Nuclear clocks put the atom’s core to work

Independent teams in Vienna and Beijing have built operating nuclear clocks, using energy changes inside atomic nuclei to measure time. The prototypes do not yet beat the best atomic clocks, but they demonstrate a technology physicists have pursued for almost 50 years.

Described in the journal Nature, both devices use lasers and thorium-229, a form of thorium embedded in crystals. Conventional atomic clocks rely on energy changes involving electrons. Nuclear clocks instead use changes within the atom’s central core, which is less sensitive to outside disturbances. Here, “nuclear” describes the timing mechanism, not a power source based on fission or fusion.

The Vienna clock kept itself stable for more than 24 hours without intervention. Its measured precision corresponds to an error of about one second in 30 million years. For comparison, the best conventional atomic clocks would gain or lose only a second over billions of years.

The promise extends beyond telling time. In an experiment searching for dark matter, the Vienna device performed at the level of the best atomic clocks, although it detected none. Researchers hope improved versions will offer sharper tests of fundamental physics. The teams also have complementary strengths: Vienna’s thorium crystals have better optical properties and a higher thorium concentration, while Beijing’s laser is stronger, suggesting a practical route towards better performance.