This work demonstrates a functioning nuclear clock based on the laser-addressable 229Th isomeric transition immobilized in CaF2 crystals. A continuous-wave, narrow-linewidth 148.4 nm vacuum-ultraviolet (VUV) laser is produced by resonance-enhanced four-wave mixing in cadmium vapour (375 nm two-photon resonance plus 710 nm input), yielding ~10 μW of VUV light and ≈5 μW delivered to the crystal. Home-grown 229Th:CaF2 crystals (fabricated from only 1.4 μg of 229Th) display both quadrupole-unresolved and quadrupole-resolved spectral structure with five narrow lines; one resolved component (line b, m_g = ±5/2 → m_is = ±3/2) serves as the clock transition. A phototube-based, frequency-modulation absorption readout produces a high-signal dispersive discriminator, and a feedback loop using an Er-fibre frequency comb and AOM locks the VUV probe to the nuclear resonance.
The locked nuclear clock achieves a fractional frequency instability of 5×10^−13 / √(τ/s). Clock-transition frequencies measured in two independently fabricated crystals agree at the 10^−13 level and match prior VUV-comb measurements on other 229Th:CaF2 samples, demonstrating reproducible solid-state references. The implementation establishes laser-addressed nuclei as operational clock references and delivers a compact, reproducible platform for nuclear clocks, nuclear-quantum sensors and precision tests of fundamental physics.
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