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Abstract
Research interest in quantum information processing is spurred by non-classical phenomena such as entanglement. This thesis focuses on Einstein-Podolsky-Rosen (EPR)-like entanglement between a pair of Gaussian continuous-mode fields, which can be produced by a nondegenerate optical parametric amplifier (NOPA). The thesis aims to exploit coherent feedback networks in the form of the feedback interconnection of multiple NOPAs to generate entanglement in a distributed and power efficient manner. Firstly, we show how EPR entanglement can be generated by a dual-NOPA coherent feedback system connecting two NOPAs over two transmission channels. We analyse stability and EPR entanglement in a lossless scenario and under the effect of transmission losses, amplification losses, time delays and phase shifts in the transmission channels. It is shown that in an ideal scenario without losses and delays, and when only transmission losses are present, the feedback connection can yield an increase in the quality of the entanglement while consuming less power, compared to a single NOPA and a two cascaded NOPA system. The thesis is then concerned with linear quantum networks of multiple NOPAs. The NOPAs are interconnected in a coherent feedback chain, connecting two communicating parties over two transmission channels. We analyse stability and EPR entanglement between two outgoing fields of interest under the effect of losses and time delays, and bipartite entanglement of two-mode Gaussian states of internal cavity modes of the multiple-NOPA networks in the lossless case. It is numerically shown that the network with more NOPAs is more power efficient for EPR entanglement generation. Finally, we study optimization of EPR entanglement of linear quantum systems consisting of two NOPAs and a static linear passive network of optical devices. The passive network has six inputs and six outputs. By employing a steepest descent method, we find an optimized static passive network made of beamsplitters. Subsequently, we look at a special case of the above configuration, where the passive network has two inputs and two outputs, and the system is considered in the idealized infinite bandwidth limit. We show that the dual-NOPA coherent feedback system has a local optimality property for generation of EPR entanglement.