Self-Correcting Nuclear Clock Ticks Steadily for 24 Hours in Vienna, a First, Nature Paper Reports

Physicists at TU Wien in Vienna have built the first nuclear clock that stabilizes itself, the way ordinary atomic clocks do, and kept it stable for more than 24 hours without intervention. The results are published in the journal Nature.

Researchers had suspected for decades that thorium atomic nuclei have a special property that makes them ideal for precision measurement. The thorium-229 nucleus has two energy states that sit extremely close together. Because the gap is so small, a laser can switch the nucleus from one state to the other. In other nuclei the gaps are much larger, so they do not respond to laser light.

In April 2024, the team led by Professor Thorsten Schumm at TU Wien's Institute of Atomic and Subatomic Physics, together with the team of Professor Ekkehard Peik at PTB Braunschweig, found this long-suspected nuclear transition for the first time. That fall, they showed it could serve as a timekeeper by coupling the thorium excitation apparatus to a conventional optical atomic clock. Strictly speaking, that was not yet the kind of clock that can set precision records.

"What you really want is a self-stabilizing nuclear clock," Schumm explains. "The basic idea is simple: you have a laser and you have thorium. The laser changes the energy state of the thorium nuclei, and the thorium nuclei are used to stabilize the frequency of the laser." At the center of the device is a thorium-doped crystal made at TU Wien, which is irradiated with the laser. The laser's frequency can drift, for example because of temperature changes, so a stabilizing mechanism is needed to keep the clock ticking at the same rhythm.

The thorium nuclei absorb laser light only when the frequency is exactly right. If the frequency slips even slightly, absorption drops measurably, and the system automatically readjusts the laser. The result is a self-regulating nuclear clock that does not have to rely on a conventional atomic clock.

Nuclei are more than 10,000 times smaller than atoms, so they react much more weakly to outside disturbances and should be more reliable timekeepers. "The great advantage of the new nuclear clock in Vienna is that, if you use atomic nuclei rather than atoms, much higher precision is possible in principle," Schumm says.

The team measured the clock's precision over a day. It came out at roughly 10 to the power of minus 15, equal to an error of about one second in 30 million years. "This is not yet at the level of the world's best optical atomic clocks, but for a first prototype it is a fantastic result," Schumm says. The researchers plan to improve it sharply with stronger lasers and better thorium crystals. The work is described in the Nature paper "A thorium-229 optical nuclear clock with feedback loop," with a companion paper by I. Morawetz and colleagues on continuous-wave laser spectroscopy of the thorium-229 nucleus, and the authors say it points toward a new kind of high-performance metrology able to measure physical quantities with precision never reached before.