  The Bialynicki-Birula-Sipe photon wave function formalism is extended to
include the interaction between photons and continuous non-absorptive media.
When the second quantization of this formalism is introduced, a new way of
describing the quantum interactions between light and matter emerges. As an
example of application, the quantum state of the twin photons generated by
parametric down conversion is obtained in agreement with previous treatments,
but with a more intuitive interpretation.
