  The purpose of this paper is to extend linear systems and signals theory to
include single photon quantum signals. We provide detailed results describing
how quantum linear systems respond to multichannel single photon quantum
signals. In particular, we characterize the class of states (which we call {\em
photon-Gaussian} states) that result when multichannel photons are input to a
quantum linear system. We show that this class of quantum states is preserved
by quantum linear systems. Multichannel photon-Gaussian states are defined via
the action of certain creation and annihilation operators on Gaussian states.
Our results show how the output states are determined from the input states
through a pair of transfer function relations. We also provide equations from
which output signal intensities can be computed. Examples from quantum optics
are provided to illustrate the results.
