DEVELOPMENT OF QUANTUM TECHNOLOGIES FOR QUANTUM COMMUNICATION

This PhD project, developed within a PNRR initiative co-funded by the Università degli Studi di Milano and Ricerca sul Sistema Energetico – RSE S.p.A., investigates quantum communication in critical industrial environments, focusing on the electrical power grid, while also advancing experimental techniques for quantum-state generation and detection in the laboratory. A preliminary survey of Quantum Key Distribution (QKD) technologies and national power-grid fiber networks has indicated that commercial solutions may work on short fiber links along medium-voltage urban lines, while long-distance communication along high-voltage transmission lines remains challenging. This has motivated the exploration of the Twin-Field (TF) QKD protocol, an interferometric scheme that allows to extend the rate-distance limit of standard QKD. Since TF-QKD encodes information in the optical phase, maintaining phase coherence is crucial. In collaboration with INRiM, an active phase-noise cancellation system for TF-QKD has been implemented and tested, demonstrating stable phase coherence over two 50-km fiber spools. Moreover, field measurements on underground fibers near medium-voltage lines have been conducted in the RSE test facility, providing a preliminary phase-noise characterization as an initial step toward real-world TF-QKD deployment in power-grid environments. Moreover, within the context of university research on the detection of entangled states in optical angular momentum generated at high-repetition-rate (100 MHz), a novel scheme for a sine-wave-gated SPAD has been proposed, distinguished by its simplicity and low implementation cost. The constructed prototypes have achieved the employed photodiode’s nominal quantum efficiency, correctly biasing the device and detecting all generated pulses, with a dead time below the source repetition period and negligible dark counts. Despite its simplicity, the detector has reached performances comparable to more complex systems already reported in the literature, and its novelty resulted in a patent. The operating principle and full electronic implementation are detailed in this thesis.