reduced the vibrational energy transferred from the pulse tube to the optical table. Doherty explains that this was done using the company’s patented technologies that minimize the transfer of vibrational energy, while at the same time maximizing thermal conductance between the pulse tube’s first stage and second stage flanges and the sample/device mounting surface(s). This includes the use of flexible, high-thermal-conductivity links and flexible vacuum bellows connections between the coldhead and the sample/device.
200mm breadboard The Cryostation 200 PT offers a large working area that can be accessed via multiple feedthrough options that support free-space optics, RF and DC electrical connections, optical fibres and a vacuum connection. (Courtesy: Montana Instruments)
Doherty adds, “we intentionally design the supporting structure to de-tune it from the pulse tube vibration source”. This was done by first measuring the pulse-tube vibrations in the lab to determine the vibrational frequencies at which energy is transferred to the optical table. Doherty and colleagues then used the ANSYS engineering/multiphysics software to simulate designs of the pulse tube support and the sample mount supporting structures.
“We optimized the supporting structure design, through material choices, assembly methods and geometry to mismatch the simulated natural frequencies of the support structure from the dominant vibrations of the source,” he explains.
As a result, the Cryostation 200 PT delivers more that 250 mW cooling power at 4.2 K, with a peak-to-peak vibrational amplitude of less than 30 nm. This is more than three times the cooling power delivered by Montana’s Cryostation s200, which offers a similarly-sized sample/device area and vibrational performance.
The control unit has a touchscreen user interface, which displays the cryostat temperature, temperature stability and vacuum pressure.
The cryostat has multiple feedthrough options that support free-space optics, RF and DC electrical connections, optical fibres and a vacuum connection. The Cryostation 200 PT supports Montana’s Cryo-Optic microscope objective and nanopositioner, which can be integrated within the cryostat. Also available is a low working distance window, which supports the use of an external microscope.
According to Montana Instrument senior product manager Patrick Gale, the higher cooling power of the Cryostation 200 PT means that it can support larger experimental payloads – meaning that a much wider range of experiments can be done within the cryostat. For example, more electrical connections can be made with the outside world than had been possible before.
“Every wire that you bring into the cryostat increases that heat load a little bit,” explains Gale, adding, “By using a 1 W pulse tube, we can cool the system down faster than any of our other systems”. While Montana’s other systems have typical cooling times of about 10 h, this has been reduced to about 6 h in the Cryostation 200. “This is particularly important for commercial users who are testing multiple samples in a week,” says Gale. “Saving that four hours per measurement allows a user to do two tests per day, versus just one per day.”
According to Gale, applications of the Cryostation 200 PT include developing ion traps for use in quantum computing, quantum sensing and atomic clocks. Other applications related to quantum technologies include the development of photonic devices; spin-based devices included those based on nitrogen-vacancies in diamond; quantum dots; and superconducting circuits.
It was a crisp, chilly morning in Bombay (Mumbai) on 4 December 2024 as delegates made their way to the Indian Institute of Technology, Bombay (IITB) for the opening day of the Women in Optics and Photonics in India-Asia (WOPI) conference. The cool, air-conditioned auditorium was soon packed with almost 300 people – mostly female postgraduate students and researchers – with notepads in hand and lanyards around their necks, eager to learn from the talks ahead.
The three-day event was organized by the IITB and sponsored by institutions, companies and publishers, including IOP Publishing, which publishes Physics World. Aimed at highlighting female voices that often go unheard, the meeting brought together scientists and experts from all over the world.
Yet not everything went according to plan. Although the room was full of curious students, each time the floor opened for questions, nervous glances were exchanged. A few moments of silence lingered as though everyone was waiting for a “better” question to come along. The silence rung of missed opportunities.
This kind of reticence is not new. From classrooms and labs to meetings, we see hesitance by students, and especially female students, everywhere. Growing up, I was told to look up the basics first and only ask so-called “high-level” questions. Good advice in theory but not if you’re already unsure of your place in the room. Add to that a dismissive answer such as “You should have known this by now” and the urge to speak up is nipped right in the bud.
Perhaps the hesitation is understandable. We live in a world where every answer is an Internet search away. Why ask a “silly” question when everyone else probably already knows better – or at least looks like they do? Yet asking questions is a fundamental part of learning. Science doesn’t move forward because people stay quiet until they have the perfect question, it moves because someone dares to ask – even when they aren’t sure.
