Entanglement enhancement from a two-port feedback optical parametric amplifier

How to achieve the maximal entanglement, for given finite entanglement resources? In this paper, we propose a two-port feedback optical parametric amplifier (TFPA) consisting of a nondegenerate optical parametric amplifier (NOPA) and two linear beam splitters. The former is an entanglement resource, and the latter serve as feedback controllers that feed the two entangled beams generated from the NOPA back into its input ports. By manipulating the reflectivity of the beam splitters and the phase delays introduced by the feedback paths, an entanglement enhancement can be achieved by the TFPA. Furthermore, we demonstrate that such entanglement enhancement can happen at all phase points for the ideal case, and the maximal entanglement can be infinitely large. For the lossy case, our TFPA also has its unique advantages in the robustness of the losses of the system.