A team led by Thorsten Schumm at the Atominstitut of TU Wien has built the first nuclear clock that stabilises itself, as atomic clocks do. The university announced the result on 7 October, with the study published in Nature. The prototype ran for more than 24 hours without anyone stepping in.
The clock relies on an unusual property of the thorium-229 nucleus: it has two energy states so close together that a laser can switch the nucleus from one to the other, while other nuclei need far more energy than laser light carries. In April 2024 Schumm's team, working with Ekkehard Peik's group at PTB, Germany's national metrology institute, showed for the first time that thorium nuclei can be excited this way. That autumn they used them to time a conventional optical atomic clock.
That earlier set-up still depended on an atomic clock. In the new one, a laser shines on a crystal containing thorium atoms, grown at TU Wien, and the nuclei keep the laser on frequency themselves: they absorb its light only when the frequency is exactly right, so any drift shows up as weaker absorption and the laser is corrected automatically.
Nuclei are more than ten thousand times smaller than atoms and far less disturbed by their surroundings, which is why nuclear clocks could in principle be more precise than atomic ones. Measured over a day, the Vienna prototype reached a precision of about one part in a million billion, roughly one second of error in 30 million years. Schumm says this is not yet at the level of the best optical atomic clocks, but a fantastic result for a first prototype, and the team plans stronger lasers and better thorium crystals.
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