Another contributing factor to such silence is imposter syndrome – the feeling that you don’t deserve to be there and aren’t good at your work. Multiple studies have shown that women consistently score higher on measures of imposter syndrome than men. Women in technical fields such as engineering, science and maths also report lower self-efficacy than men, regardless of actual performance or ability – so not only do we question whether we belong but we also underestimate ourselves.
All of this means women are less likely to ask questions or speak up when uncertain. But for a scientist, uncertainty is the norm. We spend most of our time sitting with the unknown and with it the need to ask questions and chip away at it. Yet the very process of scientific inquiry can feel to women like a trap.
A new way
So what can we do differently? Events like WOPI create time and space not just for presenting research and innovation, but for mentorship, for insight into the real-world machinery of science. It’s not just about “What are you working on?” but also “Where are you heading, and who’s with you?”
WOPI 2024 modelled a new approach to inclusivity by running panel sessions that included the families of successful leaders to showcase the kind of support that is necessary to “make it”. Women were invited who fearlessly shared their stories of pivoted careers and/or failures, while acknowledging the challenges that they encountered along the way. It reminded us that you don’t just grow by knowing but by asking, exploring and doing.
Rallying cry Organizing chair Shobha Shukla speaks at the the Women in Optics and Photonics in India meeting in 2024. Shukla is a professor in the Department of Metallurgical Engineering and Materials Science at IIT Bombay. (Courtesy: Prof. Shobha Shukla, WOPI 2024/IIT Bombay)
But we need to do more. Encouraging young female scientists today means accounting for the weight of cultural expectations, social atmosphere and gender-based tendencies. Events and conferences need to go beyond formal settings to highlight not just the science but the scaffolding that holds it all together – networking, informal mentorship, vulnerability and visibility. This must be integrated into the very fabric of all scientific events, not just those for women.
The real pulse of the WOPI conference was in the moments after the talks – students lining up to talk to speakers. Nervous, curious and determined to ask their questions anyway. That moment stayed with me. It was steadfast curiosity that had survived immense self-doubt and fear of speaking in front of an audience.
Hopefully at the next WOPI meeting to be held in India in 2026, students will be much more confident in asking questions. So, to every student: don’t wait for the perfect moment. Ask the question that’s been sitting at the edge of your mind, making you wonder. After all, that is where discovery starts.
Occupational transition network Graphical visualization of the weighted and directed labour market network in France derived from the transition probability matrix, computed from data spanning 2012 to 2020. Each node symbolizes an occupation, with links illustrating transitions between them. Node sizes correspond to the occupation’s workforce size, line widths are proportional to the transition probability. (Courtesy: M Knicker, K Naumann-Woleske and M Benzaquen, École Polytechnique Paris)
A new statistical physics analysis of the French labour market has revealed that the vast majority of occupations act as so-called condenser occupations. These structural bottlenecks attract workers from many other types of jobs, but offer very limited options for further mobility. This finding could help explain why changing jobs in response to shocks like technological change or economic crises is often so slow, say scientists at the Ecole Polytechnique in Paris, who performed the study.
“By pinpointing where mobility gets ‘stuck’, we provide a new lens to understand – and potentially improve – the adaptability of labour markets,” explains Max Knicker of the EconophysiX lab, who led this new research effort.
Knicker and colleagues borrowed a concept from statistical physics known as the “fitness and complexity” framework, which is used to study the structure of economies and ecosystems. In their work, the researchers treated occupations as nodes in a network and analysed real transition data – that is, how workers actually moved between different jobs in France from 2012 to 2020. The data came from official sources and were provided by the National Institute of Statistics and Economic Studies through the Secure Data Access Center (CASD).
“In total, we had access to information on about 30 million workers and employers in France, whom we tracked over a 10-year period,” explains Knicker. “We also worked with high-resolution administrative data from INSEE (the French National Institute of Statistics), and specifically the BTS-Postes (Base Tous Salariés-Postes).”
Two key metrics
The researchers assigned a score for each occupation and developed two key metrics. These were: accessibility, which measures how many different jobs “feed into” a given occupation; and transferability, which measures how many different jobs someone can move to from that occupation.
By studying the network of job flows with these metrics, they observed hidden patterns and constraints in occupational mobility and identified four main clusters, or categories, of jobs. The first are defined as “diffuser” occupations and have high transferability but low accessibility. “These require specific training to enter, but that training allows for transitions to many other areas,” explains Knicker. “This means they are more difficult to get into, but offer a wide range of exit opportunities.”
