Physics World

Ask me anything: Tania Kendrick – ‘There’s something really satisfying when everything starts to click’

Tania Kendrik studied physics at the University of Bristol in the UK before joining the Civil Service fast-stream development programme, which saw her have postings at the UK’s Ministry of Defence, the Department for Environment, Food and Rural Affairs, and the Cabinet Office. After completing the scheme, she decided to stay in the Cabinet Office, where she now works full-time on relations between the UK and the European Union.

What skills do you use every day in your job?

One key skill I use every day is communication. A large part of my job involves taking complex and detailed information, understanding what is most important, and communicating it clearly and concisely to different audiences. This can vary from briefing senior colleagues and ministers, where I need to distil a lot of information into the key points they need to know to make a decision, to producing guidance and written products that need to be clear and accessible to a wide range of stakeholders. Being able to adapt how I communicate depending on the audience, and being able to do this quickly, while making sure the key messages are understood, is vital.

Although a physics degree and a career in policy may not seem to have an obvious connection, I constantly use the analytical skills I developed at university. I need to look at complex information and evidence, identify the key issues, and consider how different factors should feed into decision-making. Then I need to make sure that recommendations and advice are grounded in evidence, while also considering different perspectives, potential risks and the wider context. Being able to analyse information critically – and use it to support informed, evidence-based decision-making – is crucial.

What do you like best and least about your job?

What I love most about working in the Civil Service is that I’m constantly learning something new. Policy covers such a wide range of issues, and I really enjoy the challenge of getting to grips with a new topic, understanding how it works, and gradually seeing the bigger picture. There’s something really satisfying about that moment when everything starts to click and you understand why something is done in a particular way, or realize there are considerations you would never have thought about when you started. I like that no two days are exactly the same and that there is always something new to learn.

What I like least about my job, which is something I’m still getting to grips with, is finding time to properly switch off. Policy development can take time, and it’s easy to find yourself thinking about a problem outside work, such as trying to devise a potential solution or wondering if there is a better way to frame something. It’s really important to build things into your routine that help you switch off. For me, that means setting clear boundaries, exercising straight after work, particularly running and playing hockey, and making time to catch up with friends.

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

Something I wish I could tell my past self, especially throughout university and coming up to graduation, is not to worry so much about having everything figured out. I focused too much on trying to work out my exact “dream” physics job. But looking back, I realize I didn’t necessarily need a clear plan. As a teacher once said to me, studying physics opens so many doors. You develop skills that can take you into a wide range of careers from finance and technology to consulting and, of course, the Civil Service. So my advice to others would be: you don’t need to know exactly what you want to do straight away. Try things, get some work experience, speak to people in different careers, and be open to opportunities you might not have considered. The world really is your oyster.

Quantum physicist Ignacio Cirac awarded 2026 Isaac Newton Medal and Prize

The Spanish quantum physicist Ignacio Cirac from the Max Plank Institute of Quantum Optics in Garching, Germany, has won the 2026 Isaac Newton Medal and Prize for his “pioneering contributions that founded modern quantum information science, uniting profound theoretical insight, experimental vision, and exceptional scientific leadership shaping quantum physics and technology worldwide”. Presented by the Institute of Physics (IOP), which publishes Physics World, the international award is given annually for “world-leading contributions to physics by an individual of any nationality”.

Born in 1965 in Barcelona, Spain, Cirac earned a PhD in physics from the Complutense University of Madrid in 1991. After a stint at the University of Castilla-La Mancha, he then moved to Leopold-Franzens University of Innsbruck in 1996. In 2001 Cirac became director of the Max Planck Institute of Quantum Optics the US.

Cirac is best known for his work in the 1990s in which he co-developed a proposal for building a quantum computer using trapped ions with Peter Zoller from the University of Colorado at Boulder.

The “Cirac-Zoller” proposal provided an experimentally plausible route from the theoretical idea of a quantum computer to an actual machine with trapped ions remaining one of the leading quantum-computing platforms today.

He also showed how controlled quantum many-body systems can act as simulators to solve difficult problems in physics, chemistry, and materials science.

Cirac and collaborators also pioneered long-distance quantum communication, which included the idea of a quantum repeater that offers a way of distributing entanglement across distances despite losses and imperfections.

