An optical polarisation control scheme utilising optical heterodyne detection for coherent transmission is proposed and demonstrated at a wavelength of 1.5 μm. It converts an arbitrary polarisation state to a linear state at a received optical power level of less than −70 dBm.
Laser-light (coherent-state) modulation is sufficient to achieve the ultimate (Holevo) capacity of classical communication over a lossy and noisy optical channel, but requires a receiver that jointly detects long modulated codewords with highly nonlinear quantum operations, which are near-impossible to realize using current technology. We analyze the capacity of the lossy-noisy optical channel when the transmitter uses coherent state modulation but the receiver is restricted to a general quantum-limited Gaussian receiver, i.e., one that may involve arbitrary combinations of Gaussian operations (passive linear optics: beamsplitters and phase-shifters, second order nonlinear process (squeezers), along with homodyne or heterodyne detection measurements) and any amount of classical feedforward within the receiver. Under these assumptions, we show that the Gaussian receiver that attains the maximum mutual information is either homodyne detection, heterodyne detection, or time sharing between the two, depending upon the received power level. In other words, our result shows that to exceed the theoretical limit of conventional coherent optical communications, one has to incorporate non-Gaussian (i.e. higher order nonlinear) operations in the receiver.
Abstract 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.