A nuclear clock synchronized to 229Th

Resonantly enhanced four-wave mixing in metal vapour18,23,24 and randomly quasi-phase-matched SrB4O7 (refs. 41,42) have provided routes to continuous-wave 148.4-nm radiation.

Converting nuclear laser spectroscopy into clock operation requires combining, in a single system, a traceable narrow-linewidth VUV probe, a 229Th-containing reference and a high-bandwidth, high-signal-to-noise readout of weak nuclear response, together with a demonstration of nuclear-transition-frequency reproducibility in independently prepared references.In this work, we develop and integrate all the ingredients required for 229Th clock operation.

We upgrade the cadmium vapour four-wave-mixing source18 to generate 10 μW of continuous-wave 148.4-nm radiation with an estimated sub-Hertz linewidth. We also fabricate, in a single growth run, a compact, high-quality 229Th:CaF2 crystal from a solution containing only 1.4 μg (10 kBq) of 229Th. In fluorescence and absorption spectroscopy, we resolve a broad quadrupole-unresolved (QU) and a quadrupole-resolved (QR) structure comprising five narrow transitions, including line b, which is used for clock operation. Combining this traceable high-power VUV probe with home-grown crystals, we demonstrate the operation of the first 229Th nuclear clock by using phototube-based frequency modulation absorption spectroscopy to generate the nuclear discriminator and close a feedback loop to line b. We then use the feedback-stabilized system to characterize the clock instability and test the reproducibility of the clock-transition frequency across two independently fabricated crystals and by comparison with previous VUV-comb measurements on other 229Th:CaF2 crystals17,43.A near-simultaneous independent report has also demonstrated 229Th nuclear clock operation using the randomly quasi-phase-matched SrB4O7 VUV source, a highly doped 229Th:CaF2 crystal and photomultiplier-tube-based absorption readout44, building on related continuous-wave nuclear absorption spectroscopy with sub-nanowatt 148.4-nm light45.VUV laser for nuclear clock interrogationWe generate continuous-wave 148.4-nm radiation by resonance-enhanced four-wave mixing in cadmium vapour18,23,24. The 375-nm field is tuned to the cadmium 51S0 → 61S0 two-photon resonance, which enhances the third-order non-linear response, and a 710-nm field participates in the sum-frequency four-wave-mixing process to generate VUV radiation at 148.4 nm. The 375-nm light is generated by second-harmonic generation of a fibre-based 750-nm laser in a bow-tie enhancement cavity, delivering 5 W of ultraviolet power. Together with 10 W of 710-nm light from another fibre-based laser system, the two fundamental beams are combined and sent into a cadmium oven operated at 600 °C, where 10 μW of continuous-wave 148.4-nm radiation is generated. After the oven, the VUV light is separated spatially from the fundamental beams using a Brewster-angle MgF2 prism18 and approximately 5 μW of VUV power is delivered to the front surface of the 229Th:CaF2 crystal with a Gaussian diameter of approximately 1 mm (Fig. 1a,b).Fig. 1: Experimental platform for a solid-state 229Th nuclear clock.a, Experimental setup. Continuous-wave 148.4-nm VUV light is generated by four-wave mixing in cadmium vapour and sent to a home-grown 229Th:CaF2 crystal. Nuclear fluorescence is collected with a PMT, and the transmitted VUV power is recorded with a phototube or a PMT. During absorption spectroscopy and clock operation, the 710-nm laser is modulated, and the transmitted VUV photocurrent is detected by lock-in demodulation. SHG, second-harmonic generation. b, Cadmium energy-level scheme for resonantly enhanced four-wave mixing. Two 375-nm photons drive the two-photon resonance between the 5 1S0 and 6 1S0 states, whereas one 710-nm photon completes the sum-frequency process to generate the 148.4-nm VUV field. c, Optical frequency chain. The 750-nm and 710-nm lasers are phase-locked to a self-referenced Er-fibre frequency comb, whose repetition rate (frep) is stabilized by a 4-km-long phase-stabilized fibre link to a cryogenic-silicon-cavity-stabilized laser at 1,397 nm. The fCEO is referenced to a hydrogen maser. d, Quadrupole-split nuclear energy levels of 229Th in a CaF2 host. Line b, a QR component corresponding to mg = ± 5/2 → mis = ± 3/2, is used for clock operation. Line c is a QR component corresponding to mg = ± 1/2 → mis = ± 1/2. e, Absorption-based clock discriminator and feedback.

The frequency modulation amplitude for this trace is 12 kHz. Unless otherwise stated, all error bars throughout this work represent 1σ statistical uncertainties.The 750-nm and 710-nm lasers are phase-locked to modes of a self-referenced femtosecond Er-fibre frequency comb, whose carrier-envelope-offset frequency (fCEO), is stabilized with an f–2f interferometer to a hydrogen maser-referenced direct digital synthesizer46. The repetition rate, frep, is stabilized by locking the 1,397-nm comb branch to an ultrastable 1,397-nm laser referenced to a cryogenic silicon cavity with 10−17 level stability47. The comb therefore transfers the stability of the silicon cavity to the 710-nm and 750-nm lasers, whereas frep and fCEO are counted against the hydrogen maser to provide traceability of the VUV frequency. Under the operating conditions used for spectroscopy and clock operation, the VUV linewidth is estimated to be sub-Hertz, well below the observed inhomogeneous linewidths of the nuclear transitions in the crystal (Fig. 1c).Fabrication of 229Th:CaF2 crystalsTwo 229Th:CaF2 crystals, S1 (Fig. 2g) and TS1 (Fig. 2h), are fabricated for continuous-wave laser nuclear spectroscopy and clock operation. Both crystals are prepared as elongated cylinders, providing a long VUV interaction path and hence sufficient 229Th column density for absorption readout. S1 is grown at the Shanghai Institute of Optics and Fine Mechanics using the Bridgman–Stockbarger method, with medium-frequency induction heating. S1 has a diameter of 1.44 mm, a length of 9.8 mm and an average 229Th concentration of 9.7 × 1016 cm−3. The crystal contains small bubbles along its central axis (Fig.