Coherent Enhancement Of Nonlinear Effects In A Four-Level Tripod Atomic System

Photons not only travel faster, but also less susceptible to the environment and signal external interference, therefore, it is an ideal carrier in quantum information communication. Atom as the ideal information memory unit, it is becoming a hot topic in recent years in the field of quantum information that through computing the quantum coherence in the process of interaction between light and atom for quantum entanglement. The cross-Kerr nonlinear of optical effect can be used for calculating the quantum phase, building nearly decisiving controlled-not door, achieving non-destructive detection of Bell-state, and producing photon quantum entanglement, etc. However, in the conventional nonlinear media, it is difficult to obtain large cross-Kerr nonlinearity of optics(cross-phase modulation, XPM), which has a very important role in quantum information processing.We have investigated the enhanced cross-Kerr nonlinearity in a four-level tripod system of D1 line of 87 Rb atoms. For the first time, we observed the simultaneous EIT windows(Double EIT) for probe and trigger fields induced with coupling beam by theoretical calculatoweions and Numerical Simulation. Then it has simulated that with and without trigger beam(probe beam), the absorption spectrum and dispersion spectrum of probe beam(trigger beam) have a variation trend with the frequency detuning of the probe beam. It turned out that when the frequency detuning of the trigger beam was near or equal to that of the coupling beam, the total depth of the EIT dip created by the coupling beam clearly became larger, where achieved obvious cross- Kerr nonlinear effect.