  We utilize the dark state in a {\Lambda}-type three-level system to cool an
ensemble of 85Rb atoms in an optical lattice [Morigi et al., Phys. Rev. Lett.
85, 4458 (2000)]. The common suppression of the carrier transition of atoms
with different vibrational frequencies allows them to reach a subrecoil
temperature of 100 nK after being released from the optical lattice. A nearly
zero vibrational quantum number is determined from the time-of-flight
measurements and adiabatic expansion process. The features of sideband cooling
are examined in various parameter spaces. Our results show that dark-state
sideband cooling is a simple and compelling method for preparing a large
ensemble of atoms into their vibrational ground state of a harmonic potential
and can be generalized to different species of atoms and molecules for studying
ultracold physics that demands recoil temperature and below.
