The Research Of Quantum Coherence Induced Kerr Nonlinerarity Enhancement In Four-level Atomic System

Recent studies have shown that Kerr nonlinearity can be used for quantum nondemoliton measurements, quantum logic gates, photons switch. And the effective enhancement of Kerr nonlinearity is determined by laser intensity and nonlinear coefficient of the medium, but high intensity of laser can produce the thermal effect.In a typical three-level system , one can increase the nonlinearity by decreasing the detunings. However, this will in turn strongly increase the absorption of the probe incidence, especially, destroy the quantum information.The Kerr nonlinearity, in the four-level system consisting of A-type and|4> in an EIT, can be two orders of magnitude greater than that obtained in a conventional three-level scheme at weak light intensities.We present here the theory of electromagnetically induced transparency. And the third-order susceptibilities of a four-level atomic system coupled to the three light fieldsthat are locked to hyperfine transitions of D₁ line ( 5²S_1/2 - 5²P_1/2 ) and D₂ line (5²S_1/2 -5²P_3/2)in ⁸7R_b versus the probe detuning and the signal detuning is calculated based on the matrix equation . The nonlinear absorption coefficient and XPM phase shift are yielded too. The results show that the cross Kerr nonlinearity is greatly enhanced for the highly sensitivity of quantum coherence to signal light. It can be two orders of magnitude of the Kerr nonlinearity in four-level system greater than that in three-level system .At the same time, the Kerr nonlinearity including Doppler effect and the quantum model are discussed.