Breaking the Rate-Loss Bound of Quantum Key Distribution with Asynchronous Two-Photon Interference

Abstract

Twin-field quantum key distribution can overcome the repeaterless bound via single-photon interference. However, the requirements of phase-locking and phase-tracking techniques drastically increase the experimental complexity, economic cost and prohibit free-space realization. Inspired by the duality in entanglement, we herein present an asynchronous measurement-device-independent quantum key distribution protocol that can surpass the repeaterless bound even without phase locking and phase tracking. Leveraging the concept of time multiplexing, asynchronous two-photon Bell-state measurement is realized by postmatching two interference detection events. For a 1 GHz system, the new protocol reaches a transmission distance of 450 km without phase tracking. After further removing phase locking, our protocol is still capable of breaking the bound at 270 km by employing a 10 GHz system. Intriguingly, when using the same experimental techniques, our protocol has a higher key rate than the phase-matching-type twin-field protocol. In the presence of imperfect intensity modulation, it also has a significant advantage in terms of the transmission distance over the sending-or-not-sending type twin-field protocol. With high key rates and accessible technology, our work paves the way for realistic global quantum networks across space, air, and water to the ground.

Quantum key distribution surpassing the repeaterless rate-transmittance bound without global phase locking

“…This work was supported by the National Natural Science Foundation of China Grants No. Note added.-After we submitted our work for reviewing, we became aware of a relevant work by Xie et al [54], who consider a similar MDI-QKD protocol that match the clicked data to generate key information. Under the assumption that the single-photon distributions in all the Charlie's successful detection events are independent and identically distributed, the authors simulate the performance of the protocol and show its ability to break the repeaterless rate-transmittance bound.…”