Squeezing enhancement of degenerate parametric amplifiers via coherent feedback control

The purpose of this paper is to present an efficient coherent feedback control technique for squeezing enhancement of degenerate parametric amplifiers (DPAs). The feedback controllers to be designed are in the form of linear quantum stochastic systems which can be described by quantum feedback network theory recently developed. The problem of squeezing enhancement is formulated as a non-convex constrained nonlinear programming problem. Compared with previous study, the proposed approach reveals clear advantage when DPAs are lossy, as is always the case in practice.