Vienna and Beijing Teams Build First Operating Thorium 229 Nuclear Clocks

Timekeeping has been a fundamental human pursuit for millennia, but the modern era of precision timekeeping began with atomic clocks, which use the oscillations of electrons in an atom's shell to mark intervals with extraordinary accuracy. Conventional atomic clocks, based on elements such as cesium or strontium, make electrons jump between two energy levels. The best of these devices can go billions of years losing or gaining only one second, according to Reuters. Yet physicists have long dreamed of a clock that uses an even more fundamental part of matter: the atomic nucleus.

The idea of a thorium-based nuclear clock was proposed by Peik and Tamm in Europhysics Letters in 2003. In 2016, von der Wense and colleagues measured the thorium-229 isomer, a crucial step. Then in 2024, Zhang and colleagues reported the first laser excitation of the thorium-229 nucleus. These milestones set the stage for the next leap: building an actual clock that counts time by laser-driven transitions inside the nucleus rather than in the electron shell. That leap has now been made, twice over, by independent teams on two continents.

On October 7, 2026, two research groups reported in the journal Nature the first two operating nuclear clocks. One team is based at TU Wien in Vienna, working with PTB Braunschweig in Germany. The other team is at Tsinghua University in Beijing. Both clocks are based on thorium-229 nuclei trapped in solid-state calcium fluoride crystals. The two devices represent the next generation of atomic clocks, and both teams say nuclear clocks have the potential to outperform the best conventional atomic clocks, though they do not do so yet.

The simultaneous achievement is striking because the teams worked independently and used different experimental approaches. Physicist Shiqian Ding of Tsinghua University, who helped lead the Beijing team, said, 'The two teams worked independently and reached operating thorium-229 nuclear clocks at the same time, using different experimental approaches. I think this is very encouraging because it shows that the concept is robust.' Physicist Thorsten Schumm of TU Wien, who helped lead the Vienna team, said, 'The creation of a nuclear clock was something that physicists dreamt of for almost 50 years. In my team, we have been working towards this goal since 2008.'

Key Facts

Reuters reported on October 7, 2026 that the two devices are the world's first operating nuclear clocks. The Vienna clock, described in a paper with DOI 10.1038/s41586-026-11084-4, is a self-stabilizing nuclear clock that remained stable for more than 24 hours without intervention. TU Wien announced on October 8, 2026 that the clock stabilizes itself as is customary in atomic clocks. The Beijing clock, described in a paper with DOI 10.1038/s41586-026-11122-1, is about six times as stable as the Vienna one, China Daily reported on October 8, 2026.

Both clocks use the isotope thorium-229 embedded in calcium fluoride crystals. The core of the Vienna clock is a thorium-doped calcium fluoride crystal produced at TU Wien and irradiated with a laser. The Chinese team used thorium-229 nuclei embedded in calcium fluoride crystals and a vacuum-ultraviolet laser to establish an exceptionally stable frequency reference. A nuclear clock can theoretically track oscillations occurring roughly 2 quadrillion times per second, while the best conventional atomic optical clocks are already so precise that they gain or lose only about a second over hundreds of billions of years.

The Beijing team overcame a major obstacle by developing a continuous-wave 148-nanometer vacuum-ultraviolet laser. They used a four-wave-mixing technique in metal vapor and sharply narrowed the laser's linewidth, overcoming a bottleneck that had constrained the field. Several of the PhD students involved in the Chinese project were born in the 2000s.

The Vienna team also demonstrated a precision physics experiment seeking to detect dark matter. The experiment did not detect dark matter, but the clock performed at the level of the best atomic clocks. During operation, the Vienna clock was compared with an optical atomic clock at the Austrian metrology institute BEV-PTP in Vienna.

Schumm noted key differences between the two approaches: the Vienna clock 'has slightly better thorium crystals' while 'the Beijing team has a stronger laser.' He also said the nuclear clock is still 'far from its target performance.' Nature reported on October 7, 2026 that the two papers describe the first operating nuclear clocks, with the Vienna paper and the Beijing paper published under separate DOIs.