Cirac told Physics World that he is “deeply honoured” to receive the award. “I’m very thankful to the people who have made this prize possible – it’s a team working for many years,” says Cirac. “I am immensely grateful to all of them for the privilege of having worked together and for everything we have accomplished along the way.”

Professor Cirac’s pioneering work has helped shape modern quantum information science
IOP president Paul Howarth

Cirac has many other honours to his name including the Wolf Prize in Physics in 2013, the Max Planck Medal in 2018 as well as the inaugural ICTP-IBM Richard Feynman Prize in Quantum Computing.

IOP president Paul Howarth offered his congratulations to Cirac noting that the Spanish physicist is “hugely deserving” of the award and that his influence can be seen not only in the development of quantum science, but “in the generation of researchers who have followed”.

“Professor Cirac’s pioneering work has helped shape modern quantum information science, providing crucial theoretical foundations for advances in quantum computing, simulation and communications,” adds Howarth. “His achievements demonstrate how fundamental physics can expand our understanding of the world while opening up new possibilities for technology.”

Celebrating success

Cirac’s honour forms part of the IOP’s wider 2026 awards, which recognize everyone from early-career scientists and teachers to technicians and subject specialists. Other winners include the school engagement programme Orbyts, which received the Lawrence Bragg Medal and Prize for “outstanding work empowering girls and students from socioeconomically disadvantaged backgrounds in physics education through cutting-edge research partnerships, and supporting physics teachers with novel teaching materials”.

Theoretical physicist Michael Berry wins 2025 Isaac Newton Medal and Prize

Sarah Bohndiek from the University of Cambridge has been awarded the Rosalind Franklin Medal and Prize for “pioneering the use of light and sound in early disease detection, with a focus on clinical translation and equitable healthcare outcomes”.

Natalie Lines, a PhD student at the University of Portsmouth, meanwhile, has won the Jocelyn Bell Burnell Medal and Prize for “exceptional early-career contributions to observational cosmology and the application of machine learning to large-scale astronomical surveys, with particular distinction in the field of strong gravitational lensing”.

The full list of 2026 award winners is available here.

Scottish artist marks 100 years of Schrödinger equation with new installation

Scottish artist Gregor Harvie has created a new artwork to mark 100 years since Erwin Schrödinger published his famous equation that governs how the wave function of a physical system changes over time.

Dubbed Superposition, the piece has been created in collaboration with the quantum theory group at the University of Glasgow.

The installation explores two of features of quantum physics: superposition and entanglement.

It does so via 16 aluminium panels that each have hundreds of holes drilled through them, forming complex patterns.

One side is white while the other is coloured and when someone approaches, the panels begin to spin at 400rpm so that their forms “dissolve into shifting three-dimensional illusions”.

The panels are arranged in eight pairs and the stopping positions of the panels are random but over time seven pairs show the same side as their partner half the time. This is an analogy for superposition, where the spinning stops is akin to the collapse of the wavefunction.

Yet the eighth pair is different. Individually, its two panels still appear to stop randomly, but when seen together they show the same side three quarters of the time – an analogy for entanglement.

Harvie, who has a PhD in computational fluid dynamics and previous work Beyond Absolute Zero marked the bicentenary of the birth of Lord Kelvin in 2024, says the artwork puts people inside the quantum universe.

Superposition artwork — The panels are arranged in eight pairs (courtesy: Gregor Harvie)

“I wanted to make something you could stand in front of and genuinely not know what you are looking at,” notes Harvie. “The artworks are made of holes, so in a way there is nothing there, but spun fast enough they transform into something remarkable, something the eye can’t quite hold onto. Their form changes depending on where you are looking at them from, and one pair behaves in a way that defies logic.”

Theoretical physicist Stephen Barnett at the University of Glasgow says the artwork allows everyone to experience the “strangeness” of quantum mechanics and to question what they are seeing. “Quantum physics challenges our most basic ideas about what it means for something to have a definite state,” he says. “It tells us reality is stranger than anything we encounter in everyday life.”

If you want to experience the artwork for yourself, Superposition will be displayed at the Technology and Innovation Centre at the University of Strathclyde during weekdays from 20 to 29 October.

Quiz of the week: which star could help astronomers measure the spin of Sagittarius A*?

