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.

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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One does not simply walk into the Trinity test site. Although the radioactivity from the world’s first atomic explosion has mostly dissipated over the past 81 years, the gates to this real-life Mordor are still guarded by military police, and the US Army continues to use it as a missile range.

Members of the public are allowed in just once a year, on the third Saturday in October, between 8 a.m. and 3.30 p.m. Apart from these tightly controlled “open house” events, the site where Manhattan Project participants gathered on 16 July 1945 to witness the dawn of the nuclear age is strictly off-limits.

For fans of atomic history who lack the time, money or inclination to visit this inhospitable corner of New Mexico during its annual open house, Trinity: an Illustrated History of the World’s First Atomic Test is the next best thing. In some respects, this coffee-table-sized book by Emily Seyl and her colleagues at the Los Alamos National Laboratory’s National Security Research Center (NSRC) may even be better.

As the book makes clear, there is not much to see at Trinity nowadays. A few dilapidated buildings; a commemorative obelisk; a fragment of a half-million-pound steel tank – these are all that remain of the once-bustling site where the first atomic device, known as the Gadget, was assembled and tested.

Trinity’s great achievement is to turn back the clock and make both the site and the individuals who worked there come alive. Using more than 200 photos from the test’s official archive and other sources, Seyl and her colleagues have created an important public record of a period that is fast receding from living memory and a place that is still veiled in secrecy. The result is a book that is by turns illuminating, beautiful and disturbing.

A fresh look at familiar history

Let’s start with the illuminating part. As an undergraduate intern at Los Alamos, I read Richard Rhodes’ seminal book The Making of the Atomic Bomb. Since then, I’ve read (and occasionally reviewed for this magazine) numerous other works of atomic history, from Ray Monk’s 800-page biography of the Manhattan Project’s scientific leader, J Robert Oppenheimer, to Denise Kiernan’s The Girls of Atomic City and Kate Brown’s Plutopia, both of which focus on less well-known figures.

Even so, once I got past Trinity’s introduction – which takes just six pages to cover Lise Meitner and Otto Frisch’s 1938 realizations about nuclear fission; the start of the Second World War; Enrico Fermi’s demonstration of the first nuclear chain reaction; and the launch of the Manhattan Project to weaponize these discoveries and thereby secure an Allied victory – I was into material new to me.

That steel tank, for example. Its name is Jumbo, it cost $12m in 1940s money, and it was originally designed to withstand the conventional explosives that imploded the Gadget’s plutonium core. If the explosives worked, but the chain reaction didn’t (a definite possibility – hence the need to test the Gadget’s design before deploying it in a bomb), the idea was that Jumbo would prevent plutonium from spraying all over the site.

So far, so sensible. In early 1944, Manhattan Project engineers duly drew up specifications for a containment vessel. An Ohio-based manufacturer, Babcock & Wilcox, agreed to build it, though for security reasons no-one told them what this mysterious “accumulator” was for. The finished Jumbo was then transported, under wraps, to New Mexico by heavy-duty freight train, offloaded onto a specially built carriage and wheeled to a location 800 yards from the intended ground zero.

Which is where it stayed. By the time it arrived in spring 1945, scientists working at the Manhattan Project’s nerve centre in Los Alamos, nine hours’ drive north of Trinity, had grown confident enough in the Gadget that Jumbo was deemed surplus to requirements, more likely to confuse diagnostic tests than to provide any useful safety function.

The fact that technicians had spent days hauling Jumbo across the desert is an excellent example of how the Manhattan Project combined herculean effort and ingenious engineering with tremendous cost and institutionally stupid bureaucracy

The fact that their technician colleagues had spent days hauling it across the desert is thus an excellent example of how the Manhattan Project combined herculean effort and ingenious engineering with tremendous cost and institutionally stupid bureaucracy. Despite this, I do not recall reading about Jumbo before. It’s to Seyl’s credit that she and her NSRC managers granted this potentially embarrassing episode so many pages of gorgeous black-and-white photos.

Because make no mistake, Trinity is gorgeous. I love the New Mexico desert, but you don’t need to be a fan of this austere landscape to appreciate these photographs of the people who toiled there and the machines they built. Though Oppenheimer and other well-known physicists appear in several photos, much of the book focuses on the contributions of relative unknowns such as Jack Aeby, a technician in the Army’s Special Engineer Detachment.

This organization existed to divert soldiers with technical training away from battlefields and into atomic grunt work, and Aeby, an amateur photographer, asked his group leader (the physicist Emilio Segrè) if he could document the explosion with his personal camera. Due to technical problems with the official Kodak cameras, Aeby’s snapshot is now the only decent colour image we have of the Trinity test.

