  Understanding the behavior of light in non-equilibrium scenarios underpins
much of quantum optics and optical physics. While lasers provide a severe
example of a non-equilibrium problem, recent interests in the near-equilibrium
physics of so-called photon `gases', such as in Bose condensation of light or
in attempts to make photonic quantum simulators, suggest one re-examine some
near-equilibrium cases. Here we consider how a sinusoidal parametric coupling
between two semi-infinite photonic transmission lines leads to the creation and
flow of photons between the two lines. Our approach provides a photonic analog
to the Landauer transport formula, and using non-equilbrium Green's functions,
we can extend it to the case of an interacting region between two photonic
`leads' where the sinusoid frequency plays the role of a voltage bias.
Crucially, we identify both the mathematical framework and the physical regime
in which photonic transport is directly analogous to electronic transport and
regimes in which other behavior such as two-mode squeezing can emerge.
