Effects of resonant dipole interaction between atoms on laser cooling: The case of two identical atoms

We study the effects of the resonant dipole interaction between identical atoms undergoing polarization-gradient laser cooling. A generalized master equation, which includes both the atom-field interaction and the dipole-dipole interaction between atoms, is derived. We then apply this equation to the particular case of two identical atoms cooled simultaneously by a one-dimensional cooling field. We show that, as the average center-of-mass separation between the two atoms is reduced to be less than an optical wavelength λ/2π, the population of the ground motional state of the atoms begins to decrease sharply, indicating a breakdown of the cooling mechanism due to the enhanced atom-atom interaction.