The first ticking ‘nuclear clocks’ are here — what can they do?

Two teams of physicists have made the world’s first nuclear clocks. These radical new devices use fluctuations in the energy states of an atom’s nucleus to keep time, rather than those of its electrons, which atomic clocks currently use to define the length of a second.

Working out how to extract the ‘tick’ from a nucleus and use it to keep time has taken more than 20-years. Nuclear clocks should be more robust and portable than the best available clocks today because nuclei are hard to perturb and protected in a crystal. As well as potentially someday being more precise, they also give physicists an unprecedented way to probe the forces at play inside a nucleus.

Two nuclear clocks have been presented in two studies, which were posted on the preprint server arXiv on 3 and 7 June, by teams in Europe¹ and China². They show that nuclear clocks have gone from a system with “potential” to “a functioning precision instrument” that can be used to search for new physics, says Gilad Perez, a theoretical physicist at the Weizmann Institute of Science in Rehovot, Israel.

Creating a nuclear clock is “a dream come true”, says Thorsten Schumm, an atomic physicist at the Vienna University of Technology and a lead member of the European team. Until recently the field had been “a calm niche” to work in, he says. “Now we have a fierce but friendly global competition.”

Tick tock

All clocks require a stable oscillation — like that of a swinging pendulum — to keep time. In the best atomic clocks, this oscillation is created by the visible wavelength of light that triggers an electron to jump between energy levels. Physicists determine the specific frequency of laser light required to trigger this shift in electron state, then use that frequency to keep time.

A nuclear clock is different. Rather than causing electrons to jump between energy levels, it keeps time by boosting the protons and neutrons inside the nucleus of thorium-229 atoms to a higher energy state. Most elements require an enormous amount of energy to reorganize their nuclei, but thorium is unusual because it has stable energy levels that are so close together that just the nudge of ultraviolet laser light can prompt the shift.

‘Nuclear clock’ breakthrough paves the way for super-precise timekeeping

Physicists had suspected thorium’s special properties for decades, but it wasn’t until 2024 that they finally succeeded in triggering the nuclear transition in a millimetre-sized crystal of calcium fluoride loaded with trillions of thorium-229 atoms. Later that year another team pinpointed the precise frequency at which it happens.

The only thing that was missing for a nuclear clock to work was a way to lock the frequency of the laser with the natural timekeeper and keep the clock’s tick speed from drifting over time. Both teams achieved this by monitoring how much the laser light was absorbed by the thorium-229 atoms. When the laser was in the right range the signal’s strength dipped as photons got absorbed, says Schumm. But if the frequency drifts, “you see the signal coming up again and can immediately correct for that”, he says.

The groups differed in their exact methods: the group in China, led by Shiqian Ding, a physicist at Tsinghua University in Beijing, used a laser around 100 times more powerful than the European one, but their crystal had a lower concentration of thorium-229 atoms, so overall the signals produced by both clocks were comparable.

Both teams’ clocks ticked reliably, only drifting over the course of a day by the equivalent of around 1 second in 3 million years (although, for now, that is still below the stability of the best optical atomic clocks, which gain or lose a second every 40 billion years).