Quantum key distribution protocol design based on quantum information theory

Abstract

Quantum information processing based on quantum physics has researched for several decades but attracted tremendous attention in recent days due to its potentiality which provides superior performance to its classical counterpart. One of the most feasible application of the quantum information processing is a quantum cryptography whose typical research area is quantum key distribution (QKD). QKD utilizes quantum phenomena of optical light to share secret key between remote parties. Here, key difference between quantum and classical cryptography methods comes from dependence on computational complexity. In general, current classical methods rely on the computational complexity so that it prevents an eavesdropper to successfully steal in a reasonable time, while QKD can provide security without dependence on the complexity so that it is considered to provide unconditional security. Such QKD is required to achieve some level of performances to be applied into real world by integrating conventional systems. The main measure of the performances are secret key rate and distance. In order to improve the performance, a lot of approaches about QKD protocols have been proposed under a variety of situations.

In this thesis, we deal with researches about QKD protocols depending on some degree of freedom which are discrete and continuous variables. In case of the discrete variable QKD (DVQKD), we provide security analysis in point to point scenario where a multi-qubit QKD protocol is considered under incoherent attack. We also propose a simple protocol adoptable to the multi-qubit QKD protocol to improve the performance of QKD. On the one hand, we consider the case of point to multi-point scenario for end user service of QKD. In the scenario, we propose the secret key rate formula in a passive optical network by extending the secret key rate formula based on the decoy-state BB84 protocol. For a passive optical network, we provide a way that incorporates cooperation across end users. Then, we show that the way can mitigate a photon number splitting (PNS) attack which is crucial in an well known decoy BB84 protocol. Especially, the proposed scheme enables multi-photon states to serve as secret keys unlike the conventional decoy BB84 protocol.

In the case of the continuous variable QKD (CVQKD), we provide an effective way to design CVQKD protocol based on the conventional CVQKD protocol. However it does not cover photon subtracted state which requires post selection to be generated. The photon subtracted state recently attracts attentions as a non-Gaussian state, which has a potential to improve performance of CVQKD aside from Gaussian states, and is corresponding to comparably implementable non-Gaussian states. Therefore, we additionally analyze effect of photon subtraction on secret key rate by analyzing its effect on entanglement which is a resource of quantum information processing. We also provide a effective way for the photon subtraction in terms of designing a CVQKD protocol.