My favourite photo in the book is of a chemist called Donald Hornig. As the inventor of the X unit – a heavy-duty switch that triggered the Gadget’s detonators – Hornig had the unenviable job of arming this switch a few hours before the test, with a summer lightning storm flickering all around him. He thus became the last person to see the Gadget intact, and his expression as he slumps next to it is hard to read. Is he smiling? Proud? Or merely exhausted and wishing the photographer would leave him alone?

The final pages of Trinity depict events that followed the test, including the atomic bombings of Hiroshima and Nagasaki. Only five photos are known to exist of the immediate aftermath in Hiroshima, and the book reproduces three of them. Taken by Yoshito Matsushige, a staff photographer at the regional newspaper Chugoku Shimbun, they are far more disturbing for what they don’t show than for what they do. Although Matsushige later explained that he felt a professional duty to document the bombing, he found himself unable to press the shutter on most of what he witnessed. It was simply too horrible.

Beauty and horror make an unsettling combination. Despite Trinity’s quality, it is hard to see it appealing to a broad audience; in one of the book’s two forewords, Jim Eckles, who used to work in the missile range’s public affairs office, suggests it will “make a great memento for visitors to the site”. This unambitious statement is probably correct, but I hope that some physicists – weaponeers, disarmament campaigners and everyone in between – will also find space for it on their shelves. Our atomic age may yet end with a whimper, but it began with a bang, and it is worth remembering that bang in all its complicated majesty.

• 2026 University of Chicago Press $39.00/£32.00hb 344pp

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All four major LHC experiments have found new evidence that collisions between oxygen and neon may produce the extreme state of matter that existed during the first microseconds after the Big Bang. Inside the Large Hadron Collider (LHC), collisions between relatively light oxygen and neon nuclei may be producing matter from the earliest moments of [...]

Physicists headed up by a team at the University of Tennessee, Knoxville, and the Radioactive Isotope Beam Factory (RIBF) at RIKEN in Japan say they have measured the alpha decay of tellurium-104 for the first time. The feat could help us better understand how alpha particles form inside atomic nuclei, which is one of the least understood problems in nuclear science.

Alpha radioactivity was discovered over 125 years ago and it is the process whereby an atomic nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons that are strongly bound. The alpha particle exits the nucleus by quantum-mechanical tunnelling through the energy barrier surrounding the nucleus. While this model broadly explains the lifetimes of radioactive nuclei, a big question remains, however, says study lead Robert Grzywacz at the University of Tennessee, Knoxville. This is: how do alpha particles form in the nucleus and where can they exist as “pre-formed” structures inside it before they leave?

Tellurium-104 is particularly suited to studying alpha radioactivity, he says, because it is predicted to have the highest chance of pre-forming alpha particles of all heavy nuclei. “Such a strong enhancement of preformation shouldn’t be possible in theory because the matter in heavy nuclei is uniformly distributed,” Grzywacz explains. “There must, therefore, be an extra mechanism that causes alpha particles to locally ‘clump’ or ‘cluster’.”

In their experiments, he and his colleagues set about measuring the alpha particles produced by tellurium-104. This was no easy task because this isotope of tellurium can only be observed during the decay of xenon-108, which itself is extremely difficult to make in the laboratory.

Pulses of alpha particles

The researchers did their work at Japan’s RIKEN accelerator complex, which consists of four coupled cyclotrons that accelerate a beam of  xenon-124 onto a beryllium production target. The collisions between the two produces xenon-108 and then tellurium-104. The tellurium-104 finally decays into tin-100.

Grzywacz and co-workers say they succeeded measuring pulses of alpha particles produced in short succession by the tellurium-104. They measured the half-life of the radioisotope as being 7.2 ns, which is the shortest known alpha decay half-life for alpha particle emission from a heavy nucleus. More importantly, when corrected for the tunnelling effect – using a parameter known as the reduced width, which separates quantum tunnelling from inherent alpha particle emission from the nucleus – they were able to confirm that the probability of an alpha particle pre-forming in the nucleus was much higher than expected from theory calculations.

The possibility that tellurium-104 could show such “superallowed” alpha decay was first put forward more than 60 years ago, but it has been impossible to observe experimentally, says Grzywacz. “We started our search for this decay more than 20 years ago at Oak Ridge National Laboratory and later at JAEA in Tokai (Japan). We proposed the present experiment in 2018 at RIKEN but because of the covid-19 pandemic, we had to repeat the proposal in 2022. The experiment was deemed to be of ‘very high priority’ and we performed our experiments almost exactly two years ago, in June 2024.”

The work, which is detailed in Nature, will be important for understanding how alpha particle form in nuclei and will push the theory to explain where and how nuclear cluster can form, says Grzywacz. “More than 300 nuclei – and importantly, almost all superheavy nuclei – decay via alpha particle emission. Lighter nuclei may also naturally form ‘alpha condensates’, but in heavier nuclei, it is not obvious how this happens.”