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Francis Halzen and IceCube: why neutrinos keep attracting Nobel prizes

The 2026 Nobel Prize for Physics has gone to Francis Halzen for his “decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin”.

In this episode of the Physics World Weekly podcast I am joined by the University of Oxford neutrino physicist Kirsty Duffy and Physics World’s Matin Durrani to talk about this year’s prize. This is the fifth time a Nobel prize has been awarded for work on neutrinos, and we explore why physicists are so enamoured with these elusive particles.

In this podcast we refer to my video conversation with Francis Halzen and his colleague James Roth and to Halzen’s book Quarks & Leptons. Kirsty Duffy presents the Even Bananas series of videos about neutrinos.

This podcast is supported by American Elements, the world’s leading manufacturer of engineered and advanced materials. The company’s ability to scale laboratory breakthroughs to industrial production has contributed to many of the most significant technological advancements since 1990 – including LED lighting, smartphones, and electric vehicles.

Optogenetic therapy shows promise for vision restoration

Retinitis pigmentosa is an inherited degenerative condition that destroys photoreceptors in the retina and can eventually lead to irreversible blindness. In a clinical trial first, an international research team has demonstrated that optogenetic therapy is safe to use and shows potential for restoring visual function in patients with late stages of this disorder.

In a first-in-human study in 2021, the collaboration – led by José-Alain Sahel from the University of Pittsburgh School of Medicine and Botond Roska from the Institute of Molecular and Clinical Ophthalmology Basel (IOB) – demonstrated that optogenetic therapy led to partial recovery of visual function in a blind patient with retinitis pigmentosa. This latest work, reported in the New England Journal of Medicine, expands the cohort to 10 participants.

Optogenetics is a method that uses light to characterize and manipulate the activity of neurons in the brain – and which earlier this week won its inventors the 2026 Nobel Prize in Physiology or Medicine. The optogenetic therapy developed by Sahel, Roska and collaborators combines gene therapy that makes surviving ganglion cells (neurons in the retina) sensitive to light with special goggles that stimulate the altered cells.

“Although patients with retinitis pigmentosa lose their rod and cone cells, which normally detect light, many other cells within the retina can remain present for years after vision loss,” explains Sahel, director of the UPMC Vision Institute and first author on this study. “Optogenetics enables us to introduce a light-sensitive protein into these surviving cells, allowing them to respond to light and transmit visual information to the brain. The therapy bypasses the damaged photoreceptors and takes advantage of retinal circuits that remain intact.”

Sahel notes that retinitis pigmentosa can be caused by mutations in any one of hundreds of different genes, making optogenetic therapy a particularly suitable approach as it can potentially help regardless of the underlying mutation.

Safe to use

In the study, 10 participants with blindness due to advanced retinitis pigmentosa were injected in their worse-seeing eye with a gene that produces ChrimsonR – a light-sensitive protein with a peak response at around 590 nm (amber). The team divided the subjects into three dose-escalation groups, receiving 5.0 x 10¹⁰, 1.5 x 10¹¹ or 5.0 x 10¹¹ vector genomes per eye.

The primary aim was to evaluate the safety of the optogenetic therapy. The researchers observed 33 mild and moderate adverse ocular events in nine of the participants, such as temporary inflammation and increases in eye pressure, plus one severe event, which resolved within minutes after treatment. They concluded that, within the limits of the study, the treatment was safe.

**Visual stimulation system** The specialized goggles (centre) send visual information to a processor (upper left) that converts the information into patterns of light. A projector within the glasses (lower left) sends these patterns back to the eye. Lower right: the system converts the image of a hand into a simplified light pattern that’s projected onto the retina. (Courtesy: Chloé Pagot, Streetlab, Paris)

Using the light-stimulating goggles, they then assessed the visual performance of treated eyes, before and after injection. The goggles contain a camera that detects changes in light intensities pixel-by-pixel and converts this visual data into pulses of amber light. This light is projected onto the eye to activate ChrimsonR in the modified retinal ganglion cells. Vision tests revealed that the treatment increased light sensitivity in seven of the 10 participants, by a factor of 2.0 to 62.3, with six patients showing a clinically meaningful improvement.

