Quantum Key Distribution: Security and Challenges in Practical Applications

Quantum key distribution (QKD) can unconditionally ensure the theoretical security of information transmission between two or more distant users with the three basic theories of quantum mechanics. In the process of development from theory to practice, there will be many challenges to realize remote and secure key distribution in the practical applications. With various theoretical ideas and experimental schemes being put forward, many challenges have been overcome. The first BB84 protocol [1] proposed by Bennett and Brassard realizes two-point communication and Ekert91 and BBM92 protocol have been proposed successively [2, 3]. In practical applications, we often use weak coherent source (WCS) with multi-photon component. Eve can eavesdrop with photon-number splitting (PNS) attacks [4]. The decoy-state method proposed [5, 6] can solve the PNS attacks and get the rapid development both theoretically and experimentally [7, 8, 9]. Although QKD has been proved the unconditional security in theory, the imperfect devices lead to some security loopholes that hinder the development of QKD protocol in practical application. Considering the security of QKD, Lo firstly proposed the MDI-QKD protocol [10] which thoroughly solves the security loopholes mainly at detection terminal. With the advantages of MDI-QKD protocol, the MDI-QKD protocol attracts extensive attention and has been greatly studied in theory and experiment [11, 12, 13, 14].