Observation of cross-phase shift in hot atoms with quantum coherence

We directly measure the cross-phase shift and nonlinear refractive index of a three-level Λ-type hot atomic coherence system inside an optical ring cavity. The cross-phase nonlinear index of refraction n2 is significantly enhanced and sharply changed when the conditions of atomic two-photon resonance are closed. The cross-phase shift of π induced by controlling light on the probe wave has been observed under a relatively low controlling power of 16.2$\un{mW}$. The system is valuable in developing optical devices for the optical information process.