Collective light forces on atoms in a high-finesse cavity

We solve the quantum Langevin equations of motion for N point-like two-level atoms moving in an externally pumped cavity field. In the limit of the low saturation of the atoms, we obtain analytical expressions for the dipole force, the velocity-dependent force and the momentum diffusion coefficient for each atom in the presence of other atoms. The expressions show that in general the forces on each atom depend on the position and the velocity of all the other atoms.