Reconciliation system based on polar codes for continuous-variable quantum key distribution

Abstract

Quantum key distribution is a field of research that can provide unconditionally secure communication on the basis of the law of quantum physics such as the uncertainty principle. Theoretical principles and security proofs of various protocols have been introduced and analyzed since the proposal of BB84 protocol. As technology is continuously improved, successful demonstrations of quantum key distribution have been reported actively both from academia and industry. On top of that, commercially advanced systems give positive expectations of various applications of quantum key distribution such as drone-based quantum key distribution for military use. However, there still exist dierent kinds of technical issues which are fatal not only to performance, but its security when it comes to practical implementation. The problem that is dealt throughout this thesis is a reconciliation problem in quantum key distribution system that utilizes coherent states of light or so called continuous-variable quantum key distribution. Reconciliation refers to process that two authenticated parties extract errorless secret keys from shared quantum states. The reconciliation problem in continuous-variable quantum key distribution is considered to be more dicult than its counterpart called discrete-variable quantum key distribution due to the fact that general environments for continuous-variable quantum key distribution make error correction more challenging. The dependencies of its performance, including secret key rate and transmission distance on the reconciliation process is signicant. Therefore, it is crucial to practical continuous-variable quantum key distribution systems to devise and implement ecient reconciliation protocols in continuous-variable quantum key distribution. In this paper, we propose a new reconciliation protocol with polar codes constructed by Gaussian approximation for long-distance continuous-variable quantum key distribution systems. Polar codes are linear forward error correction codes and known to be capacity-achieving. Backed up by this beautiful theory, we believe that it can help two authenticated parties achieve a secret key rate as close as to the theoretical limit. Additionally, Gaussian approximation is used in the protocol for ecient construction of polar codes by virtue of its reduced complexity. Via numerical analysis and simulations, the proposed protocol is shown to be eective for long-distance application. Furthermore, our heterogenous successive cancellation decoder for polar codes will prevent the proposed reconciliation protocol from a potential bottleneck in terms of secret key rate to guarantee high-performance continuous-variable quantum key distribution.