Quantum Theory of Optical Networks

Abstract

Passive optical networks fed with non-classical light are used in various measurement schemes, as well as in communication technology and proposed quantum computing. Such networks can be built up from simple components like beam splitters, phase shifters or optical fibre couplers. Although their action on the input state is linear, they can lead to effects that are usually associated with nonlinear system. In particular the rise of entanglement between the input modes has attracted justified interest in recent years. The entanglement of the modes can be used for instance in various methods for simultaneous measurement of canonically conjugated variables. Another challenge that passive systems hide is the question how to find a proper Hamiltonian to a prescribed linear transformation. It is straightforward to derive a transfer matrix between inputs and outputs for a given Hamiltonian. The inverse step, from a transformation to the Hamiltonian, is nontrivial.