Low frequency approximation for a class of linear complex quantum systems using cascade cavity realization

This paper presents a method for approximating a class of complex transfer function matrices corresponding to physically realizable complex linear quantum systems. The class of complex linear quantum systems under consideration includes interconnections of passive optical components such as cavities, beam-splitters, phase-shifters and interferometers. This approximation method builds on a previous result for the cascade realization of these transfer function matrices and gives good approximations at low frequencies.