The second group are called “channel” occupations. These are both hard to enter and offer few onward transitions, he says. “They often involve highly specialized skills, such as specific types of machine operation.”
The third class are “hubs” and are both widely accessible and highly transferable – so much so that they act as central nodes in the transition network. “This class includes jobs like retail sellers, which require a broad, yet not highly specialized skill set,” says Knicker.
The fourth and last category is the most common type and dubbed “condenser” occupations. “Workers from many different backgrounds can easily enter these, but they can’t easily get out afterwards,” explains Knicker. “Examples of such jobs include caregiving roles.”
A valuable tool for policymakers
The researchers explain that they undertook their study to answer a broader question: why do some economies adapt quickly to shocks while others struggle? “Despite increasing attention to issues like automation or the green transition, we still lacked tools to diagnose where worker mobility breaks down,” says Knicker. “A key challenge was dealing with the sheer complexity and size of the labour flow data – we analysed over 250 million person–year observations. Another was interpreting the results in a meaningful, policy-relevant way, since the transition network is shaped by many intertwined factors like skill compatibility, employer preferences and worker choices.”
The new framework could become a valuable tool for policymakers seeking to make labour markets more responsive, he tells Physics World. “For example, by identifying specific occupations that function as bottlenecks, we can better target reskilling efforts or job transition programmes. It also suggests that simply increasing training isn’t enough – what matters is where people are coming from and where they can go next.”
The researchers also showed that the structure of job transitions itself can limit mobility. Over time, this could inform the design of more strategic labour interventions, especially in the face of structural shocks like AI-driven job displacement, states Knicker.
Looking forward, the Ecole Polytechnique team plans to extend its approach by studying how the career paths of individual workers evolve over time. This, says Knicker, will be done using panel data, not just year-to-year snapshots as in the present analysis. He and his colleagues are also interested in linking their metrics to wage dynamics – for example, does low transferability make workers more vulnerable to exploitation or wage stagnation? “Finally, we hope to explore whether similar bottleneck structures exist in other countries, which could reveal whether these patterns are universal or country-specific.”
What does quantum physics have to do with vibrant oil paintings and the ghostly grin of a disappearing cat? Quite a lot, as it turns out. In this month’s Physics World Stories podcast, host Andrew Glester takes a colourful look at how we visualize – and try to make sense of – the curious world of quantum mechanics.
Felicity shares her journey from academia to art, and how her experience of number-colour synaesthesia – where numbers are associated with colours in her mind – shapes her creative process as she explores the elusive nature of quantum reality.
Later, Physics World features editor Tushna Commissariat introduces the Physics WorldQuantum Briefing and delves into one of its stories, ‘The curious case of quantum Cheshire cats’. It explores the strange phenomenon where a particle’s properties seem to be in a different place from the particle itself – reminiscent of Lewis Carroll’s famous feline in Alice in Wonderland, whose grin lingers even after he’s gone.
You’ll find plenty more on the history, mystery and industry of quantum mechanics in the free-to-read Quantum Briefing. Stay tuned to the Physics World quantum channel for more IYQ content throughout the year. You can already enjoy a blog series from Matin Durrani, reporting from the tiny North Sea archipelago Helgoland, where Heisenberg made his breakthrough in quantum mechanics 100 years ago.
Proton energies achievable in laser accelerators could be tripled by using specially designed micronozzle targets, according to computer simulations done by physicists in Japan and India. In their design, the electric field generated in the micronozzle would be funnelled towards the outgoing protons, allowing the acceleration to proceed for much longer. The researchers believe that the research could be useful in nuclear fusion, hadron therapy and materials science.
Conventional accelerators use oscillating electric fields to drive charged particles to relativistic speeds. The Large Hadron Collider at CERN, for example, uses radio-frequency oscillations to achieve proton energies of nearly 7 TeV.
These accelerators tend to be very large, which limits where they can be built. Laser acceleration, which involves using high-energy laser pulses to accelerate charged particle, offers a way to create much more compact accelerators.
Crucial to inertial confinement
Laser acceleration is crucial to inertial confinement fusion, and high energy proton beams produced by laser accelerators are used in scientific laboratories for a variety of scientific applications including laboratory astrophysics.
The standard techniques for laser acceleration involve firing a laser pulse at a proton target surrounded by metal foil. Solid hydrogen only exists near absolute zero, so the proton target can be a hydrogen-rich compound such as a hydride or a polymer. The femtosecond laser pulse concentrates a huge amount of energy into a tiny area and this instantly turns the target into a plasma. The light’s oscillating electromagnetic field drives electrons through the plasma, leaving behind the much heavier ions and creating a huge electric field that can accelerate protons.
