Summary
Researchers operated a thorium-229 nuclear clock by using absorption in a crystal to steer the laser probing the nuclei. They also used the clock to search for oscillations that could be caused by ultralight dark matter.
Researchers have operated a thorium-229 nuclear clock in which light absorbed by the nuclei supplies feedback to control the laser probing them. The study, published in Nature on 7 October 2026, reports a clock based on a 148-nanometre transition in thorium-229 nuclei embedded in a calcium fluoride crystal at room temperature.
The result moves the system beyond using an external reference to measure the thorium transition: the nuclear signal now helps steer the clock laser itself. The researchers also compared the clock with a ytterbium-ion atomic clock and used the measurements to search for possible signals from ultralight dark matter.
How the nuclear clock works
Most atomic clocks rely on transitions involving electrons around an atom’s nucleus. This device instead uses a low-energy transition within the thorium-229 nucleus. The nuclei are held in a millimetre-sized crystal, allowing many of them to contribute to the signal at once.
A laser is tuned to the transition’s absorption feature. The system measures absorption at nearby frequencies to determine whether the laser has drifted from the line centre, then feeds that error back to adjust the laser. A high-finesse optical cavity provides short-term stability; the nuclear transition helps correct longer-term drift. The team generated the 148-nanometre vacuum-ultraviolet light from a laser operating at a longer wavelength.
The clock’s fractional frequency instability was shot-noise limited at about 3 × 10⁻¹²/√(τ/s), where τ is the averaging time in seconds. In a continuous run, the researchers report reaching the low 10⁻¹⁴ range. They operated the system for as long as a day, and compared its frequency with that of a single-ion ytterbium clock.
This is a working nuclear clock, but its measured stability is not yet at the level of the best optical atomic clocks. The study describes future improvements as projections, including ways to increase the signal and narrow the transition linewidth.
A search for dark-matter signals
Some models propose that dark matter includes ultralight scalar particles that could cause fundamental constants to oscillate. Because the thorium transition’s energy depends on nuclear and electromagnetic effects that nearly cancel, it is expected to respond sensitively to changes in those constants.
The team analysed about 23 hours of clock-comparison data, looking for periodic changes on timescales from 20 seconds to roughly one day. No signal crossed the study’s detection threshold, so the result sets upper limits on possible oscillations. The authors report limits competitive with the best comparable atomic-clock results for coupling to photons, and bounds on couplings to the strong force and quark masses that reach further into previously unexplored parameter space than earlier atomic-clock measurements.
The experiment also measured a frequency-ratio drift of (2 ± 4) × 10⁻¹⁴ per day, consistent with zero. Extending such tests over much longer periods will depend in part on improving how reproducibly the clock measures the transition from one run to another.