Observation of a large, resonant, cross-Kerr nonlinearity in a cold Rydberg gas

We report the experimental observation of a cross-Kerr nonlinearity in a free-space medium based on resonantly excited, interacting Rydberg atoms and electromagnetically induced transparency. The nonlinearity is used to implement cross-phase modulation between two optical pulses. The nonlinear phase written onto the probe pulse is measured to be as large as 8 mrad per nanowatt of signal power, corresponding to a chi((3)) of 10(-8) m(2)/V-2 . Potential applications range from optical quantum information processing to quantum nondemolition measurement of photon number.