Study of the EPR-type entanglement characteristics of a NDPA with time-delayed coherent feedback

An open non-degenerate parametric oscillator (NDPO) is studied below threshold in the undepleted pump regime with time-delayed coherent feedback (TDCF). The figure of merit is the two-mode squeezing spectrum measuring the strength of continuous-valued Einstein-Podolsky-Rosen-type (EPR-type) entanglement between the down-converted modes in the output field. Exploring different areas of the parameter space reveals the possibility of significantly enhanced entanglement with tunable characteristic frequency. These resonance-like features are closely related to the unique dynamics emerging as a result of delayed feedback. The system is very sensitive to phase matching, which can be adjusted by various parameters of the setup. Further advantage of the current scheme is that significant enhancement of entanglement can be achieved even with finite extraneous losses.