Quantum coherence of photons at cosmological distances
  We identify potential sources of decoherence for $U(1)$ gauge bosons from a
cosmological standpoint. Besides interactions with different species in the
cosmological medium, we also consider effects due to the expansion of the
Universe, which can produce particles (especially scalars) than can potentially
interact with the photon in a quantum state. We look in particular at the case
of axion-like particles and their predicted decay channels in our analysis.
These interactions are shown to have a negligible effect as far as decoherence
goes. Interaction rates with CMB radiation or through Thomson scattering are
small, so that the interstellar medium remains the biggest decoherence factor.
Thus, quantum teleportation experiments with photon energies in the range
$1$-$10$ keV should be feasible at cosmological distances up to the galaxy
formation epoch or beyond ($z \sim 100$).