In the new work, physicist Masakatsu Murakami and colleagues at the University of Osaka in Japan, together with researchers at the Indian Institute of Technology Hyderabad, used computer modelling to examine the effect of changing the shape of the metal surrounding the target from a simple planar foil to a two-headed nozzle, with the target placed at the narrowest point. During the first stage of the acceleration process, the wide head of the nozzle behaves like a lens, concentrating the electric field from a wide area to produce an enhanced flow of hot electrons towards the centre. This electric current on the nozzle enhances ablation of protons from the hydrogen rod, kicking them forward into the vacuum.
“Just like a rocket nozzle”
Subsequently, the electrons keep moving through the “skirt” of the nozzle, creating a powerful electric field that, owing to the nozzle’s shape, remains focused on the accelerating proton pulse as it travels away into the vacuum. “With the single hydrogen rod and the single foil, the protons are accelerated only during the laser illumination,” explains Murakami. “However, interestingly with the micronozzle target, the acceleration keeps going even after the laser pulse illumination…Most of the plasma expands in a small volume together with the protons – just like a rocket nozzle,” he says. Whereas the standard proton energies achievable with a laser accelerator today are around 400 MeV, the researchers estimate that their micronozzle design could allow energies into the gigaelectronvolt regime without changing anything else.
Murakami has been studying nuclear fusion for 40 years and believes that “this method will be used for fast ignition of laser fusion”. However, he says, its potential uses go far beyond this. Proton beam therapy generally uses protons with energies of 200–300 MeV to treat cancer by delivering a high dose of radiation to the tumour and a much lower dose to surrounding healthy tissue. “Even higher energy is required to target cancers that are located in deeper parts of the body,” he says. The technique could also be useful for materials science techniques such as proton radiography or for simulation of the physics of astrophysical objects such as neutron stars. “I’m planning to do proof of principle experiments in the near future,” says Murakami.
Accelerator physicist Nicholas Dover of Imperial College London describes the work as “very interesting,” adding, “This target that they propose is a very complex thing to make. It would be a big project for a target fabrication lab to generate something like this – it’s not something we just cook up in our lab. Having these numerical optimizations is really helpful for us.” He notes, however, that one reason accelerator physicists often use planar targets (essentially pieces of kitchen foil) is the need to replace them in every shot. In scientific applications, this may not matter, he says. Applications in fields like medicine, however, would probably require the development of mass production facilities to fabricate the targets economically.
Light has always played a central role in healthcare, enabling a wide range of tools and techniques for diagnosing and treating disease. Nick Stone from the University of Exeter is a pioneer in this field, working with technologies ranging from laser-based cancer therapies to innovative spectroscopy-based diagnostics. Stone was recently awarded the Institute of Physics’ Rosalind Franklin Medal and Prize for developing novel Raman spectroscopic tools for rapid in vivo cancer diagnosis and monitoring. Physics World’s Tami Freeman spoke with Stone about his latest research.
What is Raman spectroscopy and how does it work?
Think about how we see the sky. It is blue due to elastic (specifically Rayleigh) scattering – when an incident photon scatters off a particle without losing any energy. But in about one in a million events, photons interacting with molecules in the atmosphere will be inelastically scattered. This changes the energy of the photon as some of it is taken by the molecule to make it vibrate.
If you shine laser light on a molecule and cause it to vibrate, the photon that is scattered from that molecule will be shifted in energy by a specific amount relating to the molecule’s vibrational mode. Measuring the wavelength of this inelastically scattered light reveals which molecule it was scattered from. This is Raman spectroscopy.
Because most of the time we’re working at room or body temperatures, most of what we observe is Stokes Raman scattering, in which the laser photons lose energy to the molecules. But if a molecule is already vibrating in an excited state (at higher temperature), it can give up energy and shift the laser photon to a higher energy. This anti-Stokes spectrum is much weaker, but can be very useful – as I’ll come back to later.
How are you using Raman spectroscopy for cancer diagnosis?
A cell in the body is basically a nucleus: one set of molecules, surrounded by the cytoplasm: another set of molecules. These molecules change subtlety depending on the phenotype [set of observable characteristics] of the particular cell. If you have a genetic mutation, which is what drives cancer, the cell tends to change its relative expression of proteins, nucleic acids, glycogen and so on.
