  We report on the first observation of nonlinear Faraday rotation with cold
atoms at a temperature of ~100 uK. The observed nonlinear rotation of the light
polarization plane is up to 0.1 rad over the 1 mm size atomic cloud in
approximately 10 mG magnetic field. The nonlinearity of rotation results from
long-lived coherence of ground-state Zeeman sublevels created by a
near-resonant light. The method allows for creation, detection and control of
atomic superposition states. It also allows applications for precision
magnetometry with high spatial and temporal resolution.
