Researchers working independently in Vienna and Beijing have developed the world’s first operating nuclear clocks. Built using thorium-229 trapped in solid-state calcium fluoride crystals, the devices rely on laser frequencies interacting with atomic nuclei rather than electrons, opening a new path for high-precision timekeeping and fundamental physics research.
Timekeeping has officially entered a new domain. Scientific teams in Austria and China have successfully constructed the world’s first two operating nuclear clocks, an achievement detailed in a pair of studies published in the journal Nature. The development marks a major milestone for metrology, the study of measurement, and represents the next generation of precision timekeeping since conventional atomic clocks were first created in 1949.
How Thorium-229 Crystals and Lasers Drive the Vienna and Beijing Clocks
Despite the name, these devices do not rely on nuclear fusion or fission. Traditional atomic clocks calculate time by tuning lasers or microwaves to make electrons jump between energy levels inside an atom’s outer shell, utilizing elements such as cesium or strontium. Nuclear clocks go much deeper by targeting the nucleus of an atom.
Both the Vienna and Beijing prototypes are built around an isotope of thorium known as thorium-229, which is trapped inside solid-state calcium fluoride crystals. Researchers use a high-powered laser tuned to the exact frequency required to make subatomic particles—protons and neutrons—jump between energy levels inside the atomic nucleus.
The two teams worked independently and reached operating thorium-229 nuclear clocks at the same time, using different experimental approaches.
Shiqian Ding, physicist at Tsinghua University in China
Because an atomic nucleus is tens of thousands of times smaller than the electron shell surrounding it, tracking transitions inside the nucleus promises significantly higher stability and accuracy once the technology matures.
Parallel Development and Competing Strengths Across Labs
The two laboratories pursued their goals through separate paths over many years. Work on the Vienna prototype at the Vienna University of Technology began in 2008, driven by a long-standing pursuit among physicists to harness nuclear transitions for time measurement.
“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,”
Thorsten Schumm, physicist at TU Wien in Austria
The parallel efforts in Austria and China have already created what researchers describe as a friendly rivalry, accompanied by immediate opportunities for collaboration. Testing indicates that the prototype built by Tsinghua University researchers in China is about six times more stable than the Vienna clock. Meanwhile, the Vienna device is noted as the first nuclear clock to stabilize itself in a manner similar to traditional atomic clocks.
The two systems also possess distinct material advantages. According to the Vienna research team, their apparatus features superior thorium crystals with higher concentrations and better optical properties, while the Beijing team utilized a stronger laser system. Combining these strengths could yield an even more advanced instrument.
What is really nice here: the Vienna clock has slightly better thorium crystals – higher concentration, better optical properties – while the Beijing team has a stronger laser.
Thorsten Schumm, study co-author and professor at the Vienna University of Technology
Current Performance Limits and Future Applications in Fundamental Physics
While the new instruments represent a technological breakthrough, they are still far from their target performance. The best conventional atomic clocks can run for billions of years while gaining or losing only one second, and cesium clocks currently define the official second. By comparison, the current nuclear prototypes are estimated to drift by about one second every 30 million years, trailing top-tier commercial atomic timekeepers.
Researchers expect that refinements to crystal purity and laser efficiency will eventually allow nuclear clocks to surpass conventional atomic systems. Once perfected, the hardware could also be constructed in less bulky and more durable formats than delicate atomic setups.
Beyond standard timekeeping, which underpins cellular networks, fiber-optic communications, and global navigation satellite systems like GPS, the technology offers a novel window into fundamental physics. The Vienna team already utilized their clock to conduct a precision experiment aimed at detecting dark matter, an elusive cosmic component that has resisted direct observation. Although that specific test did not find dark matter, the nuclear clock functioned at the caliber of the world’s finest atomic instruments, demonstrating its viability for high-stakes cosmic research.