Independent research teams in Vienna and Beijing have built the world’s first operating nuclear clocks, using thorium-229 to track time through transitions inside atomic nuclei rather than orbiting electrons. Detailed in Nature, the prototypes offer potential precision and a new tool to investigate dark matter.
Timekeeping has officially entered its nuclear age. Two research groups working entirely apart in Vienna and Beijing have developed the world’s first operating nuclear clocks, marking a technological pivot that moves the core mechanism of precision time from the electron shell of an atom right down to its central nucleus.
While traditional atomic clocks have anchored global navigation and digital infrastructure for decades by measuring electron shifts using cesium or strontium, these new devices shine lasers inside the nucleus of an atomic isotope. The resulting prototypes simultaneously began to tick, bringing to fruition a concept physicists have pursued for almost 50 years—with Thorsten Schumm of the Vienna University of Technology noting that his team has worked toward this goal since 2008.
How Thorium-229 Powers Nuclear Clocks
The fundamental architecture of a nuclear clock relies on a very specific atomic ingredient. Squishing an atomic nucleus normally demands an enormous amount of energy, but the isotope thorium-229 serves as a rare exception. Back in 2003, researchers realized they needed to pinpoint the resonant frequency of this heavy radioactive element to make its nucleus vibrate between different configurations.
Pinning down that exact frequency took two decades of intense global effort, culminating in the realization that the target sat at 148 nanometers, deep within the ultraviolet spectrum. Both teams had to overcome severe material bottlenecks, including the extreme scarcity of thorium-229, which is obtainable only from the decay of weapons-grade uranium. The Beijing team worked with a mere one microgram of the element, baking it into a calcium fluoride crystal while creating a powerful 148-nanometer laser using heated cadmium gas. The Vienna team lacked such a powerful laser but secured access to thorium-rich crystals with better optical properties and higher concentration.
Once the lasers fired into the thorium-doped crystals—trapped in solid-state calcium fluoride crystals, as noted by the Vienna team—both teams measured rapid oscillations as the nuclei toggled back and forth. Schumm explained that the basic idea involves a laser changing the energy state of the thorium nuclei, which in turn stabilize the frequency of the laser.
Performance Benchmarks Between Vienna and Beijing
The two completed prototypes achieved remarkably similar baseline performances despite their differing technical strengths. The Vienna prototype is reportedly the first nuclear clock that stabilizes itself the way conventional atomic clocks do, correcting for laser drift through a dedicated feedback loop. Meanwhile, testing indicates that the Beijing clock is about six times more stable than the clock in Vienna.
To contextualize their current accuracy, neither prototype has yet captured the record for most precise timepiece from the world’s finest conventional atomic clocks. While current cesium clocks can keep time with an uncertainty of one second every 300 million years—or run for billions of years losing or gaining only one second—the new nuclear clocks currently drift by roughly one second every 30 million years, which Schumm described by noting the technology is still far from its target performance.
Researchers view these figures as an impressive starting point rather than a ceiling. As Schumm pointed out, the Vienna clock features slightly better thorium crystals with higher concentration and superior optical properties, while the Beijing team supplied a stronger laser; by putting these components together, the researchers can build a significantly better clock.
Nuclear Clocks Test Dark Matter Detection
Beyond simply keeping time, nuclear clocks offer an entirely new window into fundamental physics. Because an atomic nucleus is tens of thousands of times smaller than the surrounding electron shell where electrons reside, tracking transitions inside the nucleus yields higher theoretical precision and sensitivity to environmental forces.
The Vienna research team already put their prototype to the test in a precision physics experiment designed to detect dark matter, a crucial component of the cosmos that to date has eluded observation. While that specific run did not actually detect dark matter, the clock performed at the level of the best atomic clocks, granting access to a whole new physics universe.
With both studies published in the scientific journal Nature on Oct 7, researchers emphasize that the field is accelerating at an extraordinary pace, driven in part by what Schumm described to the South China Morning Post as a fierce but friendly global competition. Experts monitoring the breakthrough note that while nuclear clocks will not immediately replace the global network of atomic timekeepers relied upon for GPS and internet synchronization, their eventual portability and reliability promise to reshape satellite-based positioning, synchronization of data transfer, surveying, and metrology.
