Motion-selective coherent population trapping

We report progress of our experiment for sub-recoil cooling of Rb atoms in a 1D optical lattice outside the Lamb-Dicke limit. We use a cooling scheme of motion-selective coherent population trapping (MSCPT). MSCPT is our adaptation of the well-known velocity-selective coherent population trapping (VSCPT) for cooling alkali-metal atoms in an optical trap. A closed λ configuration for CPT is formed by two ground hyperfine transitions with the apex state coupled to an excited state via D transition. We use a circularly polarized trap so that the vector polarizability of Rb results in different well depths and hence different vibration frequencies for the two lower states of λ. Two-photon resonance condition for CPT then depends on the motional quantum number n, and we may tune the λ fields to select the pair of states with n = 0 for CPT. Finally, we apply Raman sideband cooling by driving the hyperfine transitions using two pairs of Raman beams tuned to the red sideband.

We have observed formation of CPT dark states between various motional states as manifested by a bump or a dip in temperature as the two-photon detuning is scanned. We are working on simplifying experimental situation to unambiguously select the pair of n = 0 states for CPT formation.