Abstract
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.
Highlights
In a seminal result in 1984, Bennett and Brassard showed that it is possible for two parties to distribute a secret key in a way that is unconditionally secure against any adversary, even a quantum one [1]
It has been thoroughly studied both in theory and in practice; the rapid progress in the field has enabled the distribution of secret keys with information-theoretic security over deployed optical fiber networks [4,5], and quantum key distribution (QKD) systems are available on the market [6]
The two communicating parties of a QKD protocol [7], Alice and Bob, can in principle share an information-theoretic secret key after the exchange of a large number of quantum signals through a physical channel, known as a quantum channel, which is subject to eavesdropping, and additional information sent on a public, but authenticated classical channel
Summary
In a seminal result in 1984, Bennett and Brassard showed that it is possible for two parties to distribute a secret key in a way that is unconditionally secure against any adversary, even a quantum one [1]. It is important to emphasize that there is a significant conceptual difference between these protocols and the standard BB84 protocol proposed by Bennett and Brassard [1] and other discrete-variable protocols, even if the latter use coherent states: as we will see in detail information is encoded on non-orthogonal states, which captures the quantum nature of the CVQKD protocols; entirely different degrees of freedom are used in this case This brings the need for different security proof techniques while at the same time opening the way to very practical implementations. CVQKD protocols and implementations, but to focus on specific, well-understood examples to facilitate the understanding of the main ideas behind this approach for quantum key distribution
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