Observation of Collective Friction Forces due to Spatial Self-Organization of Atoms: From Rayleigh to Bragg Scattering

We demonstrate that emission-induced self-organization of two-level atoms can effect strong damping of the sample’s center-of-mass motion. When illuminated by far-detuned light, cold cesium atoms assemble into a density grating that efficiently diffracts the incident light into an optical resonator. We observe random phase jumps of $\pi$ in the emitted light, confirming spontaneous symmetry breaking in the atomic self-organization. The Bragg diffraction results in a collective friction force with center-of-mass deceleration up to $1000\mathrm{m}/{\mathrm{s}}^2$ that is effective even for an open atomic transition.