The patients also performed a series of tasks to detect, localize and touch a notebook and a staple-box. In the notebook task, five of eight participants had higher accuracy when wearing the goggles than when not wearing them, while four showed improved accuracy in the staple-box task. In a task determining the orientation of a single bar, accuracy was higher with the goggles in five of eight participants.

To provide an objective measurement of brain activity in response to light, the researchers obtained electroencephalographic (EEG) recordings in five patients performing two visual tests: detection of a tumbler placed in front of them (with the eyes open or closed); and passive viewing of periodic visual stimuli.

In the tumbler test, EEG decoding accuracy was higher with goggles than without when the eyes were open, but not when they were closed – implying a genuine improvement in visual processing rather than an artefact. The second test also showed increased decoding accuracy, which correlated with mean improvements in the notebook and staple-box tasks.

**Visual behavioural test** After receiving optogenetic therapy, participants used a visual-stimulation system to complete tasks. Here, a participant reaches for a notebook. (Courtesy: Chloé Pagot, Streetlab, Paris)

A bright future

The changes in visual performance were consistent across multiple tasks and testing sessions, with responses observed in all three dose groups. Participants who spent longer being trained to use the goggles performed better in the visual behavioural tests. The researchers point out, however, that functional changes were modest: improving object detection and discrimination capabilities but not restoring normal sight. The small sample size also limited their ability to characterize treatment response.

Several projects are currently underway to further enhance the technology. This includes digital holographic goggles with an eye tracker being developed by the team of Valentina Emiliani at the Institut de la Vision in Paris. The system will project 20 µm-wide images that stimulate individual cells in the retina.

“With the eye tracker, we can make sure we are projecting exactly onto the area of the retina that we want. We’ve already developed a prototype, and it is moving forward,” says Sahel.

Elsewhere, University of Pittsburgh scientists are working with Roska’s group to develop cell-specific gene therapy vectors. “There are different types of cells: some respond to the onset of light, some to the offset of light, some to movement and direction,” Sahel explains. “We are trying to selectively stimulate the ‘on’ and the ‘off’, and discriminate the signal we are sending.”

“We are optimistic about advancing this approach toward future clinical trials,” he tells Physics World.

Physics and wildlife conservation: a slide puzzle

Image courtesy: Shutterstock/Jonas Aleksander Larsen

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Disordered environments around tumour cells can promote cancer spread

A new study showing how tumour cells behave when surrounded by a disordered microenvironment could help cancer researchers by providing a biophysics model describing how cancers spread within the human body.

Researchers led by Michael Sixt, head of the Cellular Morphodynamics Group and Edouard Hannezo, leader of the Physical Principles in Biological Systems Group both at the Institute of Science and Technology Austria (ISTA), in collaboration with scientists from the UK’s Francis Crick Institute, have constructed a microfluidic chip experiment and associated computer simulation that shows how heterogeneity (disorder) surrounding a tumour can drive metastasis.

Cancers spread when individual cells detach from the tumour cell collective and invade previously healthy tissue. Whether detachment occurs depends on both genetics and the local microenvironment – including blood vessels, immune cells, signalling molecules and connective tissue. To replicate cancer cells squeezing through pores between the fibres of the tissues they are invading, the researchers created forests of pillars mounted on microfluidic devices for tumour cells to squash past.

As detailed in their recent Science Advances paper, the geometry for some forests was a square lattice of 9 µm-diameter pillars evenly spaced with 9 µm gaps between them, while other forests had their pillars arranged in a disorderly pattern created by moving each pillar from its ordered position by a random angle and distance.

“There were no suitable tools for designing mathematically defined disorder. So I had to write completely new software that can define those patterns, and then translate that into designs that can be manufactured [via standard semiconductor lithography techniques] into microfluidic devices,” explains biotech data science consultant Saren Tasciyan, who designed the microfluidic devices as part of his PhD with the Sixt Group.

The team used a combination of fluorescence imaging and light microscopy to track the behaviour of cancer cells – kept alive for up to eight days thanks to cell culture medium – after they were introduced into the centre of each respective pillar forest. These experiments revealed that the tumour cell collective was more likely to break apart when faced with a heterogeneous environment to navigate compared with a more regular geometry.