We can probe these molecules with light, and therefore determine their molecular composition. Cancer diagnostics involves identifying minute changes between the different compositions. Most of our work has been in tissues, but it can also be done in biofluids such as tears, blood plasma or sweat. You build up a molecular fingerprint of the tissue or cell of interest, and then you can compare those fingerprints to identify the disease.
We tend to perform measurements under a microscope and, because Raman scattering is a relatively weak effect, this requires good optical systems. We’re trying to use a single wavelength of light to probe molecules of interest and look for wavelengths that are shifted from that of the laser illumination. Technology improvements have provided holographic filters that remove the incident laser wavelength readily, and less complex systems that enable rapid measurements.
Raman spectroscopy can classify tissue samples removed in cancer surgery, for example. But can you use it to detect cancer without having to remove tissue from the patient?
Absolutely, we’ve developed probes that fit inside an endoscope for diagnosing oesophageal cancer.
Earlier in my career I worked on photodynamic therapy. We would look inside the oesophagus with an endoscope to find disease, then give the patient a phototoxic drug that would target the diseased cells. Shining light on the drug causes it to generate singlet oxygen that kills the cancer cells. But I realized that the light we were using could also be used for diagnosis.
Currently, to find this invisible disease, you have to take many, many biopsies. But our in vivo probes allow us to measure the molecular composition of the oesophageal lining using Raman spectroscopy, to be and determine where to take biopsies from. Oesophageal cancer has a really bad outcome once it’s diagnosed symptomatically, but if you can find the disease early you can deliver effective treatments. That’s what we’re trying to do.
Tiny but mighty (left) A Raman probe protruding from the instrument channel of an endoscope. (right) Oliver Old, consultant surgeon, passing the probe down an endoscope for a study led by the University of Exeter, with the University of Bristol and Gloucestershire Hospitals NHS Foundation Trust as partners. (Courtesy: RaPIDE Team)
The very weak Raman signal, however, causes problems. With a microscope, we can use advanced filters to remove the incident laser wavelength. But sending light down an optical fibre generates unwanted signal, and we also need to remove elastically scattered light from the oesophagus. So we had to put a filter on the end of this tiny 2 mm fibre probe. In addition, we don’t want to collect photons that have travelled a long way through the body, so we needed a confocal system. We built a really complex probe, working in collaboration with John Day at the University of Bristol – it took a long time to optimize the optics and the engineering.
Are there options for diagnosing cancer in places that can’t be accessed via an endoscope?
Yes, we have also developed a smart needle probe that’s currently in trials. We are using this to detect lymphomas – the primary cancer in lymph nodes – in the head and neck, under the armpit and in the groin.
If somebody comes forward with lumps in these areas, they usually have a swollen lymph node, which shows that something is wrong. Most often it’s following an infection and the node hasn’t gone back down in size.
This situation usually requires surgical removal of the node to decide whether cancer is present or not. Instead, we can just insert our needle probe and send light in. By examining the scattered light and measuring its fingerprint we can identify if it’s lymphoma. Indeed, we can actually see what type of cancer it is and where it has come from.
Novel needle Nick Stone demonstrates a prototype Raman needle probe. (Courtesy: Matthew Jones Photography)
Currently, the prototype probe is quite bulky because we are trying to make it low in cost. It has to have a disposable tip, so we can use a new needle each time, and the filters and optics are all in the handpiece.
Are you working on any other projects at the moment?
As people don’t particularly want a needle stuck in them, we are now trying to understand where the photons travel if you just illuminate the body. Red and near-infrared light travel a long way through the body, so we can use near-infrared light to probe photons that have travelled many, many centimetres.
We are doing a study looking at calcifications in a very early breast cancer called ductal carcinoma in situ (DCIS) – it’s a Cancer Research UK Grand Challenge called DCIS PRECISION, and we are just moving on to the in vivo phase.
Calcifications aren’t necessarily a sign of breast cancer – they are mostly benign; but in patients with DCIS, the composition of the calcifications can show how their condition will progress. Mammographic screening is incredibly good at picking up breast cancer, but it’s also incredibly good at detecting calcifications that are not necessarily breast cancer yet. The problem is how to treat these patients, so our aim is to determine whether the calcifications are completely fine or if they require biopsy.
We are using Raman spectroscopy to understand the composition of these calcifications, which are different in patients who are likely to progress onto invasive disease. We can do this in biopsies under a microscope and are now trying to see whether it works using transillumination, where we send near-infrared light through the breast. We could use this to significantly reduce the number of biopsies, or monitor individuals with DCIS over many years.
