Title: A path integral description of quantum nonlinear optics in arbitrary media

Abstract: We present a method, based on Feynman path integrals, to describe the propagation and properties of the quantised electromagnetic field in an arbitrary, nonlinear medium. We provide a general theory, valid for any order of optical nonlinearity, and we then specialise the the case of second order nonlinear processes. In particular, we show, that second-order nonlinear processes in arbitrary media, under the undepleted pump approximation, can be described by an effective free electromagnetic field, propagating in a vacuum, dressed by the medium itself. Moreover, we show,that the probability of such processes to occur is related to the biphoton propagator, which contains informations about the structure of the medium, its nonlinear properties, and the structure of the pump beam.