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Recent technological advances could make interstellar travel possible, usingultra-lightweight sails pushed by lasers or solar photon pressure, at speeds ofa few percent the speed of light. Obtaining remote observational data from suchprobes is not trivial because of their minimal instrumentation (gram scale) andlarge distances (pc). We derive the optimal communication scheme to maximizethe data rate between a remote probe and home-base. he framework requirescoronagraphic suppression of the stellar background at the level of $10^{-9}$within a few tenths of an arcsecond of the bright star. Our work includesmodels for the loss of photons from diffraction, technological limitations,interstellar extinction, and atmospheric transmission. Major noise sources areatmospheric, zodiacal, stellar and instrumental. We examine the maximumcapacity using the "Holevo bound" which gives an upper limit to the amount ofinformation (bits) that can be encoded through a quantum state (photons), whichis a few bits per photon for optimistic signal and noise levels. This allowsfor data rates of order bits per second per Watt from a transmitter of size 1 mat a distance of $\alpha\,$Centauri (1.3 pc) to an earth-based large receivingtelescope (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, dueto extinction, to a maximum of $\approx3\,\mu$m to the center of the galaxy at8 kpc.

Interstellar communication. I. Maximized data rate for lightweight space-probes