Grzywacz told Physics World that he and his colleagues will now need to measure alpha particle energies with better precision in order to constrain the preformation they have observed.

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Physicists have discovered that the quantum arrangement of protons and neutrons inside atomic nuclei plays a much bigger role in nuclear pairing than previously thought. That is the conclusion of an international team of physicists that has scattered high-energy electrons from calcium and iron nuclei. Their study improves our understanding of the strong nuclear force, which binds atomic nuclei together.

The team focused on the brief partnerships that form when a proton and a neutron come unusually close together inside a nucleus. Known as short-range correlated (SRC) pairs, these fleeting configurations involve only about 20% of all nucleons but account for almost all of the fastest-moving particles in nuclei. Because the two particles approach each other so closely, such pairs offer a rare opportunity to probe nuclear matter under extreme conditions.

The new measurements suggest that these pairs form according to quantum-mechanical rules linked to the shell structure of the nucleus rather than simply depending on how many protons and neutrons the nucleus contains.

Distance matters

Atomic nuclei contain protons and neutrons, collectively known as nucleons. In the standard shell model of the nucleus, these particles occupy different quantum states, or shells, much as electrons do in atoms.

However, the shell model does not tell the whole story. Nucleons occasionally come very close together, forming temporary pairs with exceptionally large velocities. Almost all of these pairs consist of one proton and one neutron.

According to team member Lawrence Weinstein of Old Dominion University in the US, such pairs can reveal what happens when nucleons approach each other so closely that their internal structures may begin to overlap.

“Nucleons are like people,” he says. “When they are far apart they do not interact, at moderate distances they can attract each other, but if they get too close they can repel each other violently.”

Quarks and gluons

The pairs therefore provide a way to investigate how the strong nuclear force behaves at very short distances and whether these close encounters affect the quarks and gluons inside nucleons, where quarks are the fundamental constituents of protons and neutrons and gluons are the particles responsible for binding them together.

Previous experiments had suggested that neutron-rich nuclei contain more short-range pairs than nuclei with similar numbers of protons and neutrons. However, those studies compared nuclei that also differed substantially in mass, making it difficult to determine the true cause of the effect.

To separate these possibilities, the researchers examined three carefully chosen nuclei: two calcium isotopes and an iron isotope.

“We studied calcium-40, calcium-48, and iron-54,” says Or Hen of the Massachusetts Institute of Technology, one of the authors of the study. “These let us see how the number of SRC pairs increased as we added eight neutrons from calcium-40 to calcium-48 and then added six protons from calcium-48 to iron-54.”

The measurements were carried out at the Thomas Jefferson National Accelerator Facility in Virginia. The researchers fired a beam of electrons at the nuclei and measured both the scattered electrons and protons knocked out of the target.

Reconstructed motion

By reconstructing the motion of the proton before the collision, they could determine whether it had belonged to a short-range correlated pair.

The team expected that adding large numbers of neutrons would significantly increase the number of proton-neutron pairs. Instead, the effect was surprisingly small.

“We found that adding 40% more neutrons only increased the probability of finding a proton in an SRC pair by 10%,” says Hen.

The additional neutrons occupied an outer quantum shell, while most of the protons remained in inner shells. The result suggests that the newly added neutrons rarely formed close-range pairs with protons in different shells.

The researchers then examined iron-54, which contains six additional protons occupying the same outer shell as the extra neutrons in calcium-48.

Dramatic effect

“Conversely, the added six protons in the outer orbital of iron-54 formed 50% more SRC pairs (relative to calcium-48), presumably with the outer-orbital neutrons in calcium-48,” Hen says.

The result points to an unexpected conclusion. Nucleons appear to prefer forming close-range pairs with partners occupying the same quantum shell rather than with particles located in different shells.

The finding also posed a challenge for existing theoretical models. Although some calculations reproduced part of the observed behavior, none predicted the strong increase seen in iron-54.

The work could have consequences beyond the structure of individual nuclei. Researchers have proposed that short-range pairs influence the properties of extremely dense matter, including the matter found inside neutron stars. The pairs may affect both the cooling of neutron stars and the relationship between pressure and density within these exotic objects.

Hen says that the team will now study wider range of nuclei. “We are extending this work to other stable nuclei from beryllium-9 to gold-197 to further study the effects of shell structure and mass on pair formation.”

Future experiments will also investigate unstable neutron-rich nuclei that cannot be studied using conventional targets. Those measurements should help determine whether the newly observed shell effects represent a general rule governing how short-range proton–neutron pairs form throughout nuclear matter.

The research is described in Nature.

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By recreating a nuclear fireball, scientists uncovered unexpected chemistry that could change how radioactive fallout is understood. In the first fraction of a second after a nuclear explosion or a major reactor accident, an immense surge of energy superheats the surrounding environment. Nearby materials and air are instantly vaporized, creating a rapidly expanding fireball made [...]