Observation of Collective-Emission-Induced Cooling of Atoms in an Optical Cavity

We report the observation of collective-emission-induced, velocity-dependent light forces. One-third of a falling sample containing $3\times {10}^6$ cesium atoms illuminated by a horizontal standing wave is stopped by cooperatively emitting light into a vertically oriented, confocal resonator. We observe decelerations up to $1500\mathrm{m}/{\mathrm{s}}^2$ and cooling to temperatures as low as $7\mu \mathrm{K}$, well below the free-space Doppler limit. The measured forces substantially exceed those predicted for a single two-level atom.