De-Yong He

A secure approach to determining the position of an entity can enable schemes such as position-based authorization for transactions and tracking of high-value targets. However, classical methods are incapable of providing secure position verification in the untrusted-prover case, meaning that a prover can mislead verifiers about its actual position. Here we propose and experimentally realize a secure position verification protocol that combines quantum optics and relativity within an information-theoretic framework. Using phase-randomized weak coherent states, two verifiers separated by 2 km securely verify the prover’s position with an accuracy better than 75 m. These results establish secure position-based authentication as a practical possibility, paving the way for applications in financial transactions, disaster response and authenticated secure communications.

The quantum internet promises unparalleled capabilities that are provably impossible with the classical internet. However, current quantum networks are often designed with dedicated systems for specified tasks, which hinders the openness and diversity of future quantum internet. To address these limitations, this work proposes a fully heterogeneous quantum network accompanied with several techniques. Our proposal allows users to access the network using any mainstream systems or even partial systems. The network also enables the execution of multiple distinct quantum tasks and provides opportunities for global optimization and cost-efficient design. A five-node quantum network featuring heterogeneous nodes has been implemented, demonstrating the superiority of our proposal through tasks such as quantum key distribution, quantum digital signature, quantum Byzantine agreement, and quantum conference. Notably, the experiment represents the first demonstration of multi-malicious node quantum Byzantine agreement in a quantum network.

Measurement-device-independent quantum key distribution (MDI-QKD) is considered one of the most promising protocols due to its high performance and security. To implement the MDI-QKD system in practice, clock synchronization is a crucial step to correctly generate the secret keys. However, as practical applications advance, the complexity and cost of synchronization have become significant challenges for the implementation of MDI-QKD. In this study, we introduce a qubit-based synchronization algorithm specifically designed for MDI-QKD. By utilizing the property of Hong-Ou-Mandel interference, we achieve clock synchronization between parties of MDI-QKD without the need for additional hardware. The cost-effectiveness and simplicity of this approach are expected to significantly facilitate the practical deployment of MDI-QKD.

Mode-pairing quantum key distribution (MP-QKD) is a potential protocol that is not only immune to all possible detector side channel attacks, but also breaks the repeaterless rate-transmittance bound without needing global phase locking. Here we analyze the finite-key effect for the MP-QKD protocol with rigorous security proof against general attacks. Moreover, we propose a six-state MP-QKD protocol and analyze its finite-key effect. The results show that the original protocol can break the repeaterless rate-transmittance bound with a typical finite number of pulses in practice. And our six-state protocol can improve the secret key rate significantly in long distance cases.

Decoy-state method has greatly filled in loopholes from multi-photon pulses in the field of quantum key distribution (QKD) and it has been employed in almost all practical QKD systems in recent years. However, security and performance of the practical decoy-state QKDs are still limited by inaccurate modulations such as random intensity fluctuation and correlated intensity fluctuation. In this work, an intensity tomography method is proposed as a countermeasure against these fluctuations. It has an ability to tightly bound the unknown density matrices of photon-number state and accurately calibrate the fluctuations by measuring the decoy states with a local single-photon detector. We verified the tomography method by experiments and simulations. The results indicated that our method can efficiently mitigate the flaws from these fluctuations to improve the protocol performance and practical security. Noting that these fluctuations are inevitable in practical QKDs, our method would pave the avenue for QKD's practical applications.

Gated single-photon avalanche diodes (SPADs) are practical solutions for quantum key distribution (QKD) applications. However, the gating frequency is limited by time jitter and afterpulse probability when SPADs operate at a frequency higher than 2 GHz. We find that filter distortion and variation of signal amplitude are the dominant mechanisms for time jitter in high-frequency scenarios, and design a specific low-pass filter (LPF) and a zero crossing discriminator to reduce the time jitter effectively. Specifically, the LPF extracts avalanche signals and forms accurate zero crossing points by its impulse response, and the zero crossing discriminator correctly discriminates avalanche signals to achieve low time jitter. When the SPAD is operated at a gating frequency of 2.5 GHz with a detection efficiency of 21%, the time jitter and afterpulse probability are 44 ps and 1.4%, respectively. Our results pave the way to practical QKD systems at higher clock rates.

The decoy-state method for the quantum key distribution (QKD) is a great countermeasure against the fatal photon number splitting attack. At present, the decoy-state method has been widely employed in practical QKD systems. However, the patterning effect, namely the correlated intensity fluctuations in practical QKD systems, violates the important assumption of the decoy-state method and seriously threatens the protocol security. The previous works investigated the patterning effect between the adjacent pulses and proposed the countermeasures from the degree of post selection and modulators. In this work, we investigate the patterning effect with larger correlation range and present an algorithm to calibrate the driving voltages to mitigate the patterning effect. According to our methods, we experimentally observe the higher-order patterning effect and successfully mitigate it by our calibration algorithm. The experiment results indicate that our methods efficiently improve the protocol security and are expected to be widely employed in practical decoy-state QKD systems.