Coherent optical communication in the quantum regime

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Abstract

Abstract : Quantum Key Distribution (QKD) is a critical field in communication security. Nowadays, cryptographic security relies on the mathematical complexity of certain problems, for which a brute-force resolution would require computational resources and time far exceeding current technological capabilities. However, the emergence of more efficient algorithms and the development of higher-performance technologies (notably quantum computers) poses a threat to this complexity, especially if an eavesdropper manages to intercept and store data today to decrypt it tomorrow. QKD provides an answer to this problem by shifting the security guarantee from mathematics to the fundamental laws of physics. By exploiting principles such as the no-cloning theorem, QKD allows a sender and a receiver to share an encryption key with the certainty that any interception attempt by an eavesdropper will introduce detectable statistical noise into the signal. This physical perturbation makes it possible to precisely quantify the compromised information, thus guaranteeing a key transfer whose security is theoretically unbreakable, even against unlimited computing power.

QKD protocols are classified into two main categories: those based on Discrete Variables (DV), where information is encoded among a finite dimensional space (such as the polarization of light), and those based on Continuous Variables (CV), where information is encoded within an infinite dimensional space (such as the quadratures of the electromagnetic field). Each protocol has its advantages and disadvantages; however, CV-QKD offers the possibility of working with components and algorithms used in classical telecommunications, allowing to use all knowledge and research conducted to date in that field. QKD is an evolving field that remains, for the most part, at a laboratory state-of-the-art level. Nevertheless, the European QKD ecosystem is expanding rapidly, giving rise to numerous collaborations.

The present thesis investigates and validates CV-QKD systems through experimental studies across multiple levels of integration. It contributes directly to the QKISS project, whose primary objective was the development of a French industrial CV-QKD system. The work initially focuses on laboratory experiments, exploring system architectures, practical and fundamental noise limitations, and the impact of signal processing parameters. It is then extended to field deployments, including experiments on installed fiber links and tests of complete CV-QKD systems. This work also contributes to several European initiatives. Within FranceQCI, we participated in a field demonstration of interoperability between our CV-QKD system and a DV-QKD system. In the EuroQCI project, we contributed to a live QKD networking demonstration involving more than twenty partners. Finally, within the Nostradamus project, this work supported the development of protocols and measurement methodologies aimed at the standardization and certification of QKD systems.