In the heterogeneous conditions “we found that the first cells detach, exactly as happens in a cancer metastasis,” says Sixt. The team also observed the pillars increasingly rough up the surface of the spreading cell interface, resulting in finger-like protrusions – from whose tips the cells were more likely to detach.

“When the cell interface moves through this landscape the effects accumulate over time,” continues Sixt, who asked his colleague Hannezo to create computer simulations to verify the experimental results, and study the process over longer periods than the cells could survive.

“Each cell was modelled as a bead that moves through an environment,” says Hannezo group postdoc Zuzana Dunajová, a computational scientist specializing in biological physics, who built the model. These beads are attracted to one another, and move through different geometries that mimic those on the microfluidic devices, she explains.

“I kept seeing the same behaviour in my simulations – the beads detached from the collective more often in the disordered environment compared to the ordered. This was very exciting from the physics perspective because it’s not something that we see just in this particular cell type or system, but suggests a more general principle in living, or active systems. The roughness of the cancer-cell invasion actually follows the same kind of universal behaviour as we see in other systems, such as combustion of paper, spreading of fires or drying coffee drops,” enthuses Dunajová.

Next, Sixt – who hopes these insights will help researchers developing new cancer therapies – plans to study whether cell detachment in a disordered microenvironment leads to epigenetic modifications (heritable changes that do not alter the DNA code) and then later on to genetic modifications. “I want to consider whether the detachment process can drive the single cell evolution into something malignant,” Sixt concludes.

Fastest star in the Milky Way feels the rotation of our central black hole

At the centre of our galaxy lies a supermassive black hole, Sagittarius A* (Sgr A*), that’s more than four million times the mass of the Sun. Astronomers have now discovered the fastest-known star in the Milky Way zooming around this black hole in an extremely close orbit. Observations of this star, known as S301, could shed new light on the spin of Sgr A*.

Black holes are extremely large concentrations of matter condensed into very small regions of space. As a black hole spins, it drags the surrounding space–time around it, a phenomenon known as frame dragging or the Lense-Thirring effect. This frame dragging creates a cosmic whirlpool, twisting the fabric of space–time and warping the orbits of nearby stars. However, unless the orbiting star reaches very close proximity, this subtle effect is difficult to measure.

“Our group has determined the mass of the central black hole down to sub-percent precision,” says team member Felix Mang, a PhD student at Max Planck Institute for Extraterrestrial Physics (MPE) and author of the study published in Nature. “A black hole is characterized exclusively by its mass, spin, or angular momentum and electric charge; now that we’ve measured the mass really, really accurately, the next step is to measure the spin.”

Probing Sgr A*

Astronomers can measure black hole spin through various methods, including X-ray reflection spectra and the thermal X-ray emission of the gas disk surrounding stellar-mass black holes, as well as gravitational-wave signatures from merging black hole binaries.

Lying in the galactic centre approximately 27,000 light-years from Earth, Sgr A* offers another direct way to measure its spin. There are a few dozen stars whirling around the massive black hole on nearly Keplerian orbits. The stars that come closest to Sgr A* most effectively probe its spin as they reach into the deepest parts of the gravitational potential, altering their trajectories.

For example, the star S2 is on a 16-year orbit and has been meticulously tracked using the GRAVITY instrument on the European Southern Observatory’s Very Large Telescope Interferometer. GRAVITY’s emergent near-infrared interferometry enabled the detection of strong relativistic effects during S2’s close approach in 2018, including a 12-arcminute orbital precession consistent with the Schwarzschild metric derived from Einstein’s general theory of relativity.

Despite these measurable relativistic effects, even at its pericentre when it is closest to Sgr A*, S2 remains too far away to detect the black hole’s spin without prohibitively long years of observation.

Discovery of S301

Although S2 is not a suitable candidate for Sgr A* spin measurements, the newly discovered star S301 is an ideal cosmic probe. Mang notes that the star is “a bit heavier than the Sun, and it has a radius which is a bit bigger; but overall, it’s not that special”. S301 is under 1.5 solar masses with a radius about 1.4 to 1.6 times that of the Sun, and it is too faint to be one of the giant stars in the galactic centre. Although the main-sequence star S301 may seem fairly ordinary, its extremely eccentric orbit and very close approach to the supermassive black hole are remarkable.

