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 physics^3-8. Realizing such a clock requires turning spectroscopy of the ²²⁹Th nuclear resonance^9-17 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 transition^17,19 in ²²⁹Th:CaF₂ crystals^20-22. A 10-μW VUV source generated by four-wave mixing in cadmium vapour^18,23,24 and phototube-based frequency modulation absorption readout provide a fast, high-signal-to-noise nuclear discriminator. The clock reaches a fractional frequency instability of $5\times 10^{-13}/\sqrt{\tau /s}$ 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.