Physics-based network securely distributes secret keys between any two users

Quantum key distribution (QKD) offers the potential for unconditionally secure communication, relying on the laws of physics rather than computational complexity to protect data. However, practical implementations of QKD networks face significant challenges in maintaining high key rates while ensuring security against all potential adversaries, including other network users. Yiming Bian, Yichen Zhang, et al., from Beijing University of Posts and Telecommunications, alongside colleagues at Huawei Technologies and Peking University, address this issue in their work, “Approaching the Key Rate Limit in Continuous-Variable Quantum Key Distribution Network”. They present a multi-user framework and protocol that achieves a key rate approaching the theoretical maximum within a 100 km range, demonstrating a scalable solution utilising standard telecommunications components and offering a broadly applicable method for assessing information accessibility in complex networks.

Quantum Networks Advance Secure Communication with Standard Components

Quantum key distribution (QKD) establishes a shared, secret key between parties, guaranteeing security against eavesdropping via the principles of quantum mechanics. Modern applications demand scalable networks connecting multiple users, presenting challenges in maintaining security when all users are interconnected and potentially correlated. Continuous-variable QKD (CV-QKD), utilising properties of light such as amplitude and phase, offers compatibility with existing telecommunications infrastructure, but inherently correlates all network participants, potentially exposing key information. Existing multi-user CV-QKD protocols often compromise between key rate and security, or rely on trusted users, creating vulnerabilities. A precise understanding of accessible information within the network, and accurate quantification of eavesdropper benefits from correlated users, remains critical for practical quantum networks.

This work establishes a general framework for analysing multi-user CV-QKD networks, providing an explicit formula for calculating the end-to-end key rate and enabling a more accurate assessment of security. The framework precisely quantifies information accessible to both eavesdroppers and other network users, allowing optimisation of protocols to minimise vulnerabilities and maximise performance. Applying this framework, a new multi-user protocol achieves the theoretical upper limit for key rate in practical deployments, demonstrating the potential for secure and high-performance CV-QKD networks.

This research addresses the challenge of balancing security and efficiency in multi-user quantum key distribution networks by introducing a novel framework and protocol designed to maximise both key rate and security within a continuous-variable quantum key distribution (CV-QKD) network. The core innovation lies in a comprehensive mathematical formula that accurately calculates the secure key rate achievable in a multi-user scenario, accounting for correlations between users and acknowledging that shared information can be exploited by an adversary. Crucially, the researchers move beyond considering an external eavesdropper and incorporate the possibility of collusion between network users, a realistic threat in many practical applications, determining the accessible information within the network, a critical parameter for assessing security.

By precisely quantifying the information available to potential attackers, the researchers optimise the protocol to minimise vulnerabilities and enhance the overall security of the system. The proposed protocol, tested in a three-node network simulation and experimentally validated, demonstrates a significant improvement in key rate and security, achieving megabits-per-second key rates for each user over a 100-kilometre range, approaching the theoretical upper limit of what is achievable with current technology. This performance is achieved by carefully managing the correlations between users and employing advanced data processing techniques to extract the secure key, utilising readily available, telecom-compatible components, making the system practical for real-world deployment. The system achieves 90% of the theoretical maximum key rate for the network as a whole, a substantial improvement over existing multi-user QKD systems.

This research details a successful demonstration of a multi-user continuous-variable quantum key distribution (CV-QKD) system, establishing secure communication between a central node, Alice, and two remote nodes, Bob 1 and Bob 2, via a broadcast channel. Researchers achieve this by developing a comprehensive framework for calculating key rates that accounts for potential collaboration between network users and external eavesdroppers, utilising continuous variables such as the amplitude and phase of light to encode quantum information. The core innovation lies in a novel key rate formula and a corresponding protocol that approaches the theoretical upper limit for key generation within a 100km range.

Applying this to a three-node network, the system generates a megabit-per-second (Mbps) level key rate for each user, achieving 90% of the theoretical maximum network key rate. The security analysis centres on the covariance matrix, a mathematical tool representing the correlations between quantum variables, providing a robust method for evaluating the system’s vulnerability to attacks. Furthermore, the researchers map the prepare-and-measure scheme to an equivalent entanglement-based scheme, simplifying the security proof and enhancing the clarity of the analysis. Experimental results demonstrate the practical viability of the system, achieving a secure key rate of 1.15 Mbps over a 30km fibre optic link, employing readily available, telecom-compatible components, making it a promising candidate for future quantum communication networks.

This research demonstrates a functional multi-user continuous-variable quantum key distribution (CV-QKD) system, successfully establishing secure key distribution between a central node, Alice, and two remote nodes, Bob 1 and Bob 2, utilising a broadcast channel. The system implements readily available components including IQ modulators and balanced homodyne detectors, achieving a significantly improved key rate compared to previously reported multi-user CV-QKD systems. Specifically, within a 100km range, the system attains an Mbps-level per-user key rate, with the overall network key rate reaching 90% of the theoretical upper limit.

This is achieved through a novel protocol built upon a robust security analysis based on the covariance matrix of the quantum states, providing an accurate assessment of key rates and ensuring security against collective attacks. The researchers further simplify this analysis by mapping the prepare-and-measure scheme to an equivalent entanglement-based scheme, enhancing clarity and facilitating a deeper understanding of the underlying quantum correlations. Detailed digital signal processing (DSP) techniques play a crucial role in mitigating noise and optimising signal quality, contributing to the enhanced key rates observed. The broadly applicable method for determining accessible information within the network also presents opportunities for extending this framework to other multipartite information systems.