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IET conference proceedings.

Experimental validation of DV-QKD-based Qline architecture for metropolitan network on Berlin OpenQKD testbed

M. Sena, G. Harder, R. Döring, R.-P. Braun, M. Ritter, Marc Kaplan, Marc Geitz

Published Nov 13, 2023• 2 citations
Publisher

Abstract

We demonstrate a trusted-node-free design for integration of a DV-QKD system into the Berlin OpenQKD optical testbed. By utilizing conventional telecom modulators and a standard WDM-AWG, our scheme offers flexible multi-partner communication, and full connectivity between non-adjacent nodes over multiple C-band frequencies and metropolitan distances.

DOI
10.1049/icp.2023.2351

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We demonstrate a trusted-node-free design for integration of a DV-QKD system into the Berlin OpenQKD optical testbed. By utilizing conventional telecom modulators and a standard WDM-AWG, our scheme offers flexible multi-partner communication, and full connectivity between non-adjacent nodes over multiple C-band frequencies and metropolitan distances.
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 ($< 5.55.5%) but also a rather stable key rate between nodes in the Qline system over approximately 130 hours (5.5\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.
We demonstrate for the first time a four-node trusted-node-free metro network configuration with dynamic discrete-variable quantum key distribution DV-QKD networking capabilities across four optical network nodes. The network allows the dynamic deployment of any QKD link between two nodes of the network, while a QKD-aware centralised software-defined networking (SDN) controller is utilised to provide dynamicity in switching and rerouting. The feasibility of coexisting a quantum channel with carrier-grade classical optical channels where both the quantum and classical channels are in the C-band over field-deployed metropolitan networks and laboratory-based fibres (<10 km) is experimentally explored in terms of achievable quantum bit error rate, secret key rate as well as classical signal bit error rate. Moreover, coexistence analysis over multi-hops configuration using different switching scenarios is also presented. The secret key rate dropped 43% when coexisting one classical channel with 150 GHz spacing from the quantum channel for multiple links. This is due to the noise leakage from the Raman scattering into the 100 GHz bandwidth of the internal filter of the Bob DV-QKD unit. When coexisting four classical channels with 150 GHz spacing between quantum and the nearest classical channel, the quantum channel deteriorates faster due to the combination of Raman noise, other nonlinearities and high aggregated launch power causing the QBER value to exceed the threshold of 6% leading the SKR to reach a value of zero bps at a launch power of 7 dB per channel. Furthermore, the coexistence of a quantum channel and six classical channels through a field-deployed fibre test network is examined.
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.
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.
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.