Gaussian states in quantum field theory: Exact representations of relative phase in superpositions of Gaussian states

Gaussian quantum mechanics is a powerful tool regularly used in quantum optics to model linear and quadratic Hamiltonians efficiently. Recent interest in qubit--continuous-variable (CV) hybrid models has revealed a simple, yet important gap in our knowledge, namely, how to fully manipulate superpositions of Gaussian states. In this paper, we show how to faithfully represent a quadratic Gaussian state in the Fock basis. Specifically, we evaluate the phase necessary to equate the unitary representation of a zero-mean Gaussian state with its Fock representation. This allows for the coherent manipulation of superpositions of Gaussian states, especially the evaluation of expectation values from these superposed states. We then use this method to model a simple quantum field theory communication protocol using quadratic detectors in a regime that has previously been impossible to solve. The result presented in this paper is expected to become increasingly relevant with hybrid-CV systems, especially in the strong-coupling regime.