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

TL;DR

This paper introduces an asynchronous quantum key distribution protocol that surpasses traditional capacity limits without phase tracking or locking, enabling longer-distance secure communication with simpler experimental setup.

Contribution

The authors propose a novel asynchronous measurement-device-independent protocol that breaks the rate-loss bound without phase stabilization, using time multiplexing and postmatching techniques.

Findings

Achieves 450 km transmission distance at 1 GHz without phase tracking.

Maintains capacity surpassing the secret key limit even without phase locking at 270 km.

Higher key rates than phase-matching twin-field protocols under similar conditions.

Abstract

Twin-field quantum key distribution can overcome the secret key capacity of repeaterless quantum key distribution via single-photon interference. However, to compensate for the channel fluctuations and lock the laser fluctuations, the techniques of phase tracking and phase locking are indispensable in experiment, which drastically increase experimental complexity and hinder 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 secret key capacity even without phase tracking and phase locking. 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 capacity at 270 km. 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 provides a promising candidate for practical scalable quantum communication networks.