Continuous wideband conversion of microwave and mm-wave radiation to optical domain using warm Ryberg atoms*

Bridging the radio frequency and optical domains could enable optical interconnects for quantum computers and facilitate next-generation quantum microwave sensors, detectors, and coherent imaging techniques. Efficient photon-to-photon conversion would open up the possibility of applying photon-counting measurement schemes to the previously inaccessible regimes of gigahertz and terahertz radiation, creating new opportunities for research in both fundamental and applied sciences. However, robust and field-deployable conversion remains challenging due to the natural incompatibility between microwave and optical photon energies.

Recent advances have led to several demonstrations of photon conversion, leveraging techniques such as optomechanics, electrooptics, and cold Rydberg atoms. These approaches, however, are often constrained by specific conditions, such as the need for cryogenic environments, complex impulse protocols, or narrow operational bandwidths. In contrast, microwave sensing using Rydberg atoms in warm vapor-cell systems has recently attracted significant attention due to their extraordinary sensitivity and potential for self-calibration based on atomic constants.

We demonstrate a relatively simple and robust radio-frequency to optical conversion scheme based on six-wave mixing in warm atomic media. This conversion process exploits strong dipole transitions between distinct Rydberg states to coherently absorb radio-frequency photons in the GHz or THz range and convert them into near-infrared photons with minimal added noise. We benchmark the converter across several orders of magnitude of RF field intensity, down to thermal radiation, achieving the intrinsic noise limit corresponding to 4K. Additionally, we demonstrate the ability to infer the RF photon statistics by implementing a Hanbury Brown-Twiss measurement and show that the conversion scheme can be used as a tool for absolute power calibration of various radiation sources.