Clock-line-mediated Sisyphus Cooling

We demonstrate sub-recoil Sisyphus cooling using the long-lived ^3P₀ clock state in alkaline-earth-like ytterbium. A 1388 nm optical standing wave nearly resonant with the ^3P₀\, \, ^3D₁ transition creates a spatially periodic light shift of the ^3P₀ 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 ^3D₁ and subsequent spontaneous decay to the ^1S₀ 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 ^1S₀\, \, ^3P₁ 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.