Ribezzo, Domenico (2024) Interconnecting Nations through Quantum Networks for Enhanced Quantum Key Distribution. [Tesi di dottorato]

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The term quantum network carries diverse interpretations depending on one's background. Quantum networks assume a pivotal role in interconnecting quantum processors or in the quantum metrology field. However, all the existing quantum networks, which are currently operational and have reached a noteworthy level of advancement, primarily serve communication purposes. This work aligns with the ongoing exploration and advancements within this research domain. This PhD thesis is specifically focused on the implementation of quantum key distribution (QKD) protocols. QKD is a cryptographic method used to share secure keys among different parts, whose reliability is guaranteed by quantum mechanics laws. The first two chapters present a fundamental theoretical introduction to QKD, including motivations, state-of-the-art and the introduction to practical QKD implementations. The QKD setups described in this PhD thesis are based on sources of weak coherent pulses generated by a continuous wave laser, utilizing an efficient three-state BB84 protocol and time-bin encoding. The next chapters are dedicated to experiments, including three different implementations of quantum links or quantum networks in real-world scenarios, and a method to realize the phase-randomization of weak coherent pulses. The first description reports how the cities of Trieste (Italy), Rijeka (Croatia), and Ljubljana (Slovenia) were connected in a fully operating quantum network created from scratch in just a few days. Subsequently, a link established between Malta and the city of Pozzallo (Sicily), over an underwater optical fiber, has been used to test an innovative single-photon detector produced by a research group of Politecnico di Milano, demonstrating an advantage with respect to commercial detectors. The next chapter is dedicated to a detailed description of the phase-randomization implementation, necessary for decoy-state QKD, a method that provides security even in the case of not real single-photon sources. Finally, the first field-trial QKD experiment employing high-dimensional quantum states is described. It has been proved that implementing protocols able to exploit higher-dimensional Hilbert space makes the QKD process able to deal with higher levels of noise and, since each state contains a greater amount of information, even the key rate can benefit. Here we show a record-high secret-key rate distribution in a 4-dimensional hybrid time-bin- and path-encoded QKD system with more than 100\% improvement compared to standard 2-dimensional BB84 devices. The employed infrastructure is a 52-km multicore fiber link deployed under the city of L’Aquila (Italy). In the world, several quantum networks have already been implemented and are currently operational. The works described in this PhD thesis explore various practicable paths to address the challenges currently limiting the development of quantum communications, such as the maximum distance achievable with individual links or limited key rates. In this regard, this work aligns with the perspective of implementing a European quantum network.

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