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IEEE photonics journal

Deploying the Qline System for a QKD Metropolitan Network on the Berlin OpenQKD Testbed

Matheus Ribeiro Sena, Georg Harder, Ronny Döring, Ralf-Peter Braun, Michaela Ritter, Oliver Holschke, Marc Kaplan, Marc Geitz

Published Dec 12, 2024• 4 citations
Publisher

Abstract

Quantum Key Distribution (QKD) systems have the potential to provide secure communication by offering unconditionally secure key exchange between distant parties. However, the integration of QKD into existing communication networks presents challenges, particularly when it comes to the deployment of trusted nodes and the establishment of secure connections over metropolitan distances. In this study, we present a novel approach to address these issues by introducing a trusted-node-free design on a metropolitan scale for the seamless integration of a QKD system into the Berlin OpenQKD optical fiber testbed. Our innovative design leverages conventional telecom modulators and a standard Wavelength-Division Multiplexing - Arrayed Waveguide Grating (WDM-AWG), offering a practical solution for flexible multi-partner communication and enabling full connectivity between non-adjacent nodes. In this study, we not only observed relatively low QBER ($< $5.5%) but also a rather stable key rate between nodes in the Qline system over approximately 130 hours ($\approx$5.4 days) for a lab-based back-to-back setup. Later on, a more complex experimental scheme is integrated into Berlin OpenQKD optical fiber testbed encompassing multiple C-band frequencies (191.75–195.75 THz) and extending over metropolitan distances (up to 39.8 km), providing a foundation for secure quantum communication networks with enhanced scalability and versatility.

DOI
10.1109/jphot.2024.3516138

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Quantum Key Distribution (QKD) systems have the potential to provide secure communication by offering unconditionally secure key exchange between distant parties. However, the integration of QKD into existing communication networks presents challenges, particularly when it comes to the deployment of trusted nodes and the establishment of secure connections over metropolitan distances. In this study, we present a novel approach to address these issues by introducing a trusted-node-free design on a metropolitan scale for the seamless integration of a QKD system into the Berlin OpenQKD optical fiber testbed. Our innovative design leverages conventional telecom modulators and a standard Wavelength-Division Multiplexing - Arrayed Waveguide Grating (WDM-AWG), offering a practical solution for flexible multi-partner communication and enabling full connectivity between non-adjacent nodes. In this study, we not only observed relatively low QBER (<< <5.5%) but also a rather stable key rate between nodes in the Qline system over approximately 130 hours (≈\approx≈5.4 days) for a lab-based back-to-back setup. Later on, a more complex experimental scheme is integrated into Berlin OpenQKD optical fiber testbed encompassing multiple C-band frequencies (191.75–195.75 THz) and extending over metropolitan distances (up to 39.8 km), providing a foundation for secure quantum communication networks with enhanced scalability and versatility.
Abstract Quantum key distribution (QKD) is evolving toward large-scale metropolitan networks that interoperate with modern communication infrastructures. Although wavelength division multiplexing (WDM) architectures hold great promise for high secure-key throughput and multi-user access, existing implementations suffer from security vulnerabilities introduced by trusted nodes, limited topological flexibility, and high resource overhead. Here, we experimentally demonstrate a flexible and resource-efficient mode-pairing QKD network enabled by integrated photonics, which natively supports trusted-node-free networking. Each user employs a Hz-level ultra-narrow-linewidth hybrid-integrated laser chip paired with a high-speed thin-film lithium niobate modulator chip for quantum state preparation, while a silicon-based wavelength-selective switch chip is deployed at the detection center. By introducing a novel global network-management strategy, the network fully decouples conventional physical-topology connections from resource-allocation constraints, allowing dynamic adaptation to time-varying communication demands and dynamic user access/leave. It achieves secure key sharing among all users with at most three distinct wavelengths and (N)(N)(N) Bell-state measurement units, delivering superior flexible scalability and resource-efficient full-connectivity compared with conventional WDM architectures. In a proof-of-principle three-user network, we achieve secure-key generation over both symmetric and asymmetric metropolitan hundred-kilometer-scale fiber-link configurations. Further point-to-point tests verify secure-key generation over 300 km with the asymptotic key rate exceeding the repeaterless bound. By combining photonic integrated circuits with a favorable network-management strategy, this work provides a chip-scale pathway toward compact, low-cost, and scalable large-scale metropolitan quantum networks.
The advent of cryptographically relevant quantum computers poses an existential threat to classical public-key infrastructure. Quantum Key Distribution (QKD) addresses this challenge by providing information-theoretic security for key establishment, independently of any computational hardness assumption. In this work, the deployment and experimental validation of a metropolitan-scale quantum-secure network between data centers in Milan is reported. The network operates over installed fiber infrastructure and implements a layered architecture integrating QKD hardware, standards-compliant Key Management (KM), and centralized Software-Defined Networking (SDN) orchestration. Dynamic path reconfiguration via active optical switching and trusted-node routing allow automated fail-over solutions. Application-layer validation across diverse protocols and workloads confirms the seamless interoperability of all system components. These results establish the technical and operational readiness of metropolitan QKD networks for production deployment, and offer a replicable blueprint for building quantum-secure communication infrastructure at metropolitan scale.
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Quantum Key Distribution (QKD) is maturing quickly. However, the current approaches to its application in optical networks make it an expensive technology. QKD networks deployed to date are designed as a collection of point-to-point, dedicated QKD links where non-neighboring nodes communicate using the trusted repeater paradigm. We propose a novel optical network model in which QKD systems share the communication infrastructure by wavelength multiplexing their quantum and classical signals. The routing is done using optical components within a metropolitan area which allows for a dynamically any-to-any communication scheme. Moreover, it resembles a commercial telecom network, takes advantage of existing infrastructure and utilizes commercial components, allowing for an easy, cost-effective and reliable deployment.
Current Free-Space Optical Quantum Key Distribution (FSO-QKD) systems are mainly focusing on the development of critical components needed towards the realization of satellite-based QKD, relying on bulky optical telescopes equipped with expensive optical systems. The cost and complexity of these systems hinder the establishment of a practical FSO-QKD infrastructure deployed as the wireless last-mile segment of Quantum Communication Infrastructure for applications in urban environments. In this work, we experimentally demonstrate successful transmission of polarization encoded qubits over 0.5 km free space metropolitan link with enhanced noise suppression for both nighttime and daylight operation. Βy deploying two low-cost, Single Mode Fiber-coupled, optical terminals along with a prototype quantum sender station operating in C-band, Quantum Bit Error Rate (QBER) values below 3.5% were achieved, with expected Secure Key Rates up to 1 kbps during nighttime. The FSO link was integrated with existing in-building fiber infrastructure, demonstrating stable operation and acceptable QBER values below 5.5%, even when coexisting with intense C-band light signals (> 10 mW) propagating alongside the photonic qubits. The field trial and its comprehensive verification paves the way for practical FSO-QKD in city-scale environments, enabling end users to share quantum keys over kilometer-scale wireless links between building rooftops in last-mile connectivity scenarios.