An international research team has operated an optical nuclear clock based on an energy transition inside the nucleus of the thorium-229 isotope. The clock continuously monitored the laser frequency and corrected its drift. This marks a shift from measuring the nuclear phenomenon in the laboratory to using it as a practical reference for controlling a clock, potentially supporting the development of more precise tools for measuring time, testing the fundamental laws of nature and searching for dark matter.
Stabilising the laser frequency with the nuclear transition
In a study published on 7 October 2026 in the journal Nature, the researchers announced that they had successfully and continuously locked a laser frequency to the energy transition inside a thorium nucleus.
When the laser frequency begins to move away from the correct frequency, the clock detects the deviation and automatically corrects it, making the nucleus itself the reference that maintains the system’s stability.
Luca Toscani De Col, a doctoral student in physics at TU Wien and the study’s co-first author, said the key advance was turning the thorium nuclear transition from a phenomenon measurable in the laboratory into a practical reference inside a clock capable of continuously monitoring and correcting its own frequency. In principle, the idea resembles the mechanism of conventional atomic clocks, although those clocks rely on electrons moving between different energy levels within the atom.
Why thorium-229 was chosen for the nuclear clock
The new clock uses a rare transition inside a thorium-229 nucleus, at a wavelength of 148 nanometres. The team chose this isotope because the transition in its nucleus has exceptionally low energy, making it accessible with a laser.
The atomic nucleus is also less affected by some external disturbances than electrons, while the clock’s design allows vast numbers of thorium nuclei to be placed inside a small crystal instead of relying on a single atom or ion. The researchers placed thorium nuclei inside a small calcium fluoride crystal operating at room temperature, then passed a deep-ultraviolet laser through it.
When the laser frequency moves away from the nuclear transition frequency, the response of the nuclei in the crystal changes, and the clock uses the resulting signal to automatically return the laser to the correct frequency. To verify the system’s performance, the team compared the nuclear clock with another optical clock based on a ytterbium ion.
Testing stability by comparing it with an ytterbium clock
The thorium clock operated continuously for about a day, and after that period its frequency instability was approximately one part in a thousand trillion. Toscani De Col said the result did not mean that the new clock had become more accurate than the best atomic clocks currently available.
But he noted that the achievement lay in using the nucleus, for the first time, as a practical reference inside a nuclear clock operating with a continuous correction system, rather than merely observing and measuring the nuclear transition outside a complete clock system. The researchers also used the clock to test possibilities related to dark matter.
Searching for signs of dark matter with the two clocks
According to Toscani De Col, some theories propose the existence of ultralight particles or fields that could cause extremely small periodic changes in the fundamental constants of nature, potentially appearing as a slight oscillation in the clock’s frequency. The team compared the frequency of the thorium clock with that of the ytterbium clock over a period of about 23 hours, looking for periodic changes lasting between about 20 seconds and one day.
The comparison produced no statistically significant signal indicating the presence of dark matter, but the absence of a signal enabled the researchers to establish new limits on the strength of the interaction between some hypothetical forms of dark matter and ordinary matter. The results showed that the thorium clock was now capable of competing with some of the best atomic clocks in tests involving the interaction of dark matter with light.
In certain scenarios involving the strong nuclear force and quark masses, the measurements improved the previous limits by about 100 to 1,000 times. The clock remains at an early stage of development, as the researchers found that measurements carried out on different days were not reproduced with the same precision.
The team believes this may be due to microscopic differences within the crystal that can alter the nuclear transition frequency by small amounts. The dark-matter search experiment also lasted less than a day, limiting its ability to test changes occurring within the time range covered by the comparison.
The researchers expect improvements to the laser and higher-quality crystals to significantly increase the clock’s performance in the future.
