Quantum Radar with Undetected Photons

Quantum sensing promises to revolutionize sensing applications by using quantum states of light or matter as sensing probes. Photons are a natural choice for remote sensing because they can travel to and interact with a distant target. Existing sensing schemes, currently limited to tabletop experiments, require a quantum memory to store a single photon of an initially entangled pair until its twin reflects off a target and returns. Expanding the sensing range faces challenges such as long-time quantum storage and photon loss and noise when transmitting quantum signals over long distances. We propose a novel quantum sensing framework, dubbed qCOMBPASS, to address these challenges. The proposed scheme employs quantum frequency combs—optical trains of identical laser pulses—with path identity for remote sensing of signatures. This new concept is akin to a quantum radar based on entangled frequency comb pairs that uses path identity to detect, range, and sense a remote target. They propose to measure pulses of one comb that never flew to the target yet contain target information “teleported” by quantum-induced coherence, from the other comb that did fly to the target but is not detected. This theoretical study outlines an experimental scheme to demonstrate the qCOMBPASS concept using two-mode squeezed quantum combs, in which the optical phase fluctuates less than a normal laser at the expense of an increased fluctuation in intensity. qCOMBPASS has the potential to impact remote quantum sensing, imaging, metrology, and communications. These applications include detection and ranging of low-reflectivity objects, discreet surveillance from space with low detection probability, quantum clock synchronization, and networks of distributed quantum sensors. The first qCOMBPASS paper will appear shortly in PRX and can be found in arXiv: 2410.07044.