Entanglement enhancement from coherent feedback-controlled and cascaded nondegenerate optical parametric amplifiers

The entanglement enhancement of the entangled states from the nondegenerate optical parametric amplifier (NOPA) is an important issue in quantum information science and technology. The coherent feedback-controlled NOPA (CFC-NOPA) and cascaded NOPA (C-NOPA) are two promising schemes on entanglement enhancement. In this study, the features of the quantum entanglement of the entangled states produced by the two schemes is totally studied and compared based on feasible physical parameters of realistic systems. We demonstrate that the CFC-NOPA shows better superiority on entanglement enhancement and better robustness versus the losses of the system for low frequency range (below several hundred kHz). For the high frequency range (several MHz), the situation is inverse, and the C-NOPA is more advantageous. Our results can provide useful references for the design of high-quality entanglement resources.