Microwave-to-optical conversion using room-temperature atoms

Conversion between microwave and optical photons is a challenging task with many possible applications. The potential ranges from interconnects between quantum devices such as superconducting chips, to the detection of very weak signals via upconversion and photon counting. Such detection would be beneficial for radar science, radioastronomy, and telecommunications, and the requirements are more relaxed compared with fully quantum systems. We find that in this case, upconversion in a room-temperature ensemble of Rydberg atoms provides an impressive performance, beating typical microwave amplifiers and matching the performance of cryogenic electronics in terms of intrinsic noise.

In our experiment, we select a six-wave mixing path specifically designed to minimize the generation of optical noise. This allows us to perform coherent upconversion of 13.9 GHz signal to near-infrared photons, and subsequently use a photon-counting detector. The intrinsic noise in the process corresponds to only 3.8 K equivalent noise temperature. With this device, capable of effectively counting the microwave photons, we are able to observe upconverted thermal radiation, and thus observe its second-order correlation function, as well as two-photon interference between thermal radiation and coherent signal. As additional benefits, the system has a large and tunable bandwidth, and an excellent dynamic range of almost 60 dB. The intrinsic efficiency of the device reaches 1%, and could be further improved by employing cavities, also potentially leading to strong-coupling effects at room temperature. We envisage that the presented method could be both employed as a very sensitive microwave detector in many applications, even including space-borne detectors, as well as ported to cryogenic systems and used deeply in the quantum regime.