Spontaneous and Stimulated emission from open quantum systems

We provide a systematic treatment of using the input-output formalism for computing the output photon state emitted by an initially excited open quantum system coupled to a finite number of loss channels (e.g. input and output waveguides) with or without stimulation from a few-photon input pulse. The central result of this work is a connection between the state of the photons emitted by the local system and its Green's functions. We illustrate our formalism by computing the output state for a linear system, two-level system and Jaynes-Cumming system. We also extend our formalism to analyze open quantum systems coupled to a spatial continuum (e.g. free space or photonic crystals) to predict the spatial profile of few photon emissions from such systems.