Light can also be harnessed to treat disease, for example using photodynamic therapy as you mentioned earlier. Another approach is nanoparticle-based photothermal therapy, how does this work?
This is an area I’m really excited about. Nanoscale gold can enhance Raman signals by many orders of magnitude – it’s called surface-enhanced Raman spectroscopy. We can also “label” these nanoparticles by adding functional molecules to their surfaces. We’ve used unlabelled gold nanoparticles to enhance signals from the body and labelled gold to find things.
During that process, we also realized that we can use gold to provide heat. If you shine light on gold at its resonant frequency, it will heat the gold up and can cause cell death. You could easily blow holes in people with a big enough laser and lots of nanoparticles – but we want to do is more subtle. We’re decorating the tiny gold nanoparticles with a label that will tell us their temperature.
By measuring the ratio between Stokes and anti-Stokes scattering signals (which are enhanced by the gold nanoparticles), we can measure the temperature of the gold when it is in the tumour. Then, using light, we can keep the temperature at a suitable level for treatment to optimize the outcome for the patient.
Ideally, we want to use 100 nm gold particles, but that is not something you can simply excrete through the kidneys. So we’ve spent the last five years trying to create nanoconstructs made from 5 nm gold particles that replicate the properties of 100 nm gold, but can be excreted. We haven’t demonstrated this excretion yet, but that’s the process we’re looking at.
This research is part of a project to combine diagnosis and heat treatment into one nanoparticle system – if the Raman spectra indicate cancer, you could then apply light to the nanoparticle to heat and destroy the tumour cells. Can you tell us more about this?
We’ve just completed a five-year programme called Raman Nanotheranostics. The aim is to label our nanoparticles with appropriate antibodies that will help the nanoparticles target different cancer types. This could provide signals that tell us what is or is not present and help decide how to treat the patient.
We have demonstrated the ability to perform treatments in preclinical models, control the temperature and direct the nanoparticles. We haven’t yet achieved a multiplexed approach with all the labels and antibodies that we want. But this is a key step forward and something we’re going to pursue further.
We are also trying to put labels on the gold that will enable us to measure and monitor treatment outcomes. We can use molecules that change in response to pH, or the reactive oxygen species that are present, or other factors. If you want personalized medicine, you need ways to see how the patient reacts to the treatment, how their immune system responds. There’s a whole range of things that will enable us to go beyond just diagnosis and therapy, to actually monitor the treatment and potentially apply a boost if the gold is still there.
Looking to the future, what do you see as the most promising applications of light within healthcare?
Light has always been used for diagnosis: “you look yellow, you’ve got something wrong with your liver”; “you’ve got blue-tinged lips, you must have oxygen depletion”. But it’s getting more and more advanced. I think what’s most encouraging is our ability to measure molecular changes that potentially reveal future outcomes of patients, and individualization of the patient pathway.
But the real breakthrough is what’s on our wrists. We are all walking around with devices that shine light in us – to measure heartbeat, blood oxygenation and so on. There are already Raman spectrometers that sort of size. They’re not good enough for biological measurements yet, but it doesn’t take much of a technology step forward.
I could one day have a chip implanted in my wrist that could do all the things the gold nanoconstructs might do, and my watch could read it out. And this is just Raman – there are a whole host of approaches, such as photoacoustic imaging or optical coherence tomography. Combining different techniques together could provide greater understanding in a much less invasive way than many traditional medical methods. Light will always play a really important role in healthcare.
This article is based on a session at the Institute of Physics’ Celebration of Physics in April 2025.
Dynamic PET imaging is an important preclinical research tool used to visualize real-time functional information in a living animal. Currently, however, the temporal resolution of small-animal PET scanners is on the order of seconds, which is too slow to image blood flow in the heart or track the brain’s neuronal activity. To remedy this, the Imaging Physics Group at the National Institutes for Quantum Science and Technology (QST) in Japan has developed an ultrasensitive small-animal PET scanner that enables sub-second dynamic imaging of a rat.
The limited temporal resolution of conventional preclinical PET scanners stems from their low sensitivity (around 10%), caused by relatively thin detection crystals (10 mm) and a short axial field-of-view (FOV). Thus the QST team built a system based on four-layer, depth-encoding detectors with a total thickness of 30 mm. The scanner has a 325.6 mm-long axial FOV, providing total-body coverage without any bed movement, while a small inner diameter of 155 mm further increases detection efficiency.
