A Chinese nuclear-clock prototype has achieved roughly sixfold lower frequency instability than a European counterpart on a key performance measure. The comparison concerns how steadily the devices maintain their rate, rather than a sixfold improvement in absolute timekeeping accuracy.
The studies, published in Nature on October 7, describe independently developed clocks from a Tsinghua University-led team and a European collaboration involving TU Wien and Germany’s national metrology institute, PTB. Both use thorium-229 nuclei embedded in calcium fluoride crystals as a reference for laser-based timekeeping.
Conventional atomic clocks rely on transitions involving electrons. Nuclear clocks instead use an energy transition inside the nucleus, with ultraviolet light tuned to the frequency the thorium absorbs. A feedback system detects departures from that frequency and adjusts the laser, allowing the device to maintain a consistent rate.
The approach exploits an unusual property of thorium-229: its nucleus has two energy states close enough together for available laser technology to drive transitions between them. Researchers first demonstrated direct laser excitation of the transition in 2024, after years of work to identify the required frequency.
Nuclei are less sensitive to many external disturbances than electrons, offering a possible route to more robust precision instruments. That potential has not yet translated into a record-setting timekeeper, however. The prototypes remain less stable than the best optical atomic clocks, and researchers still need to improve their lasers and crystal materials.
The Chinese team, led by physicist Shiqian Ding, also compared two separately manufactured crystals. Their reference frequencies differed by about 2.8 parts in 10 trillion, while both agreed, within measurement uncertainties, with earlier results from JILA in the United States. Such comparisons help establish whether independently built devices can reproduce a reliable frequency standard.
Crystal structure remains a practical obstacle. The Vienna researchers found that the measured frequency changed slightly depending on where the laser passed through their crystal. Realigning the equipment between runs therefore affected reproducibility, with local strain or structural unevenness identified as possible causes.
The European team also compared its nuclear clock with an atomic clock to search for signals predicted by some models of ultralight dark matter. Because nuclear and electronic transitions respond differently to changes in fundamental physical quantities, comparing the two can reveal effects that either instrument alone might miss. No dark-matter signal was detected, but the measurements placed limits on possible interactions.
For practical timekeeping, the attraction is a device that could eventually be smaller and easier to operate outside specialist laboratories. The immediate task is more modest: improving the optical systems and crystal quality enough to make nuclear clocks competitive with established technology.