Lo, Yuen San; (2025) Advancing Optical Transmitter Technology for Practical Quantum Communication Systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Quantum key distribution (QKD) allows two distant users to communicate with information theoretic security by harnessing the fundamental laws of physics. The development of such technology is of particular interest at present, as advances in quantum computing increasingly threaten the traditional security frameworks based on public-key cryptography. Over the past few decades, we have witnessed significant progress in QKD—quantum networks have transitioned from theoretical concepts to real-world applications, and practical use cases of are emerging. Despite these advancements, widespread deployment of QKD has yet to be realised. One of the primary challenges hindering its broader implementation lies in the engineering aspects—the system complexity as well as various technical issues that limits the key rate performance. In this thesis, we explore different approaches to reduce the complexity and enhance the performance of quantum transmitters, aiming to make QKD a more practical and accessible technology. In quantum transmitter, much of the complexity is contributed by numerous intensity and phase modulators, which have been essential to prepare the quantum states for BB84 QKD protocol. We show that, it is possible to generate all the required quantum states with just two gain-switched lasers configured in an optical injection locking setup, thereby removing the needs for any dedicated modulators. We further demonstrate that our laser modulation technique can be effectively extended to a more advanced protocol, measurement-device-independent (MDI) QKD, enabling a compact system design that achieves gigahertz clock rates performance. One of the main limitations on the clock rate of the QKD system is due to phase randomisation of the optical pulses. We develop a method to overcome this problem through optical seeding with spontaneous emission photons, achieving phase randomisation at a clock rate of 10 GHz. To address the stringent operating conditions required by the QKD transmitter, we develop autonomous optimisation techniques based on a genetic algorithm and reinforcement learning. Our approaches allow the system to be tuned to its optimal state without any user intervention, thus making QKD more practical and robust for real-world applications.

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