Quantum illumination
Quantum illumination is a paradigm for target detection that employs quantum entanglement between a signal electromagnetic mode and an idler electromagnetic mode, as well as joint measurement of these modes. 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. In principle, quantum illumination can be beneficial even if the original entanglement is completely destroyed by a lossy and noisy environment. Introduction Many quantum information applications, such as quantum teleportation, quantum error correction, and superdense coding, rely on entanglement. However, entanglement is a fragile quantum property between particles and can be easily destroyed by loss and noise arising from interaction with the environment, leading to quantum decoherence. Entanglement is therefore considered very hard to use in lossy and noisy environment. Lloyd, Shapiro and collaborators 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. This implies that the use of entanglement should not be dismissed in entanglement-breaking scenarios. 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. Quantum illumination is particularly useful in extremely lossy and noisy situations. History Theory The concept of quantum illumination was first introduced by Seth Lloyd and collaborators at MIT in 2008. A theoretical proposal for quantum illumination using Gaussian states was proposed by Jeffrey Shapiro and collaborators. The basic setup of quantum illumination is target detection. Here the sender prepares two entangled systems, called signal and idler. The idler is retained while the signal is sent to probe the presence of a low-reflectivity object in a region with bright background noise. The reflection from the object is then combined with the retained idler system in a joint quantum measurement providing two possible outcomes: object present or object absent. 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. The advantage of the scheme is evident at low energies where the mean number of photons in each signal system is very low (of the order of one photon or less). In this case, at fixed low energy, the probability of success in detecting a target has a remarkable improvement with respect to classical detection schemes, where entanglement is not used and signal systems are prepared in coherent states (technically, there is a 6 dB improvement in the error exponent ). A key feature of quantum illumination is that the entanglement between the idler system and the reflected signal system is completely lost in the process. However, the residual quantum correlations between these two systems (idler-reflected signal) remain so strong that they could only be created by the presence of entanglement in the initial systems (idler-signal). Because the reflected signal is quantum-correlated with the retained