Recent theoretical investigations on quantum coherent feedback networks havefound that with the same pump power, the Einstein-Podolski-Rosen (EPR)-likeentanglement generated via a dual nondegenerate optical parametric amplifier(NOPA) system placed in a certain coherent feedback loop is stronger than theEPR-like entangled pairs produced by a single NOPA. In this paper, we present alinear quantum system consisting of two NOPAs and a static linear passivenetwork of optical devices. The network has six inputs and six outputs, amongwhich four outputs and four inputs are connected in a coherent feedback loopwith the two NOPAs. This passive network is represented by a $6 \times 6$complex unitary matrix. A modified steepest descent method is used to find apassive complex unitary matrix at which the entanglement of this dual-NOPAnetwork is locally maximized. Here we choose the matrix corresponding to adual-NOPA coherent feedback network from our previous work as a starting pointfor the modified steepest descent algorithm. By decomposing the unitary matrixobtained by the algorithm as the product of so-called two-level unitarymatrices, we find an optimized configuration in which the complex matrix isrealized by a static optical network made of beam splitters.
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