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International Conference on Quantum Communications, Networking, and Computing (QCNC)

Quantum Key Distribution Over Multimode Fiber Infrastructure

Ondřej Klíčník, Grigoris Anastasiou, Nikolaos Makris, Argiris Ntanos, Aristeidis Stathis, Konstantinos Tsimvrakidis, Giannis Giannoulis, H. Avramopoulos, Petr Münster, George Theodore Kanellos, Tomáš Horváth

Published Apr 6, 2026• 0 citations
Publisher

Abstract

This paper evaluates the feasibility of off-the-shelf quantum key distribution (QKD) systems over multimode fiber (MMF) links. QKD systems with two types of encoding, polarization-based and phase/time-bin-based, are tested using OM1, OM3, and OM4 fibers, with single mode fiber (SMF) as a reference. The connection between single mode QKD components and MMF forms an interferometric single modemulti mode-single mode (SMS) structure that exhibits intensity fluctuations. These, together with modal dispersion enabled by mode coupling, lead to decoherence in phase-encoded systems and significant depolarization in polarization-encoded systems. To minimize mode excitation, the fibers are operated in an underfilled launch regime, enabling near-single mode transmission over short spans.

DOI
10.1109/qcnc69040.2026.00062

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This paper evaluates the feasibility of off-the-shelf quantum key distribution (QKD) systems over multimode fiber (MMF) links. QKD systems with two types of encoding, polarization-based and phase/time-bin-based, are tested using OM1, OM3, and OM4 fibers, with single mode fiber (SMF) as a reference. The connection between single mode QKD components and MMF forms an interferometric single modemulti mode-single mode (SMS) structure that exhibits intensity fluctuations. These, together with modal dispersion enabled by mode coupling, lead to decoherence in phase-encoded systems and significant depolarization in polarization-encoded systems. To minimize mode excitation, the fibers are operated in an underfilled launch regime, enabling near-single mode transmission over short spans.
Abstract We demonstrate the experimental feasibility of a commercially available fiber-based phase-encoding quantum key distribution system ViPNet QTS, operating over a communication link incorporating a multi-mode fiber (MMF) segment of up to 20 km. Since ViPNet QTS is designed for deployment in single-mode fiber (SMF) communication links, all experiments were conducted in an SMF-MMF-SMF configuration. We report systematic measurements of transmission losses, as well as secret key generation performance for different MMF link lengths. As a result, stable secret key generation was achieved at a rate of 1.75 kbit min −1 over MMF links of up to 20 km.
Multimode fiber (MMF) telecommunication networks are ubiquitous in metropolitan-scale settings. Since many existing links are now ‘dark’ in favour of newer single-mode fiber deployment, they present an attractive opportunity for repurposing for emerging quantum technologies, including quantum key distribution (QKD). However, the topic of QKD in MMF is rather under-explored, with the only prior work in this field achieving a very short range of 550 m. In particular, well-known signal distortion effects in MMF can be algorithmically corrected in classical communication schemes, however it is not yet clear how to deal with them in QKD protocols. In this work, we critically evaluate the prospects of MMF for high-speed metro-scale QKD, including experiment consideration of design factors affecting the performance and the introduction of a new mode-matching approach with photonic lanterns. We also report a proof-of-principle realisation of a decoy-state BB84 QKD protocol with phase encoding over 17 km of MMF fibre, as well as a key generation rate over 1 Mbit/s at a distance of 1 km. This work confirms the suitability of current QKD technology for use in existing MMF links, unlocking new opportunities for quantum applications using legacy fibre and inspiring future research directions of quantum information processing in MMF more generally.
We report a proof-of-principle realization of a decoy-state BB84 quantum key distribution (QKD) protocol with phase encoding over a record-breaking 17 km of multimode fiber (MMF) at a rate of 193 kbits/s, as well as over 1 Mbit/s at a distance of 1 km. These results suggest that QKD can be deployed over MMF in metropolitan-scale telecommunication connections. Such MMF metropolitan networks are ubiquitous—thus, this advance could pave the way to wide scale metropolitan deployment. We also assess the advantages of adapting the OM3 channel using mode-matching photonic lanterns on the quantum bit error rate, signal gain, and key rate and compare different encoding techniques in light of MMF propagation effects. This work confirms the suitability of current QKD technology for use in existing MMF links, unlocking new opportunities for quantum applications using legacy fiber.
Abstract We report on the adaptation of a commercially available fiber-based quantum key distribution (QKD) system with phase-time encoding to free-space operation. The proposed approach minimizes architectural modifications by replacing the single-mode receiving interferometer with a free-space multi-mode delay interferometer, enabling efficient detection of spatially distorted quantum states typical for atmospheric channels. Several interferometer configurations are experimentally investigated, and a multi-mode Michelson interferometer with polarization routing is identified as the optimal solution in terms of interference visibility, loss, and alignment robustness. The impact of chromatic and modal dispersion on phase-time interference is analyzed and shown to be manageable under realistic system parameters. A laboratory QKD experiment demonstrates stable operation with a quantum bit error rate of about 4.5% and a secret key rate of hundreds of bits per second, confirming the feasibility of extending fiber-based QKD platforms to free-space links.
Abstract We demonstrate a free-space quantum key distribution (QKD) system based on phase-time encoding by reconfiguring a commercially available fiber-based QKD platform. The main goal of this work is to develop a free-space QKD system with minimal modifications, primarily through the implementation of a free-space multi-mode (MM) delay interferometer at the receiver. The use of a MM interferometer is crucial for enabling phase encoding over atmospheric channels, where direct coupling into single-mode fibers suffers from prohibitive losses due to atmospheric turbulence and beam distortions. We review existing methods for detecting spatially distorted quantum states, propose technological improvements, and experimentally evaluate several interferometer designs. A free-space MM Michelson interferometer with polarization routing is identified as the optimal solution, offering high interference visibility, low loss, and ease of integration into the system. A laboratory QKD experiment conducted with the modified system achieved a quantum bit error rate of 4.5% and a stable secret key rate of hundreds bit s−1, demonstrating the feasibility of secure key distribution over free-space links using adapted fiber-based technology. Detailed theoretical and experimental studies of chromatic and modal dispersion effects confirm that their impact can be effectively managed. This approach provides a practical and scalable route for extending quantum-secure communications into scenarios where optical fiber deployment is impractical or impossible.
Intermodal quantum key distribution (IM-QKD) enables the integration of fiber networks and free-space connections, which can be ground-to-ground links or involve satellite nodes in orbit. IM-QKD permits to extend the reach of free-space links without trusting any additional node, but this requires to efficiently couple the free-space signal into a single-mode fiber (SMF). We present the implementation of different IM-QKD networks realized in Padova and Vienna, exploiting km-long deployed fibers and free-space channels up to 620 meters. We show that such an intermodal scheme is compatible with both in-house QKD systems and commercially available solutions exploiting polarization encoding at 1550 nm. Remarkably, we realized different QKD tests in daylight and also in rainy conditions.