A major advance in timekeeping, developed over many years, may change how precisely time can be measured. Scientists have constructed working clocks that rely on energy shifts inside atomic nuclei rather than electron movements, using thorium-229 atoms. The feat was accomplished separately by two research groups, one in Europe and one in China, with results posted as preprints. One group described its device as the first standalone nuclear clock. Standard atomic clocks, introduced in the 1950s, achieve extreme accuracy by tracking electron transitions triggered by lasers. Nuclear clocks, suggested in 2003, would instead follow changes within the nucleus, which normally demand higher energies beyond typical laser capabilities. The nucleus sits deeper inside the atom and resists external disturbances better than electrons, potentially yielding greater stability and new ways to study dark matter or shifts in physical constants. Thorium-229 was identified early as a suitable material due to its unusually low-energy nuclear transition. In 2024, teams in Austria and Germany first induced and observed this transition. The recent work converts that signal into operational clocks. Both groups placed thorium-229 nuclei inside calcium fluoride crystals and used vacuum-ultraviolet lasers. The European device functions independently by locking a laser frequency to the nuclear signal and was tested against a ytterbium-ion clock, showing consistent performance. It was also applied to search for ultralight dark matter, producing tighter limits on several models. The Chinese group checked two separately made crystals and found nearly matching frequencies, indicating that solid-state nuclear clocks could serve as reproducible references. The instruments do not yet surpass leading atomic clocks but prove nuclear clocks can operate outside theory. Further gains may allow them to exceed current standards within a few years.
