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The cooling of atomic motion by scattered light enables a wide variety of technological and scientific explorations. Here we focus on laser cooling from the perspective of the light specifi- cally, the scattering of light between different optical modes in the presence of the cooling beams. While optically thin modes lead to traditional laser cooling of the atoms, the dynamics of multiple scattering in optically thick modes has been more challenging to describe. We find that in an appropriate set of limits, multiple scattering leads to thermalization of the light with the atomic motion in a manner that approximately conserves total photon number between the beams and optically thick modes. In this regime, we find that the subsystem corresponding to the thermalized modes is describable by a grand canonical ensemble with a chemical potential set by the energy of a single laser photon. We consider realization of this regime using two-level atoms in Doppler cool- ing, and find physically realistic conditions for rare earth atoms. Among the possible applications of this new system is the Bose-Einstein condensation of a gas of non-interacting photons. With the addition of photon-photon interactions, this system could provide a new platform for many-body physics.
Physical Review X