“We found the star in Spring 2023,” recalls Mang. Upon detection, the team could not conclude whether it was a foreground or a background star. However, dedicated follow-up observations in 2024 revealed a pronounced acceleration in S301’s proper motion, or apparent angular change of position on the sky. “It was curving away from a linear trajectory, which gave us a hint that it may be bound to the black hole,” Mang explains. Continued observations in 2025 allowed the team to create a robust initial model of S301’s orbit, revealing it to be “indeed a very eccentric and tightly orbiting star, which was then later on confirmed”.

Armed with evidence that the star might be gravitationally bound to Sgr A*, the team looked to the past, hoping to lock down S301’s orbit. Combing through archival data, the astronomers searched for traces of the star before its close approach in 2023. As Mang notes, “getting these two pre-peri positions in 2021 and 2017 was rather challenging, [we were] digging through the data”. The meticulous hunting paid off and revealed a strong trace of S301 in 2021 and a weaker signature back in 2017.

With 19 distinct positions found across nearly a decade of data, the team outlined a clear elliptical orbit, revealing that the star had zoomed past the supermassive black hole at its closest approach in early 2023 and passed significantly closer to the black hole than S2 ever has.

Extreme path of S301

The team determined that S301 boasts an orbital period of just 8.7 years, making it the shortest known orbital period around Sgr A*. This breaks the previous record, held by the star S55, which takes about 12 years to complete its path.

But S301’s orbital period is only part of what makes its path extraordinary. With an eccentricity of about 0.98, S301’s elongated path brings the star to a pericentre distance of approximately 1.78 billion kilometres, roughly 10 times smaller than that of S2. At that closest approach, S301 speeds around at about 25,000 km/s, making it the fastest known star in the Milky Way.

Because S301 gets so close to the supermassive black hole, general relativity strongly affects its orbit. In particular, S301 experiences a phenomenon known as the Schwarzschild precession, which shifts the star’s orbit by about 2° per revolution. Over the course of roughly 1500–1600 years, those slight shifts accumulate into a full 360° rotation of the orbit.

While this dramatic shift is driven by the colossal mass of Sgr A*, S301’s extreme orbit is also subjected to a subtler spin-induced effect: the Lense-Thirring precession. As Sgr A* spins, the twisted space–time forces S301’s orbital plane to slowly wobble, making it a powerful probe of the curved space–time around Sgr A*. By precisely tracking the star’s motion, astronomers can examine how its trajectory is shifted by these two precession effects, thereby enabling them to measure the spin of the supermassive black hole.

Looking forward, the upcoming years should bring substantially better data. “The instrument we are observing with, GRAVITY, has undergone some major upgrades, which gives us more flux, and we also expect to get better astrometric precision,” Mang says. “Along with that, we expect the Extremely Large Telescope, and one of its first light instruments, MICADO, to be operational by roughly 2030.”

These advances will allow astronomers to trace the motion of S301 with incredible precision. As a result, the team is confident that these instruments will decode the mystery of Sgr A*’s spin. “Combining the astrometry or proper motion measured with GRAVITY on the sky of that star, and then having a perception of the velocity of that star in the plane or out of the plane of the sky with MICADO, will allow the measurement of SgrA*’s spin, probably within the next 10 years,” Mang explains.

Nuclear clocks finally see the light

Two independent teams have realized the first clocks based on transitions within atomic nuclei. The new nuclear clocks represent the culmination of more than 20 years of research and could become vital tools in precision tests of fundamental physics.

Today’s most accurate clocks “tick” whenever an electron transitions between two specific hyperfine levels in the ground state of an atom. These atomic clocks form a vital part of technologies such as satellite navigation and telecommunications networks. They also contribute to fundamental research in dark matter searches and gravitational wave detection.

Traditionally, the hyperfine levels used in atomic clocks are separated by gaps with frequencies in the microwave range. However, more modern atomic clocks use higher-frequency optical transitions. Known as optical clocks, they are more precise because they “tick” more often within a given period, enabling finer measurements of time passing.

Clocks based on atomic nuclei could, in principle, be more precise still. First proposed in the late 1990s, they are less prone to disturbances from electromagnetic fields, and they exploit transitions at even higher (ultraviolet) frequencies. Getting such clocks to work, however, proved no easy feat, because driving these narrow nuclear transitions requires highly stable lasers with extremely small bandwidths.

