The World's First Self-Stabilizing Nuclear Clock Starts Ticking in Vienna

Keypoints:

• Researchers at TU Wien built the first nuclear clock that stabilizes itself
• It runs on thorium-229 nuclei held in a crystal, not on electrons in atoms
• The prototype ran unaided for more than 24 hours
• Its measured precision is about one second in 30 million years
• It does not yet beat the best optical atomic clocks

Physicists at TU Wien, the Vienna University of Technology, report the first self-stabilizing nuclear clock. The team is led by Professor Thorsten Schumm at the Institute of Atomic and Subatomic Physics. The result appears in Nature. The prototype ran for more than 24 hours without outside help.

Thorium-229 is unusual among the known elements. Its nucleus has an excited state with a very low energy, about 8.4 electronvolts. A laser can reach that state. Researchers first proposed using it for a clock in 2003, but the transition stayed elusive for decades.

A breakthrough came in April 2024. Schumm's team, working with Professor Ekkehard Peik at the national metrology institute PTB in Braunschweig, showed that thorium nuclei can be excited with laser light. The new clock builds on that step.

The design is a feedback loop. A laser shines onto a crystal containing thorium, which TU Wien produces itself. The laser frequency drifts with changes such as temperature. The clock locks it to the nuclear transition, so the nucleus keeps the laser steady. Earlier tests needed a conventional atomic clock as a reference.

The numbers need careful reading. Over a single day, the clock reached a precision of roughly 10 to the power of minus 15. That equals an error of about one second in 30 million years. The figure describes measured precision, not a test lasting millions of years. The best optical atomic clocks are still more precise.

The case for nuclear clocks rests on size. Atomic nuclei are more than 10,000 times smaller than atoms, so they react much more weakly to outside disturbances. Reviews of the field suggest accuracy near 10 to the power of minus 19 is possible. Such a clock could also test whether fundamental constants of nature change over time.