Successor to the Atomic Clock The World's First Atomic Clock Is Ticking Steadily

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For decades, it was just a theoretical concept; now it’s ticking steadily: Researchers have succeeded in putting the world’s first atomic clock powered by a thorium nucleus into continuous operation. This clock is powered not by electrons, but directly by the nucleus of a thorium atom. This achievement was made possible by a significantly improved laser system.

Two recent publications in the scientific journal Nature mark a historic milestone in metrology: The team led by Ekkehard Peik (Physikalisch-Technische Bundesanstalt, PTB) and Thorsten Schumm (Vienna University of Technology) has demonstrated the first true operation of a nuclear clock. The system ran absolutely stably for 24 hours in Vienna (Austria) for the first time without any user intervention.This represents a massive technological leap. It was only in 2024—after decades of research—that the team succeeded for the first time worldwide in exciting a thorium nucleus with a laser. The fact that a functional clock is now already operational is considered an unexpectedly rapid success among experts.

The Technology: Atomic Nucleus Instead of Electron Shell

The basic principle behind any atomic clock is to excite atomic components using lasers—but until now, this has been limited exclusively to electrons. In atomic nuclei, the particles are packed much more tightly together, which is why the energy of conventional lasers is usually insufficient. The key to success lies in the isotope thorium-229. “It has two energy states that are very close together, so a single laser is enough to change the state of the atomic nucleus,” explains Ekkehard Peik, who proposed the principle as early as 2003. The technical implementation, however, was like looking for a needle in a haystack: To selectively excite the nucleus, the energy of the transition—which was only roughly known—had to be precisely targeted with a laser to within one millionth of an electronvolt. This was made possible by special thorium-doped calcium fluoride crystals developed by TU Wien, in which many thorium nuclei can be targeted simultaneously.

The technical implementation, however, was like looking for a needle in a haystack: To selectively excite the core, the energy of the transition—which was known only roughly—had to be precisely targeted with a laser to within one millionth of an electronvolt.

The Breakthrough: Compact Hardware and Instant Feedback

To turn the nuclear excitation into a working clock, the researchers had to overcome two major hurdles in the setup:

• The Laser Upgrade: The laser system initially used took up several square meters on optical benches and operated only in pulsed mode. In collaboration with the Max Born Institute in Berlin, the team developed a compact, continuously emitting solid-state laser with extremely high frequency resolution by December 2025.
• The signal delay: Until now, the researchers would irradiate the crystal, then shut off the laser, and measure the fluorescent light emitted by the excited nuclei. The problem: This process takes about 10 minutes—far too slow to continuously adjust (stabilize) the laser’s frequency.
  The solution: The new, more stable laser made it possible to measure not the delayed fluorescence, but rather the laser power absorbed by the nuclei directly in real time. The signal no longer experiences any time delay and immediately locks the laser frequency to the nuclear resonance.

Solid State Instead of a Vacuum: New Territory for Metrology

With its new architecture, the nuclear clock breaks new technological ground in several ways: Not only is it the first clock based on nuclear resonance, but it also uses a solid-state crystal as its oscillator. Previous high-end optical atomic clocks rely on atoms or ions that must be stored under complex vacuum conditions. During the first 24-hour test run, the new nuclear clock was directly synchronized with and compared to an established optical clock from the Austrian metrology institute BEV-PTP in Vienna.

Use in the Search for Dark Matter

This new level of precision offers immediate benefits for basic research. Researchers have already used the system to analyze the microscopic structure of the crystal system based on nuclear magnetic resonance spectra. Even more exciting: The nuclear clock has already undergone an initial test to detect possible couplings between the Th-229 nucleus and certain forms of dark matter.

A Global Tech Race

The research conducted in Braunschweig and Vienna has sparked tremendous momentum worldwide. Just a few weeks after the first results were announced, a consortium of Chinese institutes led by Tsinghua University reported similar experiments. The nuclear clock has not yet broken the records set by the best optical atomic clocks, but the potential is enormous: “It is clear where the current technical limits lie and which properties of the laser systems and thorium crystals need to be further improved in order to become fully competitive,” Ekkehard Peik concludes, looking toward the near future. The nuclear clock has only just begun to tick.