  Recent technological advances could make interstellar travel possible, using
ultra-lightweight sails pushed by lasers or solar photon pressure, at speeds of
a few percent the speed of light. Obtaining remote observational data from such
probes is not trivial because of their minimal instrumentation (gram scale) and
large distances (pc). We derive the optimal communication scheme to maximize
the data rate between a remote probe and home-base. he framework requires
coronagraphic suppression of the stellar background at the level of $10^{-9}$
within a few tenths of an arcsecond of the bright star. Our work includes
models for the loss of photons from diffraction, technological limitations,
interstellar extinction, and atmospheric transmission. Major noise sources are
atmospheric, zodiacal, stellar and instrumental. We examine the maximum
capacity using the "Holevo bound" which gives an upper limit to the amount of
information (bits) that can be encoded through a quantum state (photons), which
is a few bits per photon for optimistic signal and noise levels. This allows
for data rates of order bits per second per Watt from a transmitter of size 1 m
at a distance of $\alpha\,$Centauri (1.3 pc) to an earth-based large receiving
telescope (E-ELT, 39 m). The optimal wavelength for this distance is 300 nm
(space-based receiver) to 400 nm (earth-based) and increases with distance, due
to extinction, to a maximum of $\approx3\,\mu$m to the center of the galaxy at
8 kpc.
