Huygens-Fresnel principle for N-photon states of light

We show that the propagation of a N-photon field in space and time can be described by a generalized Huygens-Fresnel integral. Using two examples, we then demonstrate how familiar Fourier optics techniques applied to a N-photon wave function can be used to engineer the propagation of entanglement and to design the way the detection of one photon shapes the state of the others.