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Quantum 2.0 Conference and Exhibition

Quantum Key Distribution in Multimode Fiber on Metropolitan-scale Distances

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

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We report a proof-of-principle realization of a decoy-state BB84 QKD protocol with phase encoding over 17 km of MMF fiber, including a discussion on its stability and implementation considerations.
DOI
10.1364/quantum.2025.qm2b.5

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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.
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.
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.
Quantum key distribution (QKD) promises provably secure communications. In order to improve the secret key rate, combining a biased basis choice with the decoy-state method is proposed. Concomitantly, there is a basis-independent detection efficiency condition, which usually cannot be satisfied in a practical system, such as the time-phase encoding. Fortunately, this flaw has been recently removed theoretically and experimentally in the four-intensity decoy-state BB84 QKD protocol using the fact that the expected yields of single-photon states prepared in two bases stay the same for a given measurement basis. However, the security proofs do not fully consider the finite-key effects for general attacks. In this work, we provide the rigorous finite-key security bounds in the universally composable framework for the four-intensity decoy-state BB84 QKD protocol. We build a time-phase encoding system with 200 MHz clock to implement this protocol, in which the real-time secret key rate is more than 60 kbps over 50 km single-mode fiber.