Vienna Scientists Build First Self-Stabilising Nuclear Clock

A crystal containing thorium atoms is keeping time on its own in a Vienna laboratory. Physicists at TU Wien have built the world's first self-stabilising nuclear clock, a prototype that ran for more than a day without anyone adjusting it, according to a university announcement. The result was published in the journal Nature on October 7, 2026, in work led by Prof. Thorsten Schumm of the university's Institute of Atomic and Subatomic Physics.

The machine differs from every atomic clock in service today. Ordinary atomic clocks keep time by tracking jumps between energy states of electrons. The Vienna clock instead uses the nucleus of thorium-229, an isotope with an unusually low-energy nuclear transition, and lets the nuclei themselves correct the laser that drives the system. Over a day of operation the prototype reached a relative precision of roughly 10^-15, which corresponds to an error of about one second in 30 million years.

That figure trails the best optical atomic clocks in existence, a gap the team does not dispute. "This is not yet at the level of the world's best optical atomic clocks, but for a first prototype it is a fantastic result," says Schumm. The value of the prototype is that the closed loop works at all. Earlier thorium experiments still leaned on a conventional atomic clock to keep their lasers on frequency.

Precision timekeeping matters far beyond the laboratory. Atomic clocks already underpin satellite navigation, mobile phone networks and the power grid, all of which depend on synchronised signals. A steadier clock would sharpen those systems and open new ways to measure physical quantities that are out of reach for today's instruments.

Why the nucleus makes a better reference

The idea behind nuclear timekeeping has been around for decades, TU Wien reports. Physicists had long suspected that thorium nuclei possess two energy states whose energies sit extremely close together. The gap is so small that laser light can flip the nucleus from one state to the other, something ordinary nuclei, with their much wider energy gaps, do not respond to.

The attraction is robustness. An atomic nucleus is more than ten thousand times smaller than the atom around it, so it reacts far more weakly to outside disturbances such as stray electric fields or temperature shifts. "The great advantage of the new nuclear clock in Vienna is that, if you use atomic nuclei rather than atoms, much higher precision is possible in principle," Schumm said in the announcement.

The mechanism is a feedback loop. A thorium-doped crystal, produced at TU Wien, is illuminated with vacuum-ultraviolet laser light at a wavelength near 148 nanometers. The nuclei absorb that light only when the laser frequency matches the nuclear transition exactly. When the frequency drifts, absorption falls, and the system automatically nudges the laser back into alignment. No external clock is needed to referee the process.

The road to that loop took years. In April 2024, the team led by Schumm, working with Prof. Ekkehard Peik at PTB Braunschweig, Germany's national metrology institute, showed for the first time that thorium nuclei could be excited with laser beams, confirming a transition theorists had predicted but never observed. By autumn of that year they had coupled the apparatus to a conventional optical atomic clock and demonstrated a working nuclear timekeeper. The new paper closes the loop: the nuclei now stabilise the laser themselves. "What you really want is a self-stabilizing nuclear clock," explains Schumm. "The basic idea is simple: you have a laser and you have thorium. The laser changes the energy state of the thorium nuclei, and the thorium nuclei are used to stabilize the frequency of the laser."

A race between Vienna and Beijing

Vienna does not have the field to itself. In the same issue of Nature, a research group from China reported its own working thorium nuclear clock, making it the second group in the world to demonstrate one after the Vienna and Braunschweig team, TU Wien said. The two efforts underline that nuclear timekeeping has moved past theory into a competitive engineering phase, with several laboratories now building on the same thorium-doped crystal approach.

What comes next is better hardware. The team says precision should improve sharply with stronger lasers and improved thorium crystals. Because nuclei respond so weakly to their surroundings, the devices could eventually become the basis for measuring a range of physical quantities with precision that today's instruments cannot reach, and for testing whether the fundamental constants of nature are truly constant.

For now, the achievement rests on a modest claim. A small crystal, a laser and a feedback circuit held a steady rhythm for more than a day, and the nuclei did the stabilising. The research is published in Nature, and TU Wien's announcement of the breakthrough describes the Vienna experiment in detail.