Alessia Allevi - Como Lake Institute of Photonics, Department of Science and High Technology, University of Insubria, via Valleggio 11, 22100 Como, Italy

Hybrid SiPM-based detection system for underwater quantum optical communication

Quantum optical communication protocols demand efficient and versatile detection schemes to maximize information transfer rates. Hybrid systems combining photon-number-resolving (PNR) detection with interferometric schemes have recently emerged as a powerful alternative to conventional single-photon or homodyne approaches, enabling simultaneous access to both discrete and continuous degrees of freedom. Silicon photomultipliers (SiPMs) are particularly compelling PNR detectors in this context: they offer a wide dynamic range spanning the mesoscopic intensity regime, native sensitivity in the blue spectral region, robustness, and suitability for real-time digital readout, all at a fraction of the cost and complexity of competing technologies. 

We present a proof-of-principle quantum communication experiment designed for an underwater propagation environment, where the blue spectral window minimizes water absorption losses. The high photon detection efficiency of SiPMs in this wavelength range offers a decisive advantage over solutions optimized for telecom wavelengths, while their compatibility with MHz-rate operation matches the relaxed data-rate requirements typical of underwater channels, conventionally served by acoustic communication. 

Information is encoded in the amplitude and phase of coherent states generated by a pulsed laser at a tunable repetition rate. The detection system employs an interferometric scheme in which the signal is mixed with a local oscillator. Each output of the interferometer is measured by a SiPM coupled to an FPGA-based real-time digital processing pipeline. Processing the measurements outputs enables to retrieve the encoded information while simultaneously providing access to quadratures and photon statistics, demonstrating the full potential of SiPM-based hybrid detection for practical quantum optical communication.