Here’s what you’ll learn when you read this story:

• Atomic clocks have been around for nearly 80 years, but their successors—nuclear clocks—are ready to take the stage.
• Two independent studies, both uploaded as preprints, report reliable timekeeping devices based on oscillations of the thorium-229 nucleus.
• Because nuclear clocks are subject to different forces and less prone to disturbances by outside forces than atomic clocks, their accuracy could one day help detect ultralight dark matter particles.

Nearly 80 years ago, the atomic clock—first built by the U.S. National Bureau of Standards (now known as National Institute of Standards and Technology, or NIST)—revolutionized timekeeping. Since then, leveraging the resonant frequency of atoms, researchers around the world have steadily improved atomic clocks. One research group even created an aluminum ion clock capable of measuring time down to the 19^th decimal point.

Now, two new independent studies are heralding a new era of timekeeping—that of nuclear clocks, timekeeping devices that rely on oscillations of a nucleus rather than electrons. While this breakthrough could make for even more accurate clocks, it could also create a vital platform for exploring the true nature of dark matter.

Back in 2003, physicists Ekkehard Peik and Christian Tamm at the Physikalisch-Technische Bundesanstalt (essentially the German equivalent of NIST) first proposed that the nucleus could perhaps provide an even more accurate timekeeping platform than electrons. Because atomic nuclei are smaller than atoms, they are less reactive to outside disturbances (i.e. electromagnetic fields) than electrons, whose oscillation between quantum states serves as the basis for atomic clocks.

The outstanding problem that continued to plague scientists was that switching nuclei between two states requires more energy than a laser can provide—except, as Peik and Tamm proposed, for Thorium-229, which has two states of very similar energy. And after more than 20 years of efforts, a team of scientists from TU Wien in Vienna, Austria (along with experts from NIST and the University of Colorado Boulder’s JILA research institute) successfully created a Thorium-229 prototype in 2024. However, though the clock could “tick,” it couldn’t reliably keep time.

“Our aim was to develop a new technology. Once it’s there, the increase in quality comes naturally, that has always been the case,” Thorston Schumm, a co-author of the 2024 study from TU Wien, said at the time. “The first cars weren’t any faster than carriages. It was all about introducing a new concept. And that’s exactly what we’ve now achieved with the nuclear clock […] we expect to overtake the best atomic clocks in 2-3 years.”

Now, Schumm and his team—along with an independent research group in China—are delivering on that promise. According to Science News, the teams perfected a method for how to readjust the frequency of a laser—which sort acts like the pendulum of grandfather clock—by implementing a feedback loop.

“In clock operation, the clock laser is stabilized to the interrogated nuclear transition frequency with the help of a feedback loop that corrects for residual instability or drift of the cavity,” Schumm’s team wrote in the preprint uploaded to the arXiv server.

Because nuclei are subject predominantly to the strong nuclear force, nuclear clocks could reveal small variations in fundamental constants (which determine the strength of the strong nuclear force) that could, in turn, point toward evidence of dark matter.

“Drawing benefit from the enhanced sensitivity of the thorium-229 transition, these constraints compete with the best atomic clocks concerning dark matter coupling to photons and go beyond previous measurements regarding coupling to the strong force and quarks,” Schumm and his team wrote.

The age of the nuclear clock is upon us.