Advances in real-world applications of Quantum Key Distribution

The increasing demand for unconditionally secure communication in an era of growing computational power has positioned Quantum Key Distribution (QKD) as a critical technology for future cybersecurity infrastructures. However, transitioning QKD systems from laboratory environments to robust, high-performance, real-world applications presents challenges in terms of speed, stability, scalability, and security. This thesis addresses these challenges through a multi-faceted investigation into the practical advancement of QKD systems. First, we introduce a complete theoretical model for Sagnac-based optical modulators, which are crucial components for stable quantum state encoding. We propose and experimentally validate novel modulation strategies, including a symmetric balanced scheme, that overcome the inherent repetition-rate limitations of traditional asymmetric designs. This innovation enables the development of high-speed, calibration-free encoders that mitigate environmental noise and patterning effects, achieving modulation rates exceeding 1.5 GHz. Second, leveraging this advanced hardware, we present the design and demonstration of a complete, high-performance, polarization-based QKD system developed for the QUANGO project. The system, implementing an efficient three-state decoy protocol, was rigorously tested over both optical fiber and a 620-meter urban free-space link. The results demonstrate exceptional performance, achieving a sustained Secret Key Rate (SKR) of over 1 Mb/s with a Quantum Bit Error Rate (QBER) below 1.5% in daylight free-space operation, validating its suitability for both terrestrial and future satellite-to-ground quantum networks. Third, we detail the deployment and long-term operational analysis of VenQCI, a 4-node metropolitan QKD network in Italy. This network integrates our QKD platform with commercial telecom infrastructure, utilizing resource-efficient optical switching to connect multiple nodes. A two-month analysis of the production network confirms its long-term stability and reliability, demonstrating continuous, automated key delivery for securing 100 Gbps MACsec-encrypted classical data links. Finally, we conduct a comprehensive security analysis of the iPognac encoder against Trojan-Horse Attacks (THAs). By modeling and experimentally executing attacks in both strong- and weak-light regimes, we characterize the system’s vulnerabilities. We quantify the effectiveness of practical countermeasures, demonstrating that a combination of standard passive optical components and active monitoring can provide robust security, requiringover 65dBof attenuation to rendersuch side-channelattacksineffective. Collectively, this work provides a significant contribution to the field by advancing QKD technology from the component level to deployed networks, paving the way for scalable and secure quantum communication infrastructures.