A low noise, high fidelity cross phase modulation in multi-level atomic medium

We develop a hybrid scheme of the cross phase modulation based on the electromagnetically induced transparency (EIT) and active Raman gain (ARG) in the multi-level atomic medium. The cross phase modulation, with the low loss and without the noise, is demonstrated in a room-temperature 85Rb vapor. We show that a $\pi$ radian nonlinear Kerr phase shift of the signal light relative to a reference light is observed when the signal light is modulated by the phase control field with the low light intensity. We also show that the linear and the third-order absorption can be eliminated via the Raman gain, and the phase noise of the signal light can be ignored when the phase control light is applied in this hybrid scheme.