  Interstellar communications are achievable with gram-scale spacecraft using
swarm techniques introduced herein if an adequate energy source, clocks and a
suitable communications protocol exist. The essence of our approach to the
Breakthrough Starshot challenge is to launch a long string of 100s of
gram-scale interstellar probes at 0.2c in a firing campaign up to a year long,
maintain continuous contact with them (directly amongst each other and via
Earth utilizing the launch laser), and gradually, during the 20-year cruise,
dynamically coalesce the long string into a lens-shaped mesh network
$\sim$100,000 km across centered on the target planet Proxima b at the time of
fly-by.
  In-flight formation would be accomplished using the "time on target"
technique of grossly modulating the initial launch velocity between the head
and the tail of the string, and combined with continual fine control or
"velocity on target" by adjusting the attitude of selected probes, exploiting
the drag imparted by the ISM.
  Such a swarm could tolerate significant attrition, e.g., by collisions
enroute with interstellar dust grains, thus mitigating the risk that comes with
"putting all your eggs in one basket". It would also enable the observation of
Proxima b at close range from a multiplicity of viewpoints. Swarm
synchronization with state-of-the-art space-rated clocks would enable
operational coherence if not actual phase coherence in the swarm optical
communications. Betavoltaic technology, which should be commercialized and
space-rated in the next decade, can provide an adequate primary energy storage
for these swarms. The combination would thus enable data return rates orders of
magnitude greater than possible from a single probe.
