Observation of efficient sub-Doppler cooling under a nonzero magnetic field in a moving optical lattice

We observed efficient sub-Doppler cooling of $^{85}\mathrm{Rb}$ atoms under a nonzero magnetic field in a moving optical lattice formed in a bichromatic magneto-optical trap. Trap laser detunings were biased or set differently for the counterpropagating laser beams so that the atoms could be trapped where the magnetic field was nonzero. We investigated the center position and the temperature of the atomic cloud. We found that the sub-Doppler cooling effect, known to decrease as the nonzero magnetic field increases in a magneto-optical trap, would in fact increase under a particular trap-laser detuning difference. We first derived the condition under theoretical considerations and then verified it experimentally by conducting nondestructive measurement of atomic temperature.