  We demonstrate sub-recoil Sisyphus cooling using the long-lived
$^{3}\mathrm{P}_{0}$ clock state in alkaline-earth-like ytterbium. A 1388 nm
optical standing wave nearly resonant with the
$^{3}\textrm{P}_{0}$$\,\rightarrow$$\,^{3}\textrm{D}_{1}$ transition creates a
spatially periodic light shift of the $^{3}\textrm{P}_{0}$ 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
$^{3}\textrm{D}_{1}$ and subsequent spontaneous decay to the
$^{1}\textrm{S}_{0}$ 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
$^{1}\textrm{S}_{0}$$\,\rightarrow\,$$^{3}\textrm{P}_{1}$ 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.
