A thorium-229 optical nuclear clock with feedback loop

The laser-accessible nuclear transition in the thorium-229 isotope has been identified as a candidate for realizing an optical nuclear clock¹ that might outperform current optical clocks based on electron-shell transitions in atoms or ions². It is expected to be more robust against external perturbations^3,4 and to provide enhanced sensitivity in clock-based tests of the fundamental principles of physics^5,6. Here we realize a thorium-229 nuclear clock by stabilizing a continuous-wave laser to the 148-nm nuclear transition with rapid feedback based on absorption spectroscopy⁷. The thorium-229 nuclei are embedded in a millimetre-sized, room-temperature calcium fluoride crystal. A subharmonic of the 148-nm radiation is continuously compared with a Yb⁺ single-ion clock. The nuclear clock shows a shot-noise-limited fractional frequency instability of $3\times 10^{-12}/\sqrt{\tau /s}$ where τ is the averaging time, approaching 10^-15 instabilities over 1 day of operation. Improvements to the instability by several orders of magnitude are projected for future devices. We use the nuclear clock to constrain models of ultralight dark matter by searching for periodic fluctuations and slow drifts in the nuclear transition energy, on timescales between 20 s and 1 day. Benefitting from the enhanced sensitivity of the thorium-229 transition, these constraints compete with the best atomic clocks concerning dark matter coupling to photons and go beyond previous measurements regarding coupling to the strong force.