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 m/s(2) that is effective even for an open atomic transition.

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 x 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 m/s(2) and cooling to temperatures as low as 7 microK, well below the free-space Doppler limit. The measured forces substantially exceed those predicted for a single two-level atom.

Upper limits to submillimetre-range forces from extra space-time dimensions.

String theory is the most promising approach to the long-sought unified description of the four forces of nature and the elementary particles, but direct evidence supporting it is lacking. The theory requires six extra spatial dimensions beyond the three that we observe; it is usually supposed that these extra dimensions are curled up into small spaces. This 'compactification' induces 'moduli' fields, which describe the size and shape of the compact dimensions at each point in space-time. These moduli fields generate forces with strengths comparable to gravity, which according to some recent predictions might be detected on length scales of about 100 microm. Here we report a search for gravitational-strength forces using planar oscillators separated by a gap of 108 micro m. No new forces are observed, ruling out a substantial portion of the previously allowed parameter space for the strange and gluon moduli forces, and setting a new upper limit on the range of the string dilaton and radion forces.