Researchers at TU Wien and PTB Braunschweig report the world's first nuclear clock that stabilizes itself, without a conventional atomic clock as reference. It ran for more than 24 hours without intervention, according to a paper in Nature.
The clock uses thorium-229, whose nucleus has two states with an unusually small energy gap that a laser can drive. A laser shines on a thorium-containing crystal, and the nuclei absorb light only at one precise frequency. When the laser drifts, absorption drops, and a feedback loop steers it back.
Measured relative precision is about 10^-15, roughly one second of error in 30 million years. It does not yet match the best optical atomic clocks, but nuclei are over 10,000 times smaller than atoms and less sensitive to outside disturbances, so the approach could eventually surpass them.