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Measurement Sensors

Time and frequency dissemination and quantum key distribution on the Niedersachsen Quantum Link – A practical approach

Ann‐Kathrin Kniggendorf, Ali Hreibi, Jochen Kronjäger, S. Kück

Published Jan 15, 2025• 2 citations
Publisher

Abstract

We reconfigured an existing time and frequency dissemination (TFD) testbed on a deployed optical fibre pair for TFD and quantum key distribution (QKD) experiments using a separable fibre configuration to allow for one fibre to go dark while controlled TFD and optical communication for instrument control remains available via the second fibre. A lab-based model was used to establish and test the setup before deployment. Fractional frequency instability of the TFD available in separated fibre configuration was shown to be below 2 × 10 −13 at 1 s and at 1 × 10 −17 for 10 5 s averaging, while the looped-back configuration allows for an interferometric fibre link with a fractional frequency instability of 1 × 10 −15 (1 s averaging) and 1 × 10 −21 (10 5 s), respectively. A verified computer-based model of the link allows to test whether the photonic background from a given operation on the fibre is compatible with QKD operation in the same fibre.

DOI
10.1016/j.measen.2024.101776

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We reconfigured an existing time and frequency dissemination (TFD) testbed on a deployed optical fibre pair for TFD and quantum key distribution (QKD) experiments using a separable fibre configuration to allow for one fibre to go dark while controlled TFD and optical communication for instrument control remains available via the second fibre. A lab-based model was used to establish and test the setup before deployment. Fractional frequency instability of the TFD available in separated fibre configuration was shown to be below 2 × 10 −13 at 1 s and at 1 × 10 −17 for 10 5 s averaging, while the looped-back configuration allows for an interferometric fibre link with a fractional frequency instability of 1 × 10 −15 (1 s averaging) and 1 × 10 −21 (10 5 s), respectively. A verified computer-based model of the link allows to test whether the photonic background from a given operation on the fibre is compatible with QKD operation in the same fibre.
Quantum key distribution (QKD) is a prominent technology for exchanging secure keys between two authenticated remote parties. The security in a QKD system is unconditionally ensured by the laws of quantum mechanics which guarantee that any eavesdropping attempts will be identified. In this paper, we report the concept and structure of the Niedersachsen Quantum Link as a QKD testbed. The testbed consists of a 78 km long pair of dark fibers between the cities of Hannover and Braunschweig and is operated by Physikalisch-Technische Bundesanstalt and Leibniz Universität Hannover. One of the dark fibers is reserved for “quantum” applications, while the second fiber is dedicated to the dissemination of time and frequency reference signals as well as classical communication between two sites. This combination of dark “quantum” fiber and integrated time-frequency infrastructure is a unique feature of the Niedersachsen Quantum Link testbed.
Twin-field (TF) quantum key distribution (QKD) has rapidly risen as the most viable solution to long-distance secure fibre communication thanks to its fundamentally repeater-like rate-loss scaling. However, its implementation complexity, if not successfully addressed, could impede or even prevent its advance into real-world. To satisfy its requirement for twin-field coherence, all present setups adopted essentially a gigantic, resource-inefficient interferometer structure that lacks scalability that mature QKD systems provide with simplex quantum links. Here we introduce a technique that can stabilise an open channel without using a closed interferometer and has general applicability to phase-sensitive quantum communications. Using locally generated frequency combs to establish mutual coherence, we develop a simple and versatile TF-QKD setup that does not need service fibre and can operate over links of 100 km asymmetry. We confirm the setup's repeater-like behaviour and obtain a finite-size rate of 0.32 bit/s at a distance of 615.6 km.
Entanglement-based quantum links are the backbone of future quantum internet networks, enabling secure communication between distant cities. Realizing such networks requires addressing multiple practical challenges in long-distance quantum key distribution : time synchronisation, interferometer stabilisation and automation. Here, we report several advances. First, we maintained an operational QKD link continuously for 325hours over 50km between two remotes locations, demonstrating the feasibility of long-duration key generation. We further extended secure key distribution up to a 100km operational link connecting the University of Nice to a ground-based optical station, a setup compatible with future quantum satellite connections. Finally, by employing wavelength demultiplexing to separate photons of entangled pairs, we performed QKD across multiple ITU channels, achieving secure key exchange via the BBM92 protocol and time-energy observables.
Modern optical networking techniques have the potential to greatly extend the applicability of quantum communications by moving beyond simple point-to-point optical links, and by leveraging existing fibre infrastructures. We experimentally demonstrate many of the fundamental capabilities that are required. These include optical-layer multiplexing, switching, and routing of quantum signals; quantum key distribution (QKD) in a dynamically reconfigured optical network; and coexistence of quantum signals with strong conventional telecom traffic on the same fibre. We successfully operate QKD at 1310 nm over a fibre shared with four optically amplified data channels near 1550 nm. We identify the dominant impairment as spontaneous anti- Stokes Raman scattering of the strong signals, quantify its impact, and measure and model its propagation through fibre. We describe a quantum networking architecture which can provide the flexibility and scalability likely to be critical for supporting widespread deployment of quantum
Quantum key distribution (QKD) offers unconditional information security by allowing two distant users to establish a common encryption key resilient to hacking. Resultingly, QKD networks interconnecting critical infrastructure and enabling the secure exchange of classified information, can provide a solution to the increasing number of successful cyberattacks. To efficiently deploy quantum networks, the technology must be integrated over existing communication infrastructure, such as optical fibre links. Yet, QKD poses stringent requirements on the conditions of the network over which it is deployed. This work demonstrates the first quantum communication network in Cyprus via the deployment of a multi-node quantum network, exploiting existing commercial underground optical fibre. The network employs bidirectional occupation of fibres and wavelength multiplexing in a ring architecture to achieve, with minimal use of dark fibres, high-rate QKD. Results obtained reveal consistent key generation rates across all nodes, confirming reliable operation in a real-world environment. This deployment highlights the feasibility of leveraging existing telecom infrastructure for quantum-secured communication, marking a significant step toward scalable and cost-effective quantum networks suited for critical applications.
Modern optical networking techniques have the potential to greatly extend the applicability of quantum communications by moving beyond simple point-to-point optical links and by leveraging existing fibre infrastructures. We experimentally demonstrate many of the fundamental capabilities that are required. These include optical-layer multiplexing, switching and routing of quantum signals; quantum key distribution (QKD) in a dynamically reconfigured optical network; and coexistence of quantum signals with strong conventional telecom traffic on the same fibre. We successfully operate QKD at 1310 nm over a fibre shared with four optically amplified data channels near 1550 nm. We identify the dominant impairment as spontaneous anti-Stokes Raman scattering of the strong signals, quantify its impact, and measure and model its propagation through fibre. We describe a quantum networking architecture which can provide the flexibility and scalability likely to be critical for supporting widespread deployment of quantum applications.