The twin-field quantum key distribution (TFQKD) protocol and its variants, e.g., phase-matching (PM) QKD and TFQKD based on sending or not sending, are highly attractive since they are able to overcome the well-known rate-loss limit for QKD protocols without a repeater: $R=O(\eta )$, with $\eta$ standing for the channel transmittance. However, all these protocols require active phase randomization and postselection that play an essential role together in their security proof. Counterintuitively, we find that in TFQKD, beating the rate-loss limit is still possible even if phase randomization and postselection in the coding mode are both removed, which means our final secure key rate $R=O(\sqrt{\eta })$. Furthermore, our protocol is more feasible in practice and more promising according to its higher final key rate in the valid distance. Our security proof counters a collective attack and can also counter a coherent attack in the asymptotical case.

With the help of quantum key distribution (QKD), two distant peers are able to share information-theoretical secure key bits. Increasing the key rate is ultimately significant for the applications of QKD in the lossy channel. However, it has been proven that there is a fundamental rate-distance limit, called the linear bound, which restricts the performance of all existing repeaterless protocols and realizations. Surprisingly, a recently proposed protocol, called twin-field (TF) QKD, can beat the linear bound with no need for quantum repeaters. Here, we present one of the first implementations of the TF-QKD protocol and demonstrate its advantage of beating the linear bound at a channel distance of 300 km. In our experiment, a modified TF-QKD protocol that does not assume phase postselection is considered, and thus a higher key rate than the original one is expected. After controlling the phase evolution of the twin fields traveling through hundreds of kilometers of optical fibers, the implemented system achieves high-visibility single-photon interference and allows stable and high-rate measurement-device-independent QKD. Our experimental demonstration and results confirm the feasibility of the TF-QKD protocol and its prominent superiority in long-distance key distribution services.

Measurement-device-independent quantum key distribution (MDI QKD) is a promising protocol for removing all detector side channel attacks. However, the variation of reference frames, e.g., polarization and phase reference, would be an acute threat to the performance of MDI systems. Here, based on polarization scrambling units, we demonstrate a reference-frame-independent MDI QKD scheme that is inherently stable against volatile channel conditions; therefore, the final secure key rate will be insensitive to the random disturbances of polarization and the drifts of phase reference. Thus, calibrations of the primary reference frames are intrinsically removed in our scheme, which essentially reduces potential vulnerabilities as well as the resource consumption of the whole system. In addition, a proof-of-principle experiment with an improved fluctuation analysis method is demonstrated to verify the feasibility and advantages of the proposed scheme.

Quantum key distribution (QKD) provides an attractive solution for secure communication. However, channel disturbance severely limits its application when a QKD system is transferred from the laboratory to the field. Here a high-speed Faraday–Sagnac–Michelson QKD system is proposed that can automatically compensate for the channel polarization disturbance, which largely avoids the intermittency limitations of environment mutation. Over a 50 km fiber channel with 30 Hz polarization scrambling, the practicality of this phase-coding QKD system was characterized with an interference fringe visibility of 99.35% over 24 h and a stable secure key rate of 306 k bits/s over seven days without active polarization alignment.

The passive approach to quantum key distribution (QKD) consists of removing all active modulation from the users’ devices, a highly desirable countermeasure to get rid of modulator side channels. Nevertheless, active modulation has not been completely removed in QKD systems so far, due to both theoretical and practical limitations. In this Letter, we present a fully passive time-bin encoding QKD system and report on the successful implementation of a modulator-free QKD link. According to the latest theoretical analysis, our prototype is capable of delivering competitive secret key rates in the finite key regime.