Scientists Demonstrate First Operational Nuclear Clock Using Thorium-229

Researchers at Vienna University of Technology and collaborating institutions have demonstrated the first operational nuclear clock by stabilizing a vacuum-ultraviolet laser to a resolved nuclear transition in thorium-229 embedded in a calcium fluoride crystal. The clock achieves a fractional frequency instability of 2×10⁻¹²/√(τ/s) and demonstrates reproducibility across distinct crystals at the 10⁻¹³ level. Nuclear clocks promise superior precision to current atomic clocks because nuclei require more precise excitation due to their higher energies, potentially achieving stability of seconds over hundreds of billions of years. The thorium-229 nucleus was chosen because it has the lowest energy of all known nuclear excited states, making it accessible to table-top laser technology—a property that eluded most other nuclei. While the prototype has not yet matched the stability of the best atomic clocks, the team reports it operated continuously for 24 hours without intervention, a milestone not rapidly achieved by other optical clocks. This work opens new platforms for compact clocks, solid-state nuclear quantum sensors, and precision tests of fundamental physics.