Atto‐Watt Photo‐Detection at Mid‐Infrared Wavelengths by a Room‐Temperature Balanced Heterodyne Set‐Up

Balanced heterodyne detection (BHD) is a key technology at visible and near‐infrared wavelengths for quantum communication and quantum sensing applications based on coherent read‐out schemes. Extending BHD at mid‐infrared wavelengths (4–11 µm), given the reduced scattering and favourable transparent atmospheric windows, could enable robust earth‐satellite links and few‐photon imaging in high‐noise environments. Currently, quantum applications at these wavelengths are hindered by the lack of high‐sensitivity, room‐temperature photoreceivers; moreover, mid‐infrared single‐photon‐detectors reported to date (superconductors, single‐electron‐transistors, or avalanche‐photodiodes) require cryogenic operation, limiting practicality. Here, a room‐temperature BHD system operating at 4.6 µm‐wavelength with atto‐watt sensitivity level, corresponding to a few tens of photons per second, is demonstrated. This result is obtained by selecting commercially available photodetectors with the highest detectivity and exploiting two heterodyne setups ‐one involving a single quantum‐cascade‐laser (QCL) and an acousto‐optic‐modulator (AOM), and the other one including two QCLs with mutual coherence ensured by a phase‐locked‐loop. Combining a sufficiently high local oscillator (LO) power and the high phase‐coherence between signal and LO is crucial to push the system noise‐equivalent‐power (NEP) to values approaching the shot‐noise‐limit, as confirmed by few‐photons interferometry measurements. This work not only validates viable methods to detect ultra‐low‐intensity signals, but is also potentially scalable to the entire wavelength range already accessible by state‐of‐the‐art mid‐infrared technology.