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 second 1 to optical clocks with unprecedented precision 2 . A nuclear clock transfers the frequency reference from an electronic to a nuclear transition and the uniquely low-lying, laser-accessible, isomeric transition in 229 Th 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 229 Th nuclear resonance 9–17 into a stable discriminator for steering a traceable oscillator. Here we demonstrate a 229 Th nuclear clock by stabilizing a continuous-wave, narrow-linewidth 148.4 nm vacuum-ultraviolet (VUV) laser 18 to a resolved, weakly temperature-sensitive nuclear transition 17,19 in 229 Th:CaF 2 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× 10⁻¹³/√τ /s 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 229 Th:CaF 2 crystals 17 . 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.