Quantum Radar - A survey of the science, technology and literature

A quantum radar is a radar based on physical phenomena described by quantum physics, and not solely by classical physics. Like in many other emerging quantum technologies, e.g. quantum computing and quantum communication, entanglement is the central phenomenon used in a quantum radar. Entanglement implies a strong correlation between two (or more) particles (photons, pulses). Entanglement remain even if the particles are separated by distance. The most promising quantum radar technique today seems to be quantum illumination, theoretically presented by Seth Lloyd in 2008. Two entangled photons are generated, one, the idler, is kept in the radar. The other, the signal, is sent toward the target. If the signal photon is reflected back to the radar, it is likely to be detected due to its strong correlation with the idler. Quantum illumination, in difference to many other quantum technologies, works even in a noisy environment. Therefore, the quantum illumination protocol has been proposed even for quantum communication. The ideas behind a quantum radar is based on solid and well-accepted physics. The first experiments have been performed in the optical domain and just recently even solely in the microwave domain by use of a Josephson Parametric Amplifier. If the technology will develop and systems parameters interesting enough can be reached, is still an open question. Until now, the published research on quantum radars is small compared to other quantum technology areas, probably due to radars being a niche product. However, even relatively small efforts could give substantial results. Today's interest of a quantum radar is probably mainly military but potentially also medical.