Quantum illumination is a paradigm that uses quantum entanglement beneficially even if the original entanglement is completely destroyed by a lossy and noisy environment.
The concept of quantum illumination was first introduced by Seth Lloyd and collaborators at MIT in 2008. An experimentally feasible scheme for quantum illumination can be realized using Gaussian states as demonstrated by Jeffrey Shapiro and collaborators from the same institute. In 2009, a secure communication scheme based on quantum illumination was proposed. This scheme is a variant of the quantum cryptographic protocols based on continuous variables and two-way quantum communication introduced by Stefano Pirandola, Seth Lloyd and collaborators in 2008. In 2015 an extension of quantum illumination to microwave frequencies has been proposed by Shabir Barzanjeh, Stefano Pirandola and collaborators providing the first theoretical prototype of quantum radar.
In 2013, Lopaeva et al. exploited photon number correlations, instead of entanglement, in a sub-optimal target detection experiment. To illustrate the benefit of quantum entanglement, in 2013 Zhang et al. reported a secure communication experiment based on quantum illumination and demonstrated for the first time that entanglement can enable a substantial performance advantage in the presence of quantum decoherence. In 2015, Zhang et al. applied quantum illumination in sensing and showed that employing entanglement can yield a higher signal-to-noise ratio than the optimal classical scheme can provide, even though the highly lossy and noisy environment completely destroys the initial entanglement. This sensing experiment thus proved the original theoretical proposals of quantum illumination.
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.
Potential applications of quantum illumination include target detection in high background noise environments, but also ultra-sensitive biological imaging and sensing, and secure communication.