All-optical multiband quantum Rydberg radiometer

Radiometry at microwave (GHz) and mm-wave (THz) frequencies is a fundamental technique for measuring the power of incoherent thermal radiation, serving as a critical tool for meteorology, oceanography, and Earth observation to retrieve crucial parameters for climate models and forecasts. Furthermore, at THz frequencies, these measurements allow for the detection of faint thermal signatures from molecular transitions, which is essential for tracking ozone depletion and studying line emission from diverse objects such as molecular clouds, galaxies and comets.

Here, we introduce a novel all-optical radiometer based on coherent photon-to-photon upconversion mediated by Rydberg atoms in a hot vapour cell. This architecture utilises a compact sensing head, allowing bulky support equipment to be located far from the detection point. A key advantage is broad spectral tunability: the radiometer can operate at widely different bands without hardware modification, controlled solely by tuning the optical pumps to address different Rydberg states. We demonstrate the system’s capabilities by measuring black-body targets at various temperatures, obtaining extrinsic noise temperatures as low as 60 K at 120 GHz and 300 K at 600 GHz. Notably, the intrinsic conversion noise is measured at approximately 2 K, indicating that performance is currently limited by external coupling and conductive and dielectric losses in the sensor head, leaving significant room for optimisation. We characterise the radiometer in terms of bandwidth, and prove the thermal statistics of the received radiation by investigating photon number correlations of the converted field.

This platform unlocks detection paradigms inaccessible to classical sensors. In principle, simultaneous multi-band operation is achievable by multiplexing the optical pump fields. Furthermore, because the upconversion process preserves quantum statistics, photon number correlation measurements can be employed between radiometer nodes for enhanced calibration and noise-rejection. Finally, at higher frequencies, the ability to sense multiple spatial modes paves the way for single-photon thermal imaging for mm-waves.