Light is remarkably good at keeping its polarization state intact for long distances through single mode fiber. At least historically, the main issues with doing quantum computation with light is that’s it’s hard to store light and hard to get one photon to interact with another one in a controlled manner.
(Polarization of a photon is a two-state quantum system, otherwise known as a qubit.)
> Does that mean that an individual and unique photon travels the full distance from transmitter to receiver without interacting with anything?
Sort of. There will always be attenuation, which is more or less equivalent to losing some fraction of photons. And one can thing of the interaction of light with glass as the photons being absorbed and re-emitted, but thinking about that way doesn’t actually change the result. And, in any case, there isn’t really any such thing as an individual photon: photons are indistinguishable bosons.
In any case, the photons absolutely do interact on the way, but that interaction usually does not affect their polarization, and one of the many counterintuitive effects of quantum mechanics is that, if the polarization is intact, then no one learned it along the way.
