Nature published two papers on Wednesday describing the first working nuclear clocks, built by independent teams in Vienna and Beijing. The devices are not yet more accurate than the best conventional atomic clocks. The Vienna team used its clock in a precision experiment to search for dark matter. It found none, while the system operated at the level of leading atomic clocks.
Both clocks use thorium-229 held inside solid calcium fluoride crystals. The teams took different experimental approaches, and their parallel success shows the concept does not depend on a single technical design. The work follows a decisive advance in 2024, when teams from TU Wien and Physikalisch-Technische Bundesanstalt directly excited the thorium-229 nuclear transition with a laser for the first time.
Nuclear clocks track a change inside the atomic nucleus rather than in the surrounding electron shell. A powerful laser interacts with the nucleus, prompting protons and neutrons to move between energy levels. Conventional atomic clocks track similar transitions among electrons in elements such as cesium or strontium.
Nuclear energy levels are less exposed to some environmental disturbances than electronic ones. That could eventually bring greater long-term stability, though researchers have yet to achieve that advantage in practice. The nucleus is much smaller than the electron shell around it, so researchers expect nuclear transitions to support greater precision as the equipment improves.
The solid-state design brings its own problem: large numbers of thorium nuclei sit inside a material with local imperfections. Those imperfections can shift the transition frequency and broaden the spectral line.
Thorsten Schumm of the Technical University of Vienna said his group had pursued the goal since 2008. Shiqian Ding of Tsinghua University helped lead the Beijing project. The scientists described the simultaneous achievement as evidence that the underlying idea is technically sound.
The two systems complement each other, though they are not identical. Vienna has focused on thorium-doped crystals with a higher concentration and better optical properties. The Beijing system uses a more powerful laser.
For now, both clocks remain well short of their intended performance. The Chinese installation reached a frequency instability of roughly one part in 1015 as the averaging time increased. The Vienna system demonstrated continuous operation for approximately 24 hours. The results show the systems work, though they do not surpass the best optical atomic clocks.
The basic engineering hurdles include crystal quality and uniformity, as well as laser stability and power. Researchers also need to obtain enough of the rare radioactive isotope thorium-229 for large-scale production.
The solid-state architecture brings another problem. The measured transition frequency changed slightly depending on which part of the crystal received the laser beam. Restarting the experiment or moving the beam could so produce small differences in the clock reading.
Future systems will need a more even thorium distribution and tighter control of crystal defects, along with better optical access to the material. The International Atomic Energy Agency has identified thorium-229 as a scarce isotope, making material supply another practical constraint.
Conventional atomic clocks already support systems used every day. Their precision underpins global satellite navigation and helps synchronize internet data transfers and mobile communications. Fibre-optic networks also rely on that precision. The best of these clocks can run for billions of years with a deviation of only one second.
Nuclear clocks are not ready to replace them. A compact solid-state platform could eventually offer another route to high-precision timekeeping, using equipment that is less bulky and less delicate. Schumm also listed satellite navigation and data-transfer synchronization as possible uses, alongside surveying and metrology.
Those applications remain future prospects. The next step is more practical: researchers could combine Vienna's higher-concentration, optically improved crystals with Beijing's stronger laser. That pairing could raise performance without requiring a wholly new architecture.
The devices also give physicists a new tool for testing fundamental physics. Possible experiments include searches for dark matter and tests for changes in fundamental constants. The Vienna experiment did not detect dark matter, but it showed the clock can already support measurements at the level of the best atomic systems. The missing signal is a null result; the clock's technical performance is meaningful in its own right.
Two teams have independently made nuclear clocks work, yet neither has surpassed conventional atomic clocks. The technology is now a practical laboratory platform, while its strongest applications still depend on further improvements.
