A team in Vienna has developed the world's first self-stabilizing nuclear clock, demonstrating that it can operate independently for more than 24 hours without conventional atomic clock support. The breakthrough builds on work by Prof. Thorsten Schumm at TU Wien and Prof. Ekkehard Peik at PTB Braunschweig.
The nuclear clock uses thorium atomic nuclei as its timekeeping reference. Thorium has an unusual property: two of its nuclear energy states are separated by a small energy gap, allowing laser light to trigger transitions between them. A laser shines on a special thorium crystal, and when the laser frequency drifts due to temperature fluctuations or other environmental changes, the thorium nuclei absorb less light. The system detects this change and automatically adjusts the laser frequency back to the correct value, creating continuous self-stabilization without external intervention.
In April 2024, the researchers first demonstrated that laser beams could excite thorium nuclei. By fall 2024, they connected their apparatus to a conventional optical atomic clock and confirmed thorium could serve as a timekeeping reference. The latest breakthrough adds self-stabilization.
Over a 24-hour test, the prototype achieved a relative precision of 10 to the power of minus 15, equivalent to an error of approximately one second over 30 million years. Schumm notes this result describes the measured precision, not a test lasting millions of years. The clock is not yet as precise as the world's leading optical atomic clocks, but Schumm calls it "a fantastic result" for a first prototype.
Because atomic nuclei are more than 10,000 times smaller than atoms, they resist environmental disturbances better than electron-based transitions used in conventional atomic clocks. The researchers believe more powerful lasers and higher-quality thorium crystals could substantially increase precision and eventually allow nuclear clocks to surpass existing atomic clocks as the timekeeping standard.