  The signal half of an entangled twin-beam, generated using spontaneous
parametric downconversion, interrogates a region of space that is suspected of
containing a target, and has high loss and high (entanglement-breaking)
background noise. A joint measurement is performed on the returned light and
the idler beam that was retained at the transmitter. An optimal quantum
receiver, whose implementation is not yet known, was shown to achieve 6 dB gain
in the error-probability exponent relative to that achieved with a single
coherent-state (classical) laser transmitter and the optimum receiver. We
present two structured optical receivers that achieve up to 3 dB gain in the
error exponent over that attained with the classical sensor. These are to our
knowledge the first designs of quantum-optical sensors for target detection,
which can be readily implemented in a proof-of-concept experiment, that
appreciably outperform the best classical sensor in the low-signal-brightness,
high-loss and high-noise operating regime.
