Researchers in Austria and China have demonstrated operational clocks that keep time using transitions inside an atomic nucleus, marking an important step towards a new generation of precision timekeeping.
At TU Wien in Vienna, researchers stabilised a continuous-wave laser against the nuclear transition of thorium-229 embedded in a calcium-fluoride crystal. The feedback system operated for more than 24 hours and reached a fractional-frequency instability approaching \(10^{-15}\) over one day—roughly equivalent to an error of one second in 30 million years.
The result makes the Vienna apparatus the first self-stabilising nuclear clock: the thorium nuclei themselves provide the reference used to correct changes in the laser frequency. An independent team led by researchers at Tsinghua University in Beijing also produced an operational thorium-229 clock using a different experimental design. The separate results strengthen the case that nuclear-clock technology is reproducible rather than dependent on one particular apparatus.
The clocks are not powered by nuclear fission or fusion. They use laser light to probe changes between energy states within thorium nuclei. Conventional atomic clocks instead measure transitions involving electrons surrounding an atom.
Because an atomic nucleus is far smaller and generally less vulnerable to external disturbances than an electron shell, physicists expect mature nuclear clocks to offer exceptional stability while potentially using more compact and robust equipment. Possible applications include satellite navigation, telecommunications synchronisation, surveying and laboratory metrology.
The prototype does not yet outperform the world’s best optical atomic clocks, which can operate with errors measured at about one second over billions of years. TU Wien researchers say stronger lasers and improved thorium crystals could substantially improve the new device.
The Vienna team also used the clock to search for possible variations associated with ultralight dark matter. No dark matter was detected, but the experiment showed that the instrument could already perform competitive tests of some fundamental-physics models.
The findings are newly published in Nature, but the core results are not being disclosed for the first time. Both teams posted preprints in June 2026, when Nature reported that the first nuclear clocks had begun operating. The October development is the peer-reviewed publication and detailed institutional confirmation of the Vienna device’s self-stabilising, day-long performance.
