Cross Phase Modulation in a Δ-type Three-Level System

We propose a cross phase modulation (XPM) scheme that exhibits a pronounced, resonantly enhanced nonlinearity, and at the same time vanishing linear susceptibilities. We show that at the limit of the strong control field, the XPM can arise between two weak fields. The optical nonlinear effect in the Δ-type system can be used for the realization of the quantum phase gate. Our scheme has the possibility of generating nonlinear phase shifts of the order of π .