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Quantum Computing, Communication, and Simulation

Metropolitan-scale quantum network using multimode fiber

Adam Brzosko, Robert I. Woodward, Yuen San Lo, Mirko Pittaluga, Peter Raymond Smith, J. F. Dynes, Andrew J. Shields

Published Mar 19, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.3042155

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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.
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 Quantum communication has seen rapid progress towards practical large-scale networks, with quantum key distribution (QKD) spearheading this development. While fibre-based systems have been shown to be well suited for metropolitan scales, suitable fibre infrastructure may not always be in place. Here, we make the case for an entanglement-based free-space quantum network as a practical and efficient alternative for metropolitan applications. We developed a deployable free-space QKD system and demonstrated its use in realistic scenarios. For a representative 1.7-km free-space link, we showcase its ad hoc deployability and achieve secure key rates of up to 5.7 kbps, with 2.5 kbps in direct noon sunlight. By extrapolating experimental data, we show that kbps key rates are achievable even for 10-km distances and multi-user scenarios. We anticipate that our work will establish free-space networks as a viable solution for metropolitan applications and an indispensable complementary building block in the future global quantum internet.
Quantum key distribution is a cornerstone of quantum technology, offering information-theoretical secure keys for remote parties. With many quantum communication networks established globally, the mode-pairing protocol stands out for its efficacy over inter-city distances using simple setups, emerging as a promising solution. In this study, we employ the mode-pairing scheme into existing inter-city fiber links, conducting field tests across distances ranging from tens to about a hundred kilometers. Our system achieves a key rate of 1.217 kbit/s in a 195.85 km symmetric link and 3.089 kbit/s in a 127.92 km asymmetric link without global phase locking. The results demonstrate that the mode-pairing protocol can achieve key rates comparable to those of a single quantum link between two trusted nodes on the Beijing-Shanghai backbone line, effectively reducing the need for half of the trusted nodes. These field tests confirm the mode-pairing scheme’s adaptability, efficiency, and practicality, positioning it as a highly suitable protocol for quantum networks.
Quantum key distribution (QKD) is a well-established quantum technology with the potential to address growing security concerns associated with advancements in quantum computation. While the theoretical foundation of QKD is sufficiently understood, challenges persist in establishing long-distance connections without the necessity of costly satellite missions or the complexities of deploying quantum repeaters.We present a demonstration of QKD over a fibre with 21 dB signal attenuation, utilising a link between St. Pölten and Vienna. Overall, we achieved a secret key rate of 4 kbps, matching our prior test measurements under laboratory conditions.The implemented protocol was a four-state, one-decoy BB84 at 1550 nm, using a desktop transmitter-receiver pair configured as a flexible setup. The quantum signal was prepared via time-bin encoding, a technique that has been demonstrated to be reliable and robust against depolarisation and polarisation mode dispersion. This work demonstrates the capability of quantum communication in conventional fibre networks by showing the functionality of performing QKD over an existing single-mode fibre (SMF) link.