Two Independent Teams Demonstrate Operational Thorium-229 Nuclear Clocks

Researchers have now demonstrated an operational nuclear clock based on thorium-229, pushing a long-sought idea in physics into the realm of working hardware. In a paper published Oct. 7 in Nature, a team led by Tsinghua University reported a clock that uses a thorium nucleus, rather than an atom’s electrons, as the reference for keeping time.

It was not a lone breakthrough. Around the same time, an independent collaboration led by researchers at TU Wien, with partners including Germany’s PTB, reported a separate thorium-229 nuclear clock in Nature using a different technical approach. Taken together, the near-simultaneous results suggest the field has crossed an important threshold: nuclei can now anchor a closed-loop clock.

That matters because the world’s best clocks today are optical atomic clocks, which keep time by locking lasers to electronic transitions in atoms or ions. A nuclear clock aims to do something similar with a transition inside the nucleus itself. For decades, that was mostly a compelling concept. These new experiments show it can function as a working clock reference.

The Tsinghua-led paper, “A nuclear clock synchronized to 229Th,” describes stabilizing a continuous-wave, narrow-linewidth laser at 148.4 nanometers in the vacuum-ultraviolet part of the spectrum to a resolved nuclear transition in thorium-229-doped calcium fluoride crystals. In practice, that means the laser is continually corrected using the thorium signal so the clock stays locked to the nuclear resonance.

The team reported a fractional frequency instability of 5×10^-13 divided by the square root of averaging time in seconds, a standard measure showing that performance improves as measurements are averaged longer. The researchers also said measurements of the clock transition in two independently fabricated crystals agreed at about the 10^-13 level, and matched earlier vacuum-ultraviolet comb measurements on other thorium-doped crystals.

Generating the light was itself a major part of the advance. The 148.4-nanometer beam was produced through four-wave mixing in cadmium vapor. According to the paper, the system generated about 10 microwatts of continuous-wave light at the source and delivered about 5 microwatts to the crystal. The crystal was grown using just 1.4 micrograms of thorium-229, with activity of about 10 kilobecquerels.

The authors said the result establishes laser-addressed nuclei as operational clock references and points to a reproducible solid-state platform for compact nuclear clocks, quantum sensors and precision tests of fundamental physics.

The separate Vienna-led paper, “A thorium-229 optical nuclear clock with feedback loop,” reinforces that message. That group also demonstrated an operational thorium-229 clock, but with a different continuous-wave vacuum-ultraviolet source and a different crystal approach. The existence of two independent demonstrations, arriving essentially together, gives the claim more weight than a single result would.

“What one really wants is a self-stabilizing atomic-nucleus clock,” Prof. Thorsten Schumm of TU Wien said in a university press release.

Thorium-229 has long been considered the leading candidate for such a device because it is unique among known nuclei: it has an unusually low-energy excited state, or isomeric transition, around 148 nanometers, putting it within reach of laser techniques. That unusual property has made it a centerpiece of precision-measurement research for years. A 2024 Nature paper had already linked the thorium-229 nuclear transition to an established optical atomic clock, helping set up this next step.

Scientists are interested in nuclear clocks because nuclear transitions are expected to be less sensitive than electronic ones to many outside disturbances, which could eventually make for more robust frequency standards and new tests of basic physics.

But these are still early prototypes, not replacements for today’s best optical atomic clocks. The reported performance does not yet surpass the top atomic systems, which remain far more precise over long averaging times.

Still, the milestone is concrete. Nuclear clocks are no longer only a proposal or a spectroscopy target. Researchers have now shown that a thorium-229 nucleus can serve as the reference in an operating clock.