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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Optical frequency combs have transformed the way scientists measure light and time, earning the Nobel Prize in Physics in 2005 and underpinning technologies ranging from atomic clocks to exoplanet detection. Yet the miniature on-chip versions of these combs, known as soliton microcombs, have long suffered from a frustrating paradox: the most stable and useful operating regimes have also been the most fragile and difficult to sustain. A newly published study in Light: Science & Applications reports a significant advance toward resolving this tension. Researchers led by J. Tang, J. Yang and E. Yan describe a multimodal locking-enabled soliton microcomb that combines robust, self-starting operation with an exceptionally low repetition rate on the scale of a day of stable measurement time, opening a path toward practical high-precision metrology systems that can run unattended for extended periods.

At the heart of the advance is a technique the authors call multimodal locking, a scheme that stabilizes not just a single resonance of the optical microresonator but multiple modes simultaneously, locking the comb dynamics to a regime in which dissipative Kerr solitons can form and persist without delicate manual tuning. In conventional microcomb systems, soliton formation typically requires a precise and rapid traversal of a “chaotic” modulation instability regime, a step that is notoriously sensitive to thermal transients in the resonator chip. Laser frequency must be swept with exquisite control while the soliton’s own heat dissipation shifts the resonator’s resonances, creating a feedback loop that can destroy the soliton state within microseconds. The multimodal locking approach circumvents this fragility by engaging several mode families of the resonator at once, so that the interplay among them pins the comb to a stable soliton state even as environmental and thermal conditions fluctuate.

The significance of achieving a low repetition rate in the same device cannot be overstated. A frequency comb’s repetition rate is the spacing between its individual comb teeth, and for many applications, coarser spacing is a decisive advantage. In laser ranging and distance metrology, for example, the ambiguity range of an interferometric measurement is set by the inverse of the comb’s free spectral range. Microcombs, generated in whispering-gallery-mode or ring resonators only a few millimeters across, usually exhibit repetition rates of tens to hundreds of gigahertz, which severely limits the unambiguous measurement range available to a comb-tooth-resolved distance measurement. By engineering a low-repetition-rate soliton state, the research team extends this ambiguity range dramatically, making the microcomb directly useful for absolute distance measurement, multi-wavelength interferometry and coherent communications, where each comb line must be spectrally resolvable and addressable.

Achieving both low repetition rate and robust soliton generation in a single microresonator is a formidable engineering challenge. Low repetition rates demand large resonator diameters, which in turn produce dense spectra of resonances and increased susceptibility to mode crossings with higher-order transverse and polarization mode families. Such crossings historically destabilize soliton formation, causing comb states to collapse or preventing single-soliton operation altogether. The multimodal locking strategy turns this liability into an asset: rather than engineering mode crossings away, the researchers harness the coupling among mode families to lock the comb dynamics into a favorable basin of attraction. The result is a soliton microcomb whose low repetition rate is preserved over day-scale timeframes, a duration that places the device among the most operationally stable microcomb demonstrations reported to date.

Day-scale stability matters because it transforms the microcomb from a laboratory demonstration into an instrument. Many high-precision experiments, from astronomical spectrograph calibration to the accumulation of statistical confidence in atomic spectroscopy, require comb sources that remain locked and stable across many hours or even days of continuous operation. Conventional fiber-laser-based frequency combs achieve this, but at the cost of table-top size, high power consumption and substantial maintenance overhead. Microcombs promised to shrink the technology onto a photonic chip, yet until now their operational fragility has confined most demonstrations to short observation windows. The robust low-repetition-rate soliton microcomb described in the new work demonstrates that chip-scale frequency combs can, with appropriate locking architecture, sustain the kind of long-duration, hands-off operation that real metrology campaigns demand.

