  An optical transmitter that uses entangled light generated by spontaneous
parametric downconversion (SPDC), in conjunction with an optimal
quantum-optical receiver (whose implementation is not yet known) is in
principle capable of obtaining up to a 6 dB gain in the error-probability
exponent over the optimum-reception un-entangled coherent-state lidar to detect
the presence of a far-away target subject to entanglement-breaking loss and
noise in the free-space link [Lloyd'08, Tan'08]. We present an explicit design
of a structured quantum-illumination receiver, which in conjunction with the
SPDC transmitter is shown to achieve up to a 3 dB error-exponent advantage over
the classical sensor. Apart from being fairly feasible for a proof-of-principle
demonstration, this is to our knowledge the first structured design of a
quantum-optical sensor for target detection that outperforms the comparable
best classical lidar sensor appreciably in a low-brightness, lossy and noisy
operating regime.