It’s about time

Two independent teams are now reporting a breakthrough in this field. Writing in Nature, they describe realizing working nuclear clocks based on transitions in nuclei of thorium-229.

Members of the first team, which is led by Ekkehard Peik at the Physikalisch-Technische Bundesanstalt (PTB) in Germany and Thorsten Schumm from TU Wien in Austria and includes colleagues in Germany, Austria and Czechia, had previously made several key advances. Peik and Schumm’s group were the first to demonstrate laser excitation of the thorium-229 nuclear transition. They were also the first to embed thorium-229 within crystals of calcium fluoride (CaF₂), creating a solid-state device which, unlike (optical) atomic clocks, does not use trapped atoms or ions.

The second team, led by Shiqian Ding from Tsinghua University and including collaborators at 13 other institutions across China, has likewise chalked up notable achievements, including a demonstration of the first continuous-wave narrow-linewidth UV laser at the crucial wavelength of 148.4 nm. Because China has limited access to thorium-229, Ding and colleagues also developed an approach for growing small single crystals that requires only 1.4 μg of the isotope.

Running like clockwork

In the latest work, both teams used thorium-229 embedded in millimetre-sized CaF₂ crystals and driven by narrow-linewidth, continuous-wave UV lasers, although the nonlinear processes used to generate the laser light were different. They also both used direct absorption spectroscopy through the crystal to monitor how well they were exciting the nuclear resonance, plus a feedback loop to lock the laser frequency onto the desired transition. They thus created stabilized nuclear clocks that they could compare continuously to frequency standards derived from atomic clocks.

At this point, the teams’ efforts diverged. Whereas Schumm and colleagues were able to embed more thorium-229 into their CaF₂ crystals, Ding and colleagues generated laser light at higher powers. Both features tend to produce more stable clocks with a higher signal-to-noise ratio, although the fractional frequency instability in the Tsinghua device was lower, at 5 x 10^-13/√τ/s (where τ is the averaging time), compared to 3 x 10^-12/√τ/s in the PTB-TU Wien collaboration.

Ekkehard Peik standing next to experimental equipment. He has a fringe of white hair and is wearing a striped short-sleeve shirt, thick-rimmed glasses and a black wristwatch — Promising transition: Ekkehard Peik with the nuclear clock experiment at PTB. In 2003, Peik and his then-colleague Christian Tamm identified a transition in the radioactive isotope thorium-229m as a promising basis for a nuclear clock. (Courtesy: PTB)

The PTB-TU Wien team applied their nuclear clock to fundamental physics, using it to search for ultralight scalar dark matter couplings and constrain fluctuations/drifts in the nuclear transition energy. Their results set limits on how dark matter can couple to photons and the strong nuclear force that are comparable to those derived from today’s best atomic clocks.

The primary focus of Ding and colleagues’ work was on technical aspects such as high-power vacuum ultraviolet (VUV) interrogation, clock performance, inter-crystal reproducibility and metrological consistency. The clock transition frequencies they measured in two independently fabricated crystals agreed both with each other and with previous measurements of the thorium-229 transition made using VUV frequency combs at JILA in the US.

Heading for higher precision

Peik says that he and his team are now working on boosting their laser power, improving the signal-to-noise ratio in their detectors, and studying the structure of the thorium dopant in their crystal to understand what determines the linewidth of the nuclear resonance within it. “We have now seen 30 kHz linewidth, but the natural linewidth should be way below 1 Hz, which would enable more stable clock operation,” he tells Physics World.

Ding says that an important next step for his team is to develop better host crystals than CaF₂, which could lead to a substantially narrower nuclear transition and improve both stability and accuracy. He is also interested in creating a trapped-ion thorium nuclear clock that would be more isolated from its environment and could re-use many of the quantum control techniques developed for atomic and ion clocks. “It is technically much more difficult, but I think this route may ultimately provide the highest accuracy,” he says.

Victor Flambaum, a theorist from the University of New South Wales, Australia who was not involved in either project, calls the teams’ devices the “first prototypes of nuclear clocks”. He is excited by their potential for uncovering new physics effects, such as those produced by dark matter, which are expected to be very small. “We have shown that in nuclear clocks these effects are strongly enhanced,” he says.

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