  The possibility of achieving quantum communication using photons across
interstellar distances is examined. For this, different factors are considered
that could induce decoherence of photons, including the gravitational field of
astrophysical bodies, the particle content in the interstellar medium, and the
more local environment of the Solar System. The X-ray region of the spectrum is
identified as the prime candidate to establish a quantum communication channel,
although the optical and microwave bands could also enable communication across
large distances. Finally, we discuss what could be expected from a quantum
signal emitted by an extraterrestrial civilization, as well as the challenges
for the receiver end of the channel to identify and interpret such signals.