The physics underlying soliton microcombs rests on the Kerr nonlinearity of the resonator material, typically silicon nitride, magnesium fluoride or fused silica. When continuous-wave pump light resonantly builds up inside the ring, the Kerr effect produces a four-wave-mixing process that seeds sidebands spaced by the resonator’s free spectral range. Under the right conditions of pump detuning and intracavity power, these sidebands evolve into a coherent train of ultrashort optical pulses, dissipative Kerr solitons, that circulate around the resonator and emit a broadband comb spectrum with tooth spacing equal to the repetition rate. The comb’s output frequencies are determined by two radio-frequency quantities, the pump laser frequency and the repetition rate, which is why the combs can serve as rulers for measuring optical frequencies against a reference. Any drift in either quantity propagates directly into measurement error, and the multimodal locking mechanism suppresses precisely this drift by anchoring the comb’s underlying mode structure to the resonator’s intrinsic stability.

Applications for such a device span an unusually broad swath of science and engineering. In high-precision laser ranging, low-repetition-rate combs with tooth spacings in the low-gigahertz or even megahertz regime allow interferometric distance measurements over ranges of meters to kilometers without the need for auxiliary synthetic-wavelength techniques. In astronomical spectroscopy, comb calibration sources with well-controlled line spacing enable the detection of Earth-like exoplanets through radial velocity shifts measured in centimeters per second. In telecommunications, low-repetition-rate combs align naturally with standard channel grids used in wavelength-division multiplexed fiber networks, making them attractive as multi-carrier sources. In spectroscopy, dual-comb techniques benefit from repetition rates that are low enough to permit resolution of closely spaced molecular absorption features. A microcomb that is simultaneously robust, low-repetition-rate and day-scale stable therefore functions as a versatile enabling component across these domains rather than a single-purpose device.

The demonstration also speaks to a broader trend in photonics: the migration from delicate, expert-operated laboratory setups to self-contained, self-stabilizing systems. The self-starting character implied by multimodal locking is particularly important for this transition. A comb that requires a skilled operator to initiate soliton formation, sweeping the laser with practiced timing past the chaotic regime, cannot be deployed in an observatory dome, on a satellite platform or inside an industrial instrument. A comb that engages its soliton state reliably on demand and then holds it through thermal cycling, laser drift and mechanical perturbation removes the last practical barrier to integration. The authors’ demonstration of day-scale continuous operation is, in effect, a stress test passed by a technology that researchers have been attempting to harden for more than a decade.

The implications extend to metrology standards and timing as well. Optical atomic clocks, which now achieve fractional frequency uncertainties below one part in 10^18, rely on frequency combs to divide down their optical oscillations to countable microwave rates. Miniaturizing the comb stage of such clocks is a prerequisite for portable optical timekeeping, which would benefit navigation in GPS-denied environments, geodesy that monitors sea level and crustal motion with millimeter precision, and fundamental physics tests that track the constancy of natural constants over months and years. A robust chip-scale soliton microcomb with day-scale stability contributes a key subsystem to this vision, and the multimodal locking principle may generalize to other resonator platforms, materials and spectral regions, including the mid-infrared where molecular fingerprinting awaits compact comb sources.

Looking forward, the study suggests several directions that the photonics community is likely to pursue. Combining multimodal locking with on-chip pump lasers and integrated electronics would complete the transformation toward fully self-contained comb modules with no free-space optics. Extending the approach to lower repetition rates still, into the megahertz regime where fiber-laser combs operate, would bring microcombs into direct competition with their bulkier predecessors for the most demanding ranging and spectroscopy tasks. Improvements in resonator fabrication, reducing mode-volume disorder and engineering dispersion with greater precision, will further expand the parameter space in which robust soliton states can be locked. The present work demonstrates that the fragility once considered intrinsic to soliton microcombs is not a fundamental limit but an engineering problem, and one that can be solved by embracing, rather than avoiding, the multimodal structure of real optical resonators.

The research, published in Light: Science & Applications, marks a milestone in the maturation of microcomb technology. By uniting three properties that had previously been achieved only separately, self-starting robustness, low repetition rate and day-scale operational stability, the work delivers a soliton microcomb genuinely suited to high-precision metrology. As the demand for compact frequency references accelerates across navigation, astronomy, communications and fundamental science, the multimodal locking strategy reported by Tang, Yang, Yan and colleagues offers a clear and technically grounded route from chip-scale optical chaos to dependable, precision measurement light.