  Quantum target detection is an emerging application that utilizes
entanglement to enhance the sensing of the presence of an object. Although
several experimental demonstrations for certain situations have been reported
recently, the single-shot detection limit imposed by the Helstrom limit has not
been reached because of the unknown optimum measurements. Here we report an
experimental demonstration of quantum target detection, also known as quantum
illumination, in the single-photon limit. In our experiment, one photon of the
maximally entangled photon pair is employed as the probe signal and the
corresponding optimum measurement is implemented at the receiver. We explore
the detection problem in different regions of the parameter space and verify
that the quantum advantage exists even in a forbidden region of the
conventional illumination, where all classical schemes become useless. Our
results indicate that quantum illumination breaks the classical limit for up to
40%, while approaching the quantum limit imposed by the Helstrom limit. These
results not only demonstrate the advantage of quantum illumination, but also
manifest its valuable potential of target detection.
