# Asymmetric Measurement-Device-Independent Quantum Key Distribution through Advantage Distillation
## 1 Introduction  
Quantum key distribution (QKD) can unconditionally ensure the theoretical security of information transmission between two or more distant users with quantum mechanics. In the process of development from theory to practice, there are many challenges to realizing remote and secure quantum key distribution in the practical applications. With various theoretical ideas and experimental schemes being put forward, many challenges have been overcome. The BB84 protocol [1] proposed by [PERSON] realizes two-point communication and the Ekert91 and BBM92 protocols have been proposed successively [2, 3]. Although QKD has been proven to have unconditional security in theory, imperfect devices can lead to some security loopholes that hinder the development of QKD protocols in practical applications. In practical applications, we often use weak coherent sources (WCSs) with multi-photon components, and Eve can eavesdrop with photon-number splitting (PNS) attacks [4]. Fortunately, the decoy-state method proposed [5, 6] can solve PNS attacks and obtain rapid development both theoretically and experimentally [7, 8, 9]. Considering the imperfection of the detector, [PERSON] firstly proposed the MDI-QKD protocol [10] which thoroughly solves the security loopholes mainly at the detection terminal. With the advantages of the MDI-QKD protocol, the MDI-QKD protocol attracts extensive attention and has been greatly studied in theory and experiments [11, 12, 13, 14, 15, 16, 17, 18].  
In previous work, the MDI-QKD was mainly studied in symmetric scenarios for simplicity. With the development of theory and technology, researchers have paid moreattention to the asymmetric MDI-QKD protocol in recent years. To achieve good interference at the detection terminal, [PERSON] proposed an asymmetric seven-intensity MDI-QKD [19], which can improve the performance of MDI-QKD in practical asymmetric structures based on the four-intensity MDI-QKD [11]. Consequently, asymmetric MDI-QKD is more suitable for the common QKD networks. However, due to its asymmetric nature, its performance is inferior to that of the original symmetric scheme. Improving the performance of asymmetric MDI-QKDs has become an urgent problem that needs to be addressed.  
Inspired by the advantage distillation (AD) method [20; 21; 22; 23], we study the principle of the method and find that the AD method can be successfully applied to the asymmetric seven-intensity MDI-QKD protocol. Compared with the original protocol, the performance of the asymmetric protocol has been significantly improved, which provides another theoretical verification that the post-processing AD method can improve the performance of the QKD protocol. This method can divide the original key string into blocks of only a few bits to achieve a high key correlation and greatly improve the protocol's performance. The paper is organized as follows: In Section 2, we review the asymmetric seven-intensity MDI-QKD protocol and introduce the protocol with AD. The results of numerical simulations are shown in Section 3. Finally, summaries are given in Section 4.