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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Researchers in the Netherlands have shown that they can turn off the dipole moment of molecules that are normally polar, drastically altering the molecules’ behaviour in low-energy collisions. The discovery – which was only possible after the team cleared a major experimental hurdle – has implications for ultracold chemistry and could also lead to a better understanding of polarity, with benefits for quantum simulation and computation.

Chemical reactions are full of complexity and chaos, making them hard to study at the atomic and molecular level. Before attempting to do so, scientists typically cool the reactants to just above absolute zero, which restricts their movement and vibrations. Despite these restrictions, however, interactions between ultracold molecules remain poorly understood, especially when the molecules have an electric dipole moment.

In 2023 experimental physicist Bas van de Meerakker and theorist colleagues at the Radboud University Nijmegen in the Netherlands shone a ray of light on this murky area by studying the collision cross-section (that is, the likelihood that collisions will take place) between molecules of ammonia (NH₃) at low energies. Their simulations showed that this cross-section falls off sharply as the energy decreases. This contradicts the classical picture of polarity, which suggests that the cross-section should continuously increase.

A big turn off

The Radboud team explained this discrepancy by noting that in the classical picture, polar molecules such as ammonia have a permanently asymmetric charge distribution – a dipole moment – due to the arrangement of the bonds between their constituent atoms. The quantum mechanical picture, in contrast, depicts an isolated molecule in a specific quantum state as having a definite parity, or symmetry. Because of this symmetry, the molecule’s net dipole moment in the lab frame is exactly zero.

According to this quantum picture, collisions between polar molecules at high speeds (high energies) cause these opposite parity states to mix via interactions between the molecules’ electrostatic fields. In the process, they essentially “turn on” each other’s dipole moments, causing them to attract each other and increasing the collision cross-section. At low speeds, however, the molecules cannot get close enough to trigger this mutual mixing. As a result, their dipole moments effectively switch off, and their collision rates drop drastically.

On a collision course

The Radboud team’s next step was to prove experimentally that this is indeed what happens. However, colliding cold molecules generally requires using electric fields to merge two molecular beams. When molecules in the two beams have very similar dipole moments, this is very difficult, because using an electric field to steer one beam automatically affects the path of the other. The risk is that the beams never overlap at all.

To overcome this obstacle, van de Meerakker and colleagues combined a 2.6 m Stark decelerator, a curved hexapole guide and a merged quadrupole/hexapole trap into a device that can overlap the beams at just the right position, with minimal angle between them. In this way, they achieved sufficient overlap to observe low-energy collisions. They then experimentally verified the predicted energy-dependent changes in cross-section for NH₃-NH₃ collisions, while also showing that this dependence is different when they replace hydrogen atoms with deuterium (NH₃-ND₃ or ND₃-ND₃ collisions).

Under control

According to van de Meerakker, the fact that cross sections drop at low energies could make it hard to perform experiments that require scattering to take place in crossed or merged beams. On the other hand, experiments in which dipolar molecules are stored in traps could benefit, as lower inelastic cross-sections mean fewer molecules are lost from the trap.

As for future work, van de Meerakker says the team plans to add a controlled electric field to the beam overlap region. The mechanism they discovered is expected to respond extremely sensitively to this field, with even small electric fields changing cross sections by orders of magnitude and thereby providing a “knob” with which to control collision outcomes. “It was exactly this type of control that sparked our interest in cold and ultracold molecules two decades ago, and it is very exciting that this is now within reach in our scattering experiments,” van de Meerakker tells Physics World.

Tim Langen, a physicist at Austria’s Vienna University of Technology who was not involved in this research, describes it as important because it provides experimental evidence for a phenomenon that had mainly been discussed theoretically. In his view, the results support the idea that effective dipolar interactions become strongly suppressed at low collision energies. He also finds the experimental approach “impressive” as it enables measurements in a regime that was previously difficult to access. “Overall, I find the work convincing and expect it to be of broad interest to researchers working on cold and ultracold molecules,” Langen concludes.

The research is published in Nature Chemistry.

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