Guang-Can Guo

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.

Entanglement based quantum communication offers an increased level of security in practical secret shared key distribution. One of the fundamental principles enabling this security -- the fact that interfering with one photon will destroy entanglement and thus be detectable -- is also the greatest obstacle. Random encounters of traveling photons, losses and technical imperfections make noise an inevitable part of any quantum communication scheme, severely limiting distance, key rate and environmental conditions in which QKD can be employed. Using photons entangled in their spatial degree of freedom, we show that the increased noise resistance of high-dimensional entanglement, can indeed be harnessed for practical key distribution schemes. We perform quantum key distribution in eight entangled paths at various levels of environmental noise and show key rates that, even after error correction and privacy amplification, still exceed 1 bit per photon pair and furthermore certify a secure key at noise levels that would prohibit comparable qubit based schemes from working.

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.

At present, the performance of quantum digital signatures (QDSs) is limited by key generation protocols (e.g., BB84 or measurement-device-independent protocols), which are fundamentally limited in terms of channel capacity. Fortunately, the recently proposed twin-field quantum key distribution can overcome this limit. Here, we for the first time propose a twin-field QDS (TF-QDS) protocol and give a corresponding security analysis. It can not only possess the highest security among all existing QDS protocols, but also exhibit outstanding performance in terms of both signature rates and secure transmission distances. Therefore, our work represents another step towards practical implementation of QDSs.