  Quantum illumination is a quantum-optical sensing technique in which an
entangled source is exploited to improve the detection of a low-reflectivity
object that is immersed in a bright thermal background. Here we describe and
analyze a system for applying this technique at microwave frequencies, a more
appropriate spectral region for target detection than the optical, due to the
naturally-occurring bright thermal background in the microwave regime. We use
an electro-optomechanical converter to entangle microwave signal and optical
idler fields, with the former being sent to probe the target region and the
latter being retained at the source. The microwave radiation collected from the
target region is then phase conjugated and upconverted into an optical field
that is combined with the retained idler in a joint-detection quantum
measurement. The error probability of this microwave quantum-illumination
system, or quantum radar, is shown to be superior to that of any classical
microwave radar of equal transmitted energy.
