The signal mode is propagated toward a region of space, and it is either lost or reflected, depending on whether a target is absent or present, respectively.
Some showed that, even though entanglement itself may not survive, the residual correlation between the two initially-entangled systems remains much higher than any initial classical states can provide.
Quantum illumination takes advantage of this stronger-than-classical residual correlations between two systems to achieve a performance enhancement over all schemes based on transmitting classical states with comparable power levels.
More precisely, the probing process is repeated many times so that many pairs of signal-idler systems are collected at the receiver for the joint quantum detection.
Because the reflected signal is quantum-correlated with the retained idler system, it can be distinguished among all the uncorrelated background thermal photons that are also received by the detector.
The original proposal from [3] was analyzed in the Bayesian setting of hypothesis testing, in which prior probabilities are assigned to the hypotheses that the target is absent or present.
The first experimental effort to perform microwave quantum illumination was based on using Josephson parametric amplifier and a digital receiver.
Potential applications of quantum illumination include target detection in high background noise environments, but also ultra-sensitive biological imaging and sensing, and secure communication.