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

Abstract

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 nW of signal power, corresponding to a ${\chi }^{(3)}$ of $10^{-8}{\text{m}}^2/{\text{V}}^2$. Potential applications range from optical quantum information processing to quantum non-demolition measurement of photon number.