  Achieving the ultimate Holevo limit of optical communication capacity
requires a joint-detection receiver which makes a collective quantum
measurement over multiple modulated symbols. Such superadditivity -- a higher
communication rate than that achievable by symbol-by-symbol optical detection
-- is a special case of the well-known nonlocality without entanglement and has
yet to be demonstrated. In this article, we propose and demonstrate a design of
joint-detection receivers, the Green Machine, that can achieve superadditivity.
We build this receiver and show that its capacity surpasses any
symbol-by-symbol receivers in the photon-starved regime with
binary-phase-shift-keying (BPSK). Our Green Machine receiver can also
significantly reduce the transmitter peak power requirement compared with the
pulse-position modulation (the conventional modulation format used for deep
space laser communication). We further show that the self-referenced phase
makes it immune to phase noise, e.g., atmospheric turbulence or platform
vibrations.
