Zhen-Qiang Yin

Quantum key distribution (QKD) networks represent a foundational step toward the future quantum internet and have become a focus of intensive research. By analogy with the architecture of classical optical networks, we can observe that the high-capacity backbone typically relies on complex external modulation, while the cost-sensitive access layer prefers simpler, more compact, and more economical direct modulation. As the cornerstone of quantum networks, today’s QKD networks share this backbone-access hierarchy, yet their implementations remain heavily reliant on external modulators, leaving a clear ecological niche unfilled for directly modulated QKD in the access layer. Here, we propose and experimentally demonstrate the first fully modulator-free, full-degree-of-freedom direct-modulation QKD architecture that fills this niche. The transmitter combines optical injection locking with coherent optical interference to achieve simultaneous phase and intensity control without external modulators, enabling arbitrary quantum-state preparation while preserving inherent compatibility with standard time-bin-encoding receivers. Built with relatively inexpensive laser diodes and passive optical components, our experimental implementation achieves a secure key rate of 4.78 kbps at 30 dB channel loss, matching the performance of conventional modulator-based systems. By filling the long-standing direct-modulation niche in the QKD ecosystem, this work delivers a practical solution to the access-layer deployment challenge and opens a viable new direction for compact, scalable QKD implementation.

The study of quantum information processing seeks to characterize the resources that enable quantum information processing to perform tasks that are unfeasible or inefficient for classical information processing. Quantum cryptography is one such task, and researchers have identified entanglement as a sufficient resource for secure key generation. However, quantum discord, another type of quantum correlation beyond entanglement, has been found to be necessary for guaranteeing secure communication. Despite this, it remains a long-standing problem to clearly understand the role that discord plays and how to effectively utilize discord for analyzing security in specific quantum cryptography protocols. Here, we strictly prove that discord determines the secrecy and show its practical interest by considering a three-state quantum key distribution protocol. Its robustness against imperfections in qubit sources and measurements as well as basis misalignment, results in better key rates than BB84 protocol. Those advantages are experimentally demonstrated via photonic phase encoding systems.

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.

Fully connected quantum networks enable simultaneous connection of every user to every other user and are a highly versatile and robust networking architecture. However, the scalability of such networks remains a great challenge for practical applications. Here we construct a large-scale fully connected quantum network founded on two-photon Hong-Ou-Mandel (HOM) interference, where user-to-user security is guaranteed even with an untrusted network provider. Using integrated soliton microcomb (SMC) and photonic encoding chips, we realize precise, massively parallel frequency generation and locking, high-visibility HOM interference and measurement-device-independent (MDI) quantum key distribution. The proposed architecture enables a fully connected quantum network over 200 kilometers with strict information-theoretic security via an untrusted network provider. The implemented networking architecture paves the way for large-scale fully connected MDI quantum networks across metropolitan and intercity regions.

Quantum random number generators (QRNGs) exploit quantum mechanics to produce true randomness, but practical implementations suffer from device imperfections that can compromise security. We propose a semi–device-independent QRNG based on measurement uncertainty under purity constraints for conditional min-entropy estimation. Our protocol requires no detailed modeling of the two-dimensional source and the countable-dimensional measurement and remains secure against quantum attacks on measurement devices. We uncover an implicit uncertainty relation arising from naturally satisfied purity conditions, define a randomness parameter C using test states, and connect it to an analytical bound on extractable randomness via unitary rotation symmetries of states and measurements. Using the generalized entropy accumulation theorem, we extend the single-round bound to multirounds and obtain composable security against coherent attacks. Implemented on a standard commercial fiber system, our high-tolerance protocol achieves exponential secure randomness expansion and is suitable for integrated-photonic platforms.

Quantum key distribution (QKD) provides information-theoretic security based on quantum mechanics, yet its implementation is hindered by source imperfections. Among these, correlations between transmitted pulses present a critical but underexplored threat to QKD’s theoretical security. We propose a general framework for QKD security under correlated sources, achieving the first finite-key analysis by extending and reorganizing QKD rounds using the generalized chain rule. Inspired by side-channel-secure QKD, we design a protocol secure against correlated leakage sources, requiring only the correlation range and lower bounds on the vacuum component of the prepared states. Our framework also applies to other QKD protocols, providing a general approach to address correlation-induced vulnerabilities. Simulations demonstrate the effectiveness of our protocol and its significantly superior tolerance to imperfections compared to existing protocols. This work provides a crucial step toward closing security loopholes in QKD, enhancing its practicality, and ensuring long-distance, high-performance secure communication under real-world constraints.

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.