  Breakthrough Starshot is an initiative to propel a sailcraft to Alpha
Centauri within the next generation. As the sailcraft transits Alpha Centauri
at 0.2 c, it looks for signs of life by imaging planets and gathering other
scientific data. After the transit, the 4.1-meter diameter sailcraft downlinks
its data to an Earth-based receiver. The present work estimates the raw data
rate of a 1.02 {\mu}m, 100 Watt laser that is received at 1.25 {\mu}m by a
30-meter telescope. The telescope receives 288 signal photons per second (-133
dBm) from the sailcraft after accounting for optical gains (+296 dBi),
conventional losses (-476 dB), relativistic effects (-3.5 dB), and link margin
(-3.0 dB). For this photon-starved Poisson channel with 0.1 nm equivalent noise
bandwidth, 90% detector quantum efficiency, 1024-ary PPM modulation, and 10^-3
raw bit error rate, the raw data rate is 260 bit/s (hard-decision) to 1.5
kbit/s (ideal) raw data rate, which is 8-50 Gbit/year. This rate is slowed by
noise, especially starlight from Alpha Centauri A scattered into the detector
by the atmosphere and receiver optics as sailcraft nears the star. Because this
is a flyby mission (the sailcraft does not stop in the Centauri system), the
proper motion of Alpha Centauri relative to Earth carries it away from the
sailcraft after transit, and the noise subsides over days to weeks. The
downlink can resume as soon as a day after transit, starting at 7-22 bit/s and
reaching nearly full speed after 4 months. By using a coronagraph on the
receiving telescope, full-rate downlink speed could be reached much sooner.
