Two independent research teams have achieved a longstanding goal in physics: building a working nuclear clock. The devices, developed by Beichen Huang and colleagues at Tsinghua University and by Luca Toscani De Col and colleagues at the Vienna Center for Quantum Science and Technology in Austria, exploit the nucleus of a thorium-229 atom to keep time with extraordinary precision—possibly surpassing even the best atomic clocks available today.
The Chinese and European studies have both been published in preprint on arXiv.
Harnessing nuclear energy levels
Atomic clocks are the most accurate clocks available to date, measuring time by registering the frequencies emitted as electrons jump between atomic levels. Since these frequencies are extraordinarily stable, they are also highly predictable, allowing observers to track how much time has passed simply by counting the number of oscillations.
In theory, nuclear clocks could work on the same principle—but instead of atomic energy levels, they anchor their timekeeping to transitions between the energy levels of protons and neutrons inside the nucleus. Because the nucleus is far more isolated from the environment than its surrounding electrons, it is less vulnerable to disturbances from stray electric and magnetic fields. If achieved practically, this would enable clocks that are even more accurate and robust than their atomic counterparts.
Of all the nuclei in the periodic table, only one is suitable for this purpose: thorium-229. The energy jump available inside its nucleus happens to be just the right size to be triggered and measured using laser light, which isn’t true for any other known nucleus.
Despite this, building a working nuclear clock has so far proved a formidable challenge. This is largely because the required laser light sits in the vacuum ultraviolet part of the spectrum, a region that is technically challenging to generate and control.
Same method, different validations
In their studies, Huang and Toscani De Col’s teams each overcame this challenge by embedding thorium-229 nuclei in crystals of calcium fluoride and probing them with a finely tuned continuous-wave laser operating at around 148 nanometers. The two approaches differed in their details: While the Chinese team used a more powerful laser, the European team worked with a crystal containing a higher concentration of thorium nuclei.
To validate their clocks, each team took a different approach. Huang’s team demonstrated that its device could stabilize the frequency of its vacuum ultraviolet laser, locking it to the nuclear transition with a fractional frequency instability approaching one part in 10 trillion after a day of operation.
In contrast, Toscani De Col’s team put its clock to work searching for signatures of ultralight dark matter, theoretical particles that have been proposed to comprise much of the universe’s as-yet unexplained mass. To do this, the researchers searched for tiny, periodic shifts in the thorium transition energy. No dark matter signal was found, but the sensitivity achieved matched or exceeded that of the best existing atomic clocks.
A step toward unprecedented timekeeping
Together, the results could mark a profound turning point. Beyond precision timekeeping, nuclear clocks offer a new window onto some of the deepest questions in physics, including whether the fundamental constants governing the forces of nature are truly constant.
If the technology can be refined and miniaturized, both teams are hopeful that compact nuclear clocks could eventually find their way into navigation systems, gravitational sensing and tests of fundamental physics that are simply beyond the reach of today’s instruments.
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Publication details
L. Toscani De Col et al, A thorium-229 optical nuclear clock with feedback loop, arXiv (2026). DOI: 10.48550/arxiv.2606.04997
Beichen Huang et al, A nuclear clock based on 229Th, arXiv (2026). DOI: 10.48550/arxiv.2606.08870
Journal information:
arXiv
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Nuclear clocks tick for the first time (2026, June 12)
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