The first “nuclear clocks” are here

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

Figuring out how to extract the “tick” from a core and use it to keep time took some time over 20 years old. Nuclear clocks should be more robust and portable than the best clocks available today, because the nuclei are difficult to disrupt and are protected in a crystal. In addition to potentially one day being more precise, they also offer physicists an unprecedented way to probe the forces at play inside a nucleus.

Two nuclear clocks were presented in two studies published on the preprint server arXiv on June 3 and 7 by teams in Europe¹ and China². They show that nuclear clocks have evolved from a system with “potential” to “a working 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.

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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 was “a quiet niche” in which to work, he says. “Now we have a fierce but friendly global competition.”

Tick ​​tock

All clocks require a steady swing – like that of a swinging pendulum – to keep time. In the best atomic clocks, this oscillation is the oscillation of the visible wavelength of light that is absorbed as electrons jump between energy levels. Physicists determine the specific frequency of laser light required to trigger this change in electronic state, then use that frequency to keep time.

A nuclear clock is different. Rather than making electrons jump between energy levels, it maintains time by propelling protons and neutrons inside the core of thorium-229 atoms to a higher energy state. Most elements require an enormous amount of energy to rearrange their cores, but thorium is unusual because its stable energy levels are so close together that just the boost of ultraviolet laser light can cause the change.

Physicists had suspected the special properties of thorium for decades, but it was not until 2024 that they finally succeeded. trigger the nuclear transition in a millimeter crystal of calcium fluoride loaded with billions of thorium-229 atoms. Later that year, another team identified the precise frequency with which this occurs.

The only thing missing for a nuclear clock to work was a way to lock the frequency of the laser with the natural timer and prevent the ticking speed of the clock from drifting over time. The two teams achieved this by monitoring the amount of laser light absorbed by the thorium-229 atoms. When the laser is in the right range, the signal strength decreases as photons are absorbed, Schumm explains. But if the frequency drifts, “you see the signal come back and you can immediately correct it,” he says.

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

Both teams’ clocks worked reliably, drifting over the course of a day by only the equivalent of about a second over three million years (although, for now, this remains short of the stability of the best optical atomic clocks, which gain or lose a second). every 40 billion years).

New window

Plans to further develop the clocks are now accelerating. Compared with atomic clockscrystal nuclear clocks are less sensitive to environmental disturbances and can operate without extreme cooling. This “opens the way for compact and robust optical clocks,” says Ding, for use in navigation and communications devices. Nuclear clocks using crystals are already being developed commercially, Schumm says.

Other researchers are working to create nuclear clocks that could be more accurate than the best atomic clocks. Since the light that triggers the nuclear transition is of a higher frequency than that used for an atomic clock, in theory, nuclear clocks should be able to slice time more finely. But this will require the thorium-229 to be isolated rather than incorporated into a crystal. This is an “important avenue that remains to be explored”, believes Ekkehard Peik, physicist at PTB, the German national metrology institute in Braunschweig, who co-led the European team.

Even today, nuclear clocks offer a new way to study fundamental physics. Theorists predict that some forms of dark matter would change the strength of the fundamental forces that bind an atom’s nucleus, causing a measurable change in the transition frequency. Having an operational clock creates a continuously running sensor that allows for cleaner, faster studies than before, Perez adds. “It’s incredible,” he said. “I think we’re seeing the birth of a new field.”

Schumm says he receives several emails a week from theoretical physicists who want to use the clocks to probe their own exotic theory creating specific observable effects. “Eventually, we will have to use many different types of clocks, which correspond to the different effects.”

This article is reproduced with permission and has been published for the first time June 22, 2026.