  The position-representation wave function for multi-photon states and its
equation of motion are introduced. A major strength of the theory is that it
describes the complete evolution (including polarization and entanglement) of
multi-photon states propagating through inhomogeneous media. As a demonstration
of the two-photon wave function's use, we show how two photons in an
orbital-angular-momentum entangled state decohere upon propagation through a
turbulent atmosphere.