“The main application of the total-body small-animal PET (TBS-PET) scanner will be assessment of new radiopharmaceuticals, especially for cardiovascular and neurodegenerative diseases, by providing total-body rodent PET images with sub-second temporal resolution,” first author Han Gyu Kang tells Physics World. “In addition, the scanner will be used for in-beam PET imaging, and single-cell tracking, where ultrahigh sensitivity is required.”
Performance evaluation
The TBS-PET scanner contains six detector rings, each incorporating 10 depth-of-interaction (DOI) detectors. Each DOI detector comprises a four-layer zirconium-doped gadolinium oxyorthosilicate (GSOZ) crystal array (16×16 crystals per layer) and an array of multi-anode photomultiplier tubes. The team selected GSOZ crystals because they have no intrinsic radiation signal, thus enabling low activity PET imaging.
The researchers performed a series of tests to characterize the scanner performance. Measurements of a 68Ge line source at the centre of the FOV showed that the TBS-PET had an energy resolution of 18.4% and a coincidence timing resolution of 7.9 ns.
Imaging a NEMA 22Na point source revealed a peak sensitivity of 45.0% in the 250–750 keV energy window – more than four times that of commercial or laboratory small-animal PET scanners. The system exhibited a uniform spatial resolution of around 2.6 mm across the FOV, thanks to the four-layer DOI information, which effectively reduced the parallax error.
In vivo imaging
Kang and colleagues next obtained in vivo total-body PET images of healthy rats using a single bed position. Static imaging using Na18F and 18F-FDG tracers clearly visualized bone structures and glucose metabolism, respectively, of the entire rat body.
Moving to dynamic imaging, the researchers injected an 18F-FDG bolus into the tail vein of an anesthetized rat for 15 s, followed by a saline injection 15 s after injection. They acquired early-phase dynamic PET data every second until 27 s after injection. To enable sub-second PET imaging, they used custom-written software to subdivide the list-mode data (1 s time frame) into time frames of 0.5 s, 0.25 s and 0.1 s.
Dynamic PET images with a 0.5 s time frame clearly visualized the blood stream from the tail to the heart through the iliac vein and inferior vena cava for the first 2 s, after which the tracer reached the right atrium and right ventricle. At 4.0 s after injection, blood flowed from the left ventricle into the brain via the carotid arteries. The cortex and kidneys were identified 5.5 s after injection. After roughly 17.5 s, the saline peak could be identified in the time-activity curves (TACs).
At 0.25 s temporal resolution, the early-phase images visualized the first pass blood circulation of the rat heart, showing the 18F-FDG bolus flowing from the inferior vena cava to the right ventricle from 2.25 s. The tracer next circulated to the lungs via the pulmonary artery from 2.5 s, and then flowed to the left ventricle from 3.75 s.
The TACs clearly visualized the time dispersion between the right and left ventricles (1.25 s). This value can change for animals with cardiac disease, and the team plans to explore the benefit of fast temporal resolution PET for diagnosing cardiovascular and neurodegenerative diseases.
The researchers conclude that the TBS-PET scanner enables dynamic imaging with a nearly real-time frame rate, visualizing cardiac function and pulmonary circulation of a rat with 0.25 s temporal resolution, a feat that is not possible with conventional small-animal PET scanners.
“One drawback of the TBS-PET scanner is the relatively low spatial resolution of around 2.6 mm, which is limited by the relatively large crystal pitch of 2.85 mm,” says Kang. “To solve this issue, we are now developing a new small-animal PET scanner employing three-layer depth-encoding detectors with 0.8 mm crystal pitch, towards our final goal of sub-millimetre and sub-second temporal resolution PET imaging in rodent models.”
A bumper crop of measurements of the expansion rate of the universe have stretched the Hubble tension as taut as it has ever been, with scientists grappling with trying to find a solution.
Over 500 researchers have come together in the “CosmoVerse” consortium to produce a new white paper that delves into the various cosmological tensions between theory and observation. These include the Hubble tension, which is the bewildering discrepancy in the expansion rate of the universe, referred to as the Hubble constant (H0).
Predictive measurements made by applying the standard model of cosmology to the cosmic microwave background (CMB) give H0 as 67.4 km/s/Mpc. In other words, every volume of space a million parsecs across (one parsec is 3.26 light years) should be expanding by 67.4 kilometres every second.
Yet that’s not what Hubble’s law – which tells us the expansion rate based on a given object’s velocity away from us and its distance – says, as demonstrated by the CosmoVerse White Paper.
