Quantum key distribution offers the possibility of cryptography whose security is demonstrated by the laws of quantum physics. The first commercial systems of this technology are now available. This thesis focuses on continuous variable protocols, whose practical implementation is close to modern digital transmission techniques over optical fibers. By exploiting these techniques, we realize an experimental system for high speed continuous variable quantum key distribution.
Quantum key distribution offers the possibility of cryptography whose security is demonstrated by the laws of quantum physics. The first commercial systems of this technology are now available. This thesis focuses on continuous variable protocols, whose practical implementation is close to modern digital transmission techniques over optical fibers. By exploiting these techniques, we realize an experimental system for high speed continuous variable quantum key distribution.
Techniques avancées de traitement du signal pour les systèmes de distribution quantique de clés sur fibre optique basés sur des variables continues La distribution quantique de clés offre la possibilité d’une cryptographie dont la sécurité est démontrée par les lois de la physique quantique. De premiers systèmes commerciaux de cette technologie sont aujourd’hui disponibles. Cette thèse s’intéresse plus particulièrement aux protocoles à variables continues, dont l’implémentation pratique s’approche des techniques modernes de transmissions numériques sur fibre optique. En exploitant ces techniques, on réalise un système expérimental de distribution quantique de clés à variables continues à haut débit.
Quantum key distribution provides secure keys with information-theoretic security ensured by the principle of quantum mechanics. The continuous-variable version of quantum key distribution using coherent states offers the advantages of its compatibility with telecom industry, e.g., using commercial laser and homodyne detector, is now going through a booming period. In this review article, we describe the principle of continuous-variable quantum key distribution system; focus on protocols based on coherent states, whose systems are gradually moving from proof-of-principle lab demonstrations to in-field implementations and technological prototypes. We start by reviewing the theoretical protocols and the current security status of these protocols. Then, we discuss the system structure, the key module, and the mainstream system implementations. The advanced progresses for future applications are discussed, including the digital techniques, system on chip, and point-to-multipoint system. Finally, we discuss the practical security of the system and conclude with promising perspectives in this research field.
Digital signal processing technology has paved the way for the realization of high-speed continuous-variable quantum key distribution systems. However, existing security proofs are limited to static digital signal processing algorithms, while practical systems rely on dynamic multiple-input multiple-output algorithms to compensate for time-varying channel impairments. Our analysis reveals that the conventional dynamic algorithm, due to its non-unitary nature, systematically underestimates the excess noise, which in turn leads to security issues and the generation of insecure keys. To close this gap, we propose a secure algorithm model, mapping the dynamic algorithm to an equivalent physical optical model whose security can be rigorously assessed. Simulations illustrate the algorithm's non-unitary property and provide a quantitative analysis of the excess noise underestimation caused by the conventional algorithm. We further experimentally validate the necessity of the proposed modeling for dynamic digital signal processing, achieving a secret key rate of 14.4 Mbps based on estimated excess noise of 0.07 shot noise unit; whereas the conventional algorithm would have dangerously overestimated the key rate to 28.2 Mbps with noise of 0.008 shot noise unit. This work provides the essential security framework for dynamic digital signal processing, overcoming a critical impediment for the development of high-performance continuous-variable quantum key distribution systems.
Quantum key distribution is the first major area of quantum information science to find practical use outside of its field. Quantum key distribution involves transmitting cryptographic keys using quantum states. Any attempt to eavesdrop on the transmission will, necessarily by the laws of quantum mechanics, disturb or destroy the states. By identifying and only using keys that have not been disturbed, a perfectly secure cryptographic system can be realised. ¶ Discrete variable quantum key distribution was proposed in 1984. Since then attempts to exploit this technology have been beset with the difficulties of working with the discrete variables in question—single photon states. In the late 90’s the idea of continuous variable quantum key distribution was developed, in which the key-carrying quantum information states are properties of a bright, continuous wave laser beam. Bright laser beams are relatively simple to manipulate and can carry large amounts of information even when subject to high loss. ‘Broadband’ continuous variable quantum key distribution refers to the large information rates obtainable by using broad laser modulation sidebands. ¶ In 2005 the ANU Quantum Optics group published an experimental demonstration of broadband continuous variable quantum key distribution. This thesis documents the project work undertaken to rebuild an improved version of this experiment, and also the work undertaken to prepare for more advanced experiments. The main goal of the improved experiment was to increase the key distribution rate, while the advanced experiments were directed towards practical implementation of the system in optical fibre. ¶ The experiment rebuild was achieved with most improvements for higher bandwidth successfully implemented. Some improvements were unplanned, being necessitated by problems with the initial improvements. Although preliminary experimental data was acquired, time constraints precluded a successful cryptographic key distribution. Pilot design and major purchasing for future experiments was also completed.
Continuous-variable quantum key distribution (CV-QKD) is a promising solution for providing high secure key rates in moderate loss channels. A great advantage with respect to discrete-variable (DV) systems is the use of a technology similar to the one used in classical coherent communication, in particular for the detection system, which can operate at room temperature and benefits from an easier integration process. In addition to this, the use of advanced digital signal processing (DSP) techniques developed for classical communication allows for bandwidth-efficient temporal shaping, which optimizes the performance of the CV-QKD system. These techniques applied to the detected signal are also fundamental for using a locally generated local oscillator, correcting frequency and phase differences using frequency-multiplexed pilots generated by the transmitter. In this presentation, we will describe how these DSP techniques can be applied to a CV-QKD system and show some recent experimental results obtained by our research group, including results for a receiver based on a Photonic Integrated Circuit (PIC).
The ability to distribute secret keys between two parties with information-theoretic security, that is, regardless of the capacities of a malevolent eavesdropper, is one of the most celebrated results in the field of quantum information processing and communication. Indeed, quantum key distribution illustrates the power of encoding information on the quantum properties of light and has far reaching implications in high-security applications. Today, quantum key distribution systems operate in real-world conditions and are commercially available. As with most quantum information protocols, quantum key distribution was first designed for qubits, the individual quanta of information. However, the use of quantum continuous variables for this task presents important advantages with respect to qubit based protocols, in particular from a practical point of view, since it allows for simple implementations that require only standard telecommunication technology. In this review article, we describe the principle of continuous-variable quantum key distribution, focusing in particular on protocols based on coherent states. We discuss the security of these protocols and report on the state-of-the-art in experimental implementations, including the issue of side-channel attacks. We conclude with promising perspectives in this research field.