A nuclear clock synchronized to 229Th.

Atomic clocks have made time and frequency the most precisely measured quantities in physics, progressing from microwave standards that realize the SI second1 to optical clocks with unprecedented precision2. A nuclear clock transfers the frequency reference from an electronic to a nuclear transition and the uniquely low-lying, laser-accessible, isomeric transition in 229Th currently offers the most practical route to compact, robust timekeeping and sensitive tests of fundamental physics3-8. Realizing such a clock requires turning spectroscopy of the 229Th nuclear resonance9-17 into a stable discriminator for steering a traceable oscillator. Here we demonstrate a 229Th nuclear clock by stabilizing a continuous-wave, narrow-linewidth 148.4 nm vacuum-ultraviolet (VUV) laser18 to a resolved, weakly temperature-sensitive nuclear transition17,19 in 229Th:CaF2 crystals20-22. A 10-μW VUV source generated by four-wave mixing in cadmium vapour18,23,24 and phototube-based frequency modulation absorption readout provide a fast, high-signal-to-noise nuclear discriminator. The clock reaches a fractional frequency instability of 5 × 1 0 - 13 / τ / s for averaging time τ. Clock-transition frequencies measured in two independently fabricated crystals agree at the 10-13 level and are consistent with previous VUV-comb measurements on other 229Th:CaF2 crystals17. These results establish laser-addressed nuclei as operational clock references and provide a reproducible solid-state platform for compact nuclear clocks, nuclear quantum sensors and precision tests of fundamental physics.