  A downlink for the return of scientific data from space probes at
interstellar distances is studied. The context is probes moving at relativistic
speed using a terrestrial directed-energy beam for propulsion, necessitating
very-low mass probes. Achieving simultaneous communication from a swarm of
probes launched at regular intervals to a target at the distance of Proxima
Centauri is addressed. The analysis focuses on fundamental physical and
statistical communication limitations on downlink performance rather than a
concrete implementation. Transmission time/distance and probe mass are chosen
to achieve the best data latency vs volume tradeoff. Challenges in targeting
multiple probe trajectories with a single receiver are addressed, including
multiplexing, parallax, and target star proper motion. Relevant sources of
background radiation, including cosmic, atmospheric, and receiver dark count
are identified and estimated. Direct detection enables high photon efficiency
and incoherent aperture combining. A novel burst pulse-position modulation
(BPPM) beneficially expands the optical bandwidth and ameliorates receiver dark
counts. A canonical receive optical collector combines minimum transmit power
with constrained swarm-probe coverage. Theoretical limits on reliable data
recovery and sensitivity to the various BPPM model parameters are applied,
including a wide range of total collector areas. Significant near-term
technological obstacles are identified. Enabling innovations include a high
peak-to-average power ratio, a large source extinguishing factor, the shortest
atmosphere-transparent wavelength to minimize target star interference,
adaptive optics for atmospheric turbulence, very selective bandpass filtering
(possibly with multiple passbands), very low dark-count single-photon
superconducting detectors, and very accurate attitude control and pointing
mechanisms.
