Observation of a large, resonant, cross-Kerr nonlinearity in a free-space Rydberg medium

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 (3) −822 probe pulse is measured to be as large as 8 mrad per nW of signal power, corresponding to a χ of 10 m /V . Potential applications range from optical quantum information processing to quantum non-demolition measure- ment of photon number.