Microwave quantum radar based on Rydberg atoms

We investigate the feasibility of optical-to-microwave photon conversion, intended for quantum radar (QR), using a six-wave mixing scheme in Rb-87 Rydberg state atomic vapor. Coherence between entangled microwave photons affords QR a significant gain in remote sensing performance relative to conventional radar at the same power level. The challenge of QR comes from developing compact sources and detectors for entangled microwave photons. Existing demonstrations of entangled microwave photon generation require bulky dilution refrigerators. Our approach explores entangled, 84GHz microwave photon generation through Rydberg atom-based quantum frequency conversion of optical entangled photons. The high frequency (84GHz) relaxes the cooling requirement for suppressing thermal background and lies in the W-band (75GHz – 110GHz) atmospheric transparency window with 0.1dB/km loss. Microwave photon detection can be performed by reversing the six-wave mixing process for microwave-to-optical conversion.