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 clock 1 that might outperform current optical clocks based on electron-shell transitions in atoms or ions 2 . 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 spectroscopy 7 . 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 \(3\times 1{0}^{-12}/\sqrt{\tau /{\rm{s}}}\) where τ is the averaging time, approaching 10 −15 instabilities over 1 day of operation.