“The paper’s been getting a lot of attention in our field,” Joe Jensen of Utah Valley University tells Physics World. “You can easily see that the vast majority of measurements fall around 73 km/s/Mpc, with varying uncertainties.”
There’s no known reason why local measurements of H0 (based on supernovae observations) should differ from the CMB measurement. This discrepancy leads to two possibilities. Either there are unknown systematic uncertainties in measurements that skew the results, or cosmology’s standard model is wrong and new physics is needed.
A lot at stake
The highest rung on the cosmic distance ladder is a type Ia supernova – a white dwarf explosion. They have a standardizable brightness that makes them perfect for judging how far away they are, based on their luminosity curve. These measurements are calibrated by lower rungs on the ladder, such as Cepheid variable stars or the peak brightness of red giant stars (referred to as the “tip of the red giant branch”, or TRGB).
If the tension is real, then different calibrators should still give the same result. One of the few outliers is found in a new paper published in The Astrophysical Journal by the Chicago–Carnegie Hubble Program (CCHP) led by the University of Chicago’s Wendy Freedman.
CCHP’s latest paper uses the TRGB to arrive at a best value of 70.39 km/s/Mpc when combining measurements from the James Webb Space Telescope (JWST) – which is able to better resolve red giant stars in other galaxies – with Hubble Space Telescope data.
The CCHP team argue that this result is in line with the CMB measurements and removes the tension. However, their conclusion has met opposition.
“Their result is sort of in the middle of the Hubble tension, so I’m surprised that they would say they rule it out,” Dan Scolnic, an astrophysicist at Duke University in the United States, tells Physics World.
At a meeting of the American Astronomical Society in January 2025, Scolnic declared that the Hubble tension was now a crisis. CCHP’s results do not dissuade him from this conclusion.
“For some reason they don’t include a number of supernovae in their sample that they could have,” says Scolnic. “Siyang Li [of Johns Hopkins University] led a paper [on which Scolnic is a co-author] that showed that if one uses their TRGB measurements, and the complete sample of supernovae, one goes back to higher H0.”
Freedman did not respond to Physics World‘s request for an interview.
Different approaches
Jensen has also led a team that recently conducted measurements of H0 using TRGB stars, but in a different way by looking for surface brightness fluctuations (SBF).
“SBF is a statistical method that measures the brightnesses of red giant stars even when they cannot be measured individually,” says Jensen.
Individual stars in galaxies cannot be resolved at great distance – their light blends together, and the more distant the galaxy, the smoother this blend is. We describe this blended light as the galaxy’s surface brightness, and fluctuations are statistical in nature and result from the discrete nature of stars.
In old elliptical galaxies, the surface brightness is dominated by red giant stars, which are evolved Sun-like stars. Measuring the SBF therefore provides a value for the TRGB, from which a distance can be determined.
Using JWST images to measure the SBF of 14 elliptical galaxies, then using those to calibrate the distances to 60 more distant ellipticals, and then using that calibration to determine H0, Jensen’s team arrived at a value of 73.8 km/s/Mpc.
“The reason that we don’t get the same answer [as CCHP] is that we are not using the same JWST calibrators, and we don’t use type Ia to measure H0,” says Jensen.
This contradicts CCHP’s main assertion, which is that there must be unknown systematic uncertainties in either the type Ia supernovae or the Cepheids. Jensen’s team use neither, yet still find a tension.
Perhaps the most convincing evidence for the tension comes from the TDCOSMO (time-delay cosmography) team, who utilize gravitationally lensed quasars to measure H0.
Quasars fluctuate in brightness over a matter of days. When light from a quasar takes paths of varying lengths around a lensing object, it produces multiple images that have time lags relative to one another. The expansion of space can extend this time delay, providing a completely independent measure of H0.
In 2019 the H0LiCOW project used six gravitational lenses to arrive at a value of 73.3 km/s/Mpc. This result came with some scepticism. So they formed the new TDCOSMO consortium and “went on a six-year journey to see if their original measurement was okay,” says Scolnic.
TDCOSMO’s final conclusion is 72.1 km/Mpc/s, strongly supporting the tension. However, in all these measurements there’s wriggle room from various known measuring uncertainties.
“It’s important to remember that the uncertainties put us in only mild disagreement,” says Jensen. “I expect that we will soon know if the disagreement can be explained by the mundane choices of calibration galaxies and processing techniques.”
If it cannot, then the inescapable conclusion is that there’s something wrong with our understanding of the universe. Figuring that out could be the next great quest in cosmology.
