Optical-density enhanced quantum entanglement via four-wave mixing process

We theoretically propose a scheme to generate a strong continuous-variable quantum entangled light source in four-wave mixing (FWM) process by increasing the optical density of atomic medium. By properly choosing the input coupling field Rabi frequency and detuning, the optimized entanglement can be achieved to be better than -17 dB at an optical density of approximately 1, 000, which has been realized in atomic media. Besides, with the optimized one-photon detuning and coupling Rabi frequency, the optimum entanglement degree can be greatly enhanced with the increment of optical density. We also examine the effects of atomic decoherence rate and two-photon detuning on entanglement in a realistic setting, and evaluate the experimental feasibility. We find that the entanglement can be further improved by considering two-photon detuning. In addition, with optimum parameters the entanglement is robust against the decoherence. The strong entanglement provides a promising applications in continuous-variable quantum communications.