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High-Dimensional Quantum Key Distribution via Full Core-Mode Encoding over Deployed Multicore Fibers

G. H. dos Santos1,2,*, K. B. Sawada1,2,*, N. Villalba1,2, C. Jara1,2, N. Guerrero1,2, C. Melo3, M. H. Magiotto4, D. Martinez5,6, G. B. Xavier7 et al.

J. Cariñe8, G. Saavedra3, E. S. Gómez1, S. P. Walborn1,2, and G. Lima1,2,†

  • *These authors equally contributed to this work.
  • †Contact author: glima@udec.cl
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PRX Quantum 7, 043012 – Published 19 October, 2026

DOI: https://doi.org/10.1103/y779-t6ry

Abstract

Quantum key distribution (QKD) provides information-theoretic security rooted in quantum physics, while high-dimensional (HD) encoding increases both noise tolerance and secret-key yield. Multicore fibers (MCFs), a leading platform for next-generation telecom networks, are a natural substrate for HD-QKD. Field demonstrations over deployed MCFs have so far relied on a hybrid qudit encoding strategy that combines two path (core modes) with the time-bin photonic degree of freedom, rather than exploiting the full set of available core modes. Although practical, this current approach incurs intrinsic efficiency penalties that grow with dimension. Encoding a qudit in the full set of core modes removes these penalties, but it requires the coherent control of all cores at once and has so far been restricted to laboratory settings. Here we implement a four-dimensional (d=4) QKD protocol that directly exploits the full set of core modes of a four-core MCF, operating over an installed MCF network across the Universidad de Concepción campus under continuous environmental perturbations. We further benchmark the scheme using superconducting nanowire detectors at 10 dB channel loss, achieving a composable finite-key rate of R=6.19×10−3  bits/pulse, the highest per-pulse rate reported to date for deployed HD-QKD at comparable loss. This result establishes core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.

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Popular Summary

Quantum key distribution (QKD) allows two parties to share secret cryptographic keys whose security is guaranteed by the laws of physics, making it a strong candidate for protecting communications against future quantum computers. In this work we demonstrate a four-dimensional QKD system operating over an installed multicore fiber network. Rather than encoding information in ordinary quantum bits, we use all four cores of the fiber simultaneously so that each photon carries a four-dimensional quantum state. Tested under continuous real-world perturbations including temperature changes and vehicle vibrations, the system achieved a secret-key rate of 6.19×10−3  bits per pulse, the highest ever reported for high-dimensional QKD at comparable channel loss, establishing multicore fiber as a practical platform for next-generation quantum-secure communications.

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PRX Quantum 7, 043012– Published 19 October, 2026
  • Received 2 June 2026
  • Revised 13 August 2026
  • Accepted 10 September 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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