Analysis

What this really means is that nuclear clocks have crossed the threshold from theoretical dream to operating instruments, but they are not yet ready to dethrone the best atomic clocks. The two papers, published simultaneously in Nature on October 7, 2026, show that the concept is robust because independent teams using different methods both succeeded.

The competitive dynamic is also revealing. China Daily reported on October 8, 2026 that the Beijing clock is about six times as stable as the Vienna one, yet the Vienna clock is the first to stabilize itself for more than 24 hours without intervention. Schumm's comment that Vienna has better crystals while Beijing has a stronger laser points to a healthy division of labor: each team has pushed a different part of the technology. The Vienna team's dark matter search, which found no dark matter but matched the best atomic clocks, shows that even a first-generation nuclear clock can already do competitive precision physics.

The bigger picture here is that nuclear clocks offer a fundamentally different way to measure time, because the nucleus is far smaller and less sensitive to external electromagnetic fields than an atom's electron shell. That promises higher precision and robustness, and it also offers an independent way to test whether the fundamental constants of nature change over time. The 2003 proposal by Peik and Tamm, the 2016 isomer measurement, and the 2024 laser excitation were all necessary steps. Now the field has two working clocks to compare and improve. The next phase will likely see rapid iteration, with each team borrowing ideas from the other and from the broader metrology community.

However, it is important not to overstate the current performance. Schumm himself said the nuclear clock is 'far from its target performance,' and both teams acknowledge that nuclear clocks do not yet outperform the best conventional atomic clocks. The best conventional clocks lose or gain only one second over billions of years. A nuclear clock that is six times more stable than another nuclear clock is still not necessarily better than a mature optical lattice clock. The value of these first operating clocks is that they provide a platform for improvement and a proof of principle. They also demonstrate that the technology can be self-stabilizing, as the Vienna clock did for more than 24 hours, which is a prerequisite for any practical clock.

Why It Matters

The practical implications could be substantial. More precise timekeeping underpins satellite navigation, communications, surveying, and deep-space exploration. Even a modest improvement in clock stability can translate into better positioning and longer autonomous operation for spacecraft. Nuclear clocks could eventually provide that improvement, because they are less susceptible to environmental disturbances than electron-shell clocks.

Beyond navigation, nuclear clocks offer a new tool for testing the laws of physics and searching for dark matter. The Vienna team's dark matter search, while unsuccessful in detecting dark matter, shows that the clock can perform at the level of the best atomic clocks. Because nuclear transitions are sensitive to possible changes in fundamental constants, nuclear clocks could help answer whether constants such as the fine structure constant vary over time or space. That is a question at the heart of modern physics.

The international nature of the achievement is also significant. The Vienna team included researchers working with PTB in Germany. The Beijing team was led by Ding Shiqian of Tsinghua University. The fact that both teams reached operating clocks at the same time, using different approaches, underlines that frontier science remains an international enterprise. It also suggests that the field will advance faster because of the friendly competition and the open exchange of results in Nature.

Next Up

The immediate next step is to improve the performance of both clocks and to compare them directly. Researchers will likely focus on reducing laser noise, improving crystal quality, and extending self-stabilized operation beyond the 24 hours demonstrated in Vienna. The Beijing team's 148-nanometer vacuum-ultraviolet laser and the Vienna team's thorium-doped calcium fluoride crystal are two key technologies that could be combined or refined. The dark matter search will continue, and more precision tests of fundamental physics are expected.

It is also likely that other groups will join the field, building on the published methods and the two DOIs. The fact that the clocks are based on thorium-229 in calcium fluoride crystals means that the required materials and techniques are accessible. Now that two clocks are ticking, the question is no longer whether nuclear clocks can work, but how precise they can become and what new physics they will reveal.