Security and performance of continuous-variable quantum key distribution systems. (Performance et sécurité de dispositifs de distribution quantique de clés à variables continues)

This thesis focuses on a cryptographic primitive that allows two distant parties, usually called Alice and Bob, to generate an arbitrary amount of secret key even in the presence of an eavesdropper, provided that they share a short initial secret message. This primitive is called quantum key distribution, or in short QKD; and differs from classical key distribution primitives in the sense that no assumption on the capacities of the eavesdropper is required to prove the security of the scheme. While several QKD protocols exist, we focus our study on continuous-variable protocols, which encode the information on the quadratures of the electromagnetic field. Some of these protocols are of particular interest because they can be implemented with standard fiber-based telecommunication components and feature an interferometric detection scheme that exhibits a high resistance to the noise induced by potential adjacent classical channels that propagate on the same optical fiber. The advantage of QKD over classical primitives relies on fundamental quantum physics principles such as the uncertainty principle: in a carefully designed QKD protocol, an eavesdropper cannot interact with the quantum system without introducing some noise which can be quantified by the parties of the protocol. However, even when there is no active eavesdropper, some errors inevitably occur during the transmission of quantum states between Alice and Bob. Alice and Bob must correct these errors to agree on a common bit string. Since this step is done by revealing some information on a public channel, it results in a limitation of the protocol in terms of tolerance to noise and losses. To overcome this, we develop efficient errorcorrecting codes that allow to extend the range of a QKD protocol based on a Gaussian modulation of the quadratures of the electromagnetic field. Security proofs for QKD protocols typically consider an ideal description of the protocol. In practice, however, one has to consider the distance between a practical implementation of a QKD protocol and its theoretical description. We study different aspects of a practical implementation of a Gaussian continuous-variable QKD protocol, such as the imperfect statistical estimation (in the finite-size scenario) of the parameters that are relevant to the security of the protocol and the quality of the practical Gaussian modulation of coherent states. We also consider a calibration attack that can lead to a powerful eavesdropping strategy. We demonstrate experimentally the distribution of secret keys over a 80 km fiber link when taking into account all the known imperfections of our system. Finally, we make initial deployment tests involving our QKD system and wavelength division multiplexed (WDM) classical channels on the same fiber link. We also demonstrate the long-term stability of our system in combination with high-speed classical encryptors.