Twin-Field Quantum Key Distribution on the telecommunication optical fiber network. Analysis and experimental implementation of the asymmetric SNS protocol.

Abstract This thesis, developed during an experimental research project at the Italian Na- tional Metrology Research Institute (INRIM) in Turin, focuses on the practical implementation of the Twin-Field Quantum Key Distribution (TF-QKD) protocol. This approach is promising for realizing quantum-secure communication over long distances, as it suffers less from transmission loss compared to more conventional QKD protocols. However, it requires ultrastable lasers and fully coherent photon propagation over noisy channels. The main objective of my research was to address the key technical challenges that have so far impeded its long-distance application in real-world optical fiber networks, with a particular focus on maintaining phase coherence. After introducing the fundamentals of QKD, this work presents a theoretical analysis of the phase coherence problem, showing the impairments associated to the application of TF-KD in real-world optical networks. Based on the analyzed criticalities, we present an experimental architecture that applies the dual-band stabilization technique to an Independent laser scheme to detect and compensate for fiber noise in real-time without affecting quantum transmission. In particular, this approach uses commercial technologies, such as narrow-linewidth fiber lasers, to replace the complex and expensive ultra-stable lasers based on Fabry-Pérot cavities and vacuum systems used in previous demonstrations. This allowed for the realization of a compact and transportable setup suitable for the real-world application of TF-QKD in modern telecommunication networks. During my experimental work, I had the opportunity to characterize the optical components of the setup. This included an analysis of the laser sources to evaluate Raman scattering as a function of their power, thereby limiting noise. I also char- acterized the phase and intensity modulators by evaluating their Extinction Ratio (ER), and the detectors in terms of their dark counts. For all components, their adequacy for the protocol’s critical requirements was verified. Furthermore, specific control software was developed for the advanced Field-Programmable Gate Arrays (FPGAs), which are used within each communication node for intensity/phase modulation and temporal synchronization. This latter function is accomplished by also leveraging advanced protocols like White Rabbit for nanosecond-order synchronization The thesis also includes an estimation of the expected secret key rate for a TF-QKD implementation on a deployed fiber network under realistic conditions, using the decoy-state method. This simulation is based on a complete mathematical analysis of the asymmetric SNS protocol variant, which is considered the most robust and secure TF-QKD implementation to date. Overall, the setup is distinguished by its robustness and compactness. By employ- ing a complex optical system based on four active feedback controls, the setup demonstrates its ability to effectively compensate for fast and slow phase drift, polarization mismatch, and temporal delay between the twin fields. These features make it suitable for TF-QKD implementation on optical fiber links of up to 200 km in noisy environments, ensuring effective fast phase control for over 4 s with residual noise correction occurring in windows of about 100 ms. Ultimately, this work validates the setup as a highly promising solution, paving the way for the system’s integration into future global quantum networks.