Clock-Line-Mediated Sisyphus Cooling

We demonstrate subrecoil Sisyphus cooling using the long-lived P03 clock state in alkaline-earth-like ytterbium. A 1388-nm optical standing wave nearly resonant with the P03→D13 transition creates a spatially periodic light shift of the P03 clock state. Following excitation on the ultranarrow clock transition, we observe Sisyphus cooling in this potential, as the light shift is correlated with excitation to D13 and subsequent spontaneous decay to the S01 ground state. We observe that cooling enhances the loading efficiency of atoms into a 759-nm magic-wavelength one-dimensional (1D) optical lattice, as compared to standard Doppler cooling on the S01→P13 transition. Sisyphus cooling yields temperatures below 200 nK in the weakly confined, transverse dimensions of the 1D optical lattice. These lower temperatures improve optical lattice clocks by facilitating the use of shallow lattices with reduced light shifts while retaining large atom numbers to reduce the quantum projection noise. This Sisyphus cooling can be pulsed or continuous and is applicable to a range of quantum metrology applications.