Quantum key distribution can establish information-theoretically secure keys based on quantum physics. There are two main issues in practice --- implementation security and system performance. The former is primarily enhanced by the proposal of the measurement-device-independent scheme. The latter is characterized by the dependence of the key rate on the channel transmittance, $R(\eta)$, which was previously believed to be upper bounded by a linear function. Recently, schemes based on single-photon interference have been proposed to improve the key rate to overcome the linear bound, $R=O(\sqrt{\eta})$. Unfortunately, all of these schemes require challenging global phase locking to realize a stable long-arm single-photon interferometer with a precision of approximately 100 nm over hundreds of kilometers long fibers. In this talk, I shall present a mode-pairing measurement-device-independent scheme, in which the key bits and bases are determined during data post-processing. Using conventional second-order interference, this scheme can achieve a key rate of $R=O(\sqrt{\eta})$ without global phase-locking. We then implement this high-performance scheme with off-the-shelf optical devices. The theoretical proposal is published in [Nat. Comm. 13, 3903, (2022)] and its experimental demonstration in [Phys. Rev. Lett. 130, 030801, (2023)].

Xiongfeng Ma obtained his B.Sc. degree at Peking University in 2003 and his Ph.D. degree at the University of Toronto in 2008. In 2012, he joined Tsinghua University. Xiongfeng is currently an associate professor and holds a Changjiang Scholarship. He is an Editorial Board Member of Physical Review Letters. Xiongfeng’s main research interest lies in quantum information science, particularly in quantum cryptography, quantum computing, and quantum foundation. He is one of the main contributors of the decoy-state method, which becomes a standard technique for commercial quantum key distribution systems. According to the Scientometric Assessment of Global Publications from 1992 to 2019, Xiongfeng was one of the most productive researchers worldwide in quantum cryptography by ResearchGate.

Please contact phweb@ust.hk should you have questions about the talk.