  The information capacity of an optical channel under power constraints is
ultimately limited by the quantum nature of transmitted signals. We discuss
currently available and emerging photonic technologies whose combination can be
shown theoretically to enable nearly quantum-limited operation of a noisy
optical communication link in the photon-starved regime, with the information
rate scaling linearly in the detected signal power. The key ingredients are
quantum pulse gating to facilitate mode selectivity, photon-number-resolved
direct detection, and a photon-efficient high-order modulation format such as
pulse position modulation, frequency shift keying, or binary phase shift keyed
Hadamard words decoded optically using structured receivers.
