Zhen-Qiang Yin

Recent studies have revealed critical source-side vulnerabilities in practical quantum key distribution systems. Despite their demonstrated risks, these threats receive limited attention in both academic discussions and practical implementations. To highlight the urgency of addressing source-side vulnerabilities, we will report two widespread but overlooked loopholes: the induced-photorefractive effect and the pattern effect, including a report of the first-time system-level attack against a running MDI-QKD. Except for the attack, we will also report countermeasures against the loopholes, including a fully-passive QKD architecture resistant to encoding side-channels and a correlation-immune QKD protocol mitigating the pattern effect. These works provide essential insights and solutions for advancing the practical deployment of secure QKD systems.

Measurement-device-independent quantum key distribution (MDIQKD) can resist all attacks on the detection devices, but there are still some security issues related to the source side. One possible solution is to use the passive protocol to eliminate the side channels introduced by active modulators at the source. Recently, a fully passive QKD protocol was proposed that could simultaneously achieve passive encoding and passive decoy-state modulation using linear optics. In this work, we propose a fully passive MDIQKD scheme that can protect the system from both side channels of source modulators and attacks on the measurement devices, which can significantly improve the implementation security of the QKD systems. We provide a specific passive encoding strategy and a method for decoy-state analysis, followed by simulation results for the secure key rate in the asymptotic scenario. Our work offers a feasible way to improve the implementation security of QKD systems and serves as a reference for achieving passive QKD schemes using realistic devices.

Intensity correlations between neighboring pulses open a prevalent yet often overlooked security loophole in decoy-state quantum key distribution (QKD). As a solution, we present and experimentally demonstrate an intensity-correlation-tolerant QKD protocol that mitigates the negative effect that this phenomenon has on the secret key rate according to existing security analyses. Compared to previous approaches, our method significantly enhances the robustness against correlations, notably improving both the maximum transmission distances and the achievable secret key rates across different scenarios. By relaxing constraints on correlation parameters, our protocol enables practical devices to counter intensity correlations. We experimentally demonstrate this first practical solution that directly overcomes this security vulnerability, establish the feasibility and efficacy of our proposal, taking a major step towards loophole-free and high-performance QKD.

Twin-field quantum key distribution (TF-QKD) and its variants provide a promising solution for sharing information-theoretic secure keys between intercity peers since they are able to overcome the fundamental rate-transmittance bound without quantum repeaters. In this paper, we propose to improve the key rate at long distances and the maximum achievable distance for TF-QKD by deriving the error rates under three mutually unbiased bases, i.e., σX, σY , and σZ in two-dimensional Hilbert space. Moreover, learning these error rates, one can add noisy preprocessing to further improve its performance. We also observe that higher bit error rates do not necessarily imply lower key rates when noisy preprocessing is added. Our method does not change the existing physical implementation or experimental operation, but only requires simple postprocessing of the experimental data, which can be directly used to improve the key rate performance of the existing QKD system. The simulation results demonstrate its notable enhancements in terms of key rate at long distances and the maximum achievable distance for the phase-encoded TF-QKD protocol.

Among all existing quantum key distribution (QKD) protocols, the round-robin-differential-phase-shift (RRDPS) protocol is one of the unique protocols. Because it can be running without monitoring signal disturbance, which improves its tolerance of error rate and does well in the finite-key scenario. Considering that a tight finite-key analysis with a practical phase-randomized source is still missing, we propose an improved security proof of RRDPS against the most general coherent attack based on the entropic uncertainty relation. We also introduce Azuma’s inequality into our proof, which can tackle finite-key effects. The results indicate experimentally acceptable numbers of pulses are sufficient to approach the asymptotic bound closely. This method may be the optimal one in the finite-key analysis for the RRDPS protocol.

The security proofs of continuous-variable quantum key distribution are based on the assumptions that the eavesdropper can neither act on the local oscillator nor control Bob’s beam splitter. These assumptions may be invalid in practice due to potential imperfections in the implementations of such protocols. In this paper, we consider the problem of transmitting the local oscillator in a public channel and propose a wavelength attack which can allow the eavesdropper to control the intensity transmission of Bob’s beam splitter by switching the wavelength of the input light. Specifically we target continuous-variable quantum key distribution systems that use the heterodyne detection protocol using either direct or reverse reconciliation. Our attack is proved to be feasible and renders all of the final key shared between the legitimate parties insecure, even if they have monitored the intensity of the local oscillator. To prevent our attack on commercial systems, a simple wavelength filter should be added before performing the monitoring detection.