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

Gustavo Henrique dos Santos, Kei Braz Sawada, N. Villalba, C. Jara, Nayda Guerrero, C. A. R. Sá de Melo, M. H. Magiotto, Daniel Martínez, GUILHERME BARRETO XAVIER, Jaime Cariñe, G. Saavedra, Esteban S. Gómez, Stephen Patrick Walborn, G. Lima

Published Sep 10, 2026• 0 citations
Publisher

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.

DOI
10.1103/y779-t6ry

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Quantum Computing, Communication, and Simulation•Mar 12, 2024•2 citations
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.
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 approach incurs intrinsic efficiency penalties that grow with dimension. Here we implement a four-dimensional (d=4d=4d=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 10\,10dB channel loss, achieving a composable finite-key rate of R=6.19×10−3 R = 6.19\times 10^{-3}\,R=6.19×10−3bits/pulse, the highest per-pulse rate reported to date for HD-QKD at comparable loss. This result establishes core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.
Quantum key distribution (QKD) is a secure communication scheme for sharing symmetric cryptographic keys based on the laws of quantum physics, and is considered a key player in the realm of cyber-security. A critical challenge for QKD systems comes from the fact that the ever-increasing rates at which digital data are transmitted require more and more performing sources of quantum keys, primarily in terms of secret key generation rate. High-dimensional QKD based on path encoding has been proposed as a candidate approach to address this challenge. However, while proof-of-principle demonstrations based on lab experiments have been reported in the literature, demonstrations in realistic environments are still missing. Here we report the generation of secret keys in a 4-dimensional hybrid time-path-encoded QKD system over a 52-km deployed multicore fiber link forming by looping back two cores of a 26-km 4-core optical fiber. Our results indicate that robust high-dimensional QKD can be implemented in a realistic environment by combining standard telecom equipment with emerging multicore fiber technology.
HD-QKD(high-dimensional quantum key distribution) is an alternative to two-dimensional QKD with advantages such as higher data transmission per pulse [54] and noise tolerance [13, 66]. It has been shown that MCFs (multicore fibers) are a viable platform for performing HD-QKD through phase encoding across modes of the same fiber [10]. MCFs have also gathered attention due to their increased data transmission rates and are likely to become part of classical telecom infrastructure in the future [35, 50, 14, 51]. In this thesis we report a proof-of-concept decoy-state BB84 HD-QKDexperiment with an all-fiber setup using installed 4C-MCFs (four core multicore fibers) in a realistic environment. We introduce the theory of QKDand the motivations for decoy states as well as the security bounds for systems with different dimensions. We describe the experiment in detail, including active phase stabilization, state preparation, MCF-based technologies, channels and the detection system. We report a maximum short-range key rate of 7.8 kbit/s and a maximum secure key transmission distance of 105 km, as well as a short-range stability of over 16 hours. A comparison of our experiment with several others is provided, showing that our results are comparable to other works with similar detection technology. Wealso include two papers where we used our experimental platform for experiments simulating non-Markovian noise and implementing single setting quantum state tomography, showing the flexibility of our setup and potential for further work.
By exploiting the core aspects of quantum mechanics, quantum key distribution (QKD) offers a solution to the critical challenge of secure communication, facilitating the exchange of a provably secure secret key between remote parties. Despite its potential, QKD faces challenges such as low secret key rates, short propagation distances, and low noise tolerance; however, high-dimensional QKD is a promising strategy to mitigate these constraints. High-dimensional QKD extends the Hilbert space dimension beyond two by utilizing multiple degrees of freedom of the electromagnetic field. While several studies address high-dimensional QKD in multicore fibers, which provide degrees of freedom associated with a particular core choice, these works do not consider cases with significant crosstalk between cores. In this work, we present the estimated secret key rate and quantum bit error rate as functions of the parameters of a four-dimensional hybrid time-path QKD system, considering the crosstalk between cores.
Multicore-fibres (MCF) have emerged as an attractive solution for capacity scaling in optical communication. Securing such links with Quantum Key Distribution (QKD) is critical with the advent of Quantum computers. For practical deployment, QKD is required to operate alongside high-speed classical communication over field-deployed fiber infrastructure. In this work, the performance of QKD is experimentally evaluated in coexistence with high-speed classical data transmission over field-deployed underground and aerial multicore fiber links. The QKD system is implemented with Coherent One-Way Slim protocol, with dedicated Quantum and classical channels for Quantum communication and QKD post-processing. The deployed 4-core MCF infrastructure at IIT Madras provides an effective transmission length of 48 km through loopback operation. QKD performance over the MCF is evaluated in terms of quantum bit error rate (QBER) under varying attenuation, controlled noise injection, and inter-core crosstalk over both singlecore fiber and MCF links. The experimental results show that inter-core crosstalk has a stronger impact in MCF, leading to increased excess noise at the quantum receiver. For a 32-GBaud PM 16-QAM classical modulation format, stable QKD operation is maintained when the spectral separation is approximately six to eight times the occupied bandwidth.
Higher Dimensional Quantum Key Distribution (HD-QKD) improves information capacity and security compared to binary QKD systems. Previous HD-QKD implementations have required d+1 (d refers to encoding dimension) single-photon detectors, limiting large-scale implementation. In contrast, another HD-QKD implementation using coherent one-way (COW) protocol hardware with two detectors achieved a secret key rate (SKR) of 155 kbps for an 8-D system and 2.7% QBER tolerance for a 16-D system with qubit scrambling across a time window. In this paper, we theoretically simulate and experimentally demonstrate time-bin encoded HD-QKD using COW protocol hardware with two single-photon detectors operating at standard telecom wavelengths, achieving a remarkable maximum encoding of 6 bits per photon (i.e., 56-d system). Our HD-QKD system incorporates a practical decoy-state technique to counter the photon number splitting attack, allowing greater flexibility in choosing higher mean photon number thereby achieving higher SKR and distances. This implementation demonstrates a remarkable ninefold increase in SKR for a 16-d system (210 kbps over 25 km of standard single-mode fiber) compared to qubit-based systems, with a QBER tolerance of 13.5% for d = 16. Notably, we achieved positive key rates for emulated loss up to 131 km of fiber length for d = 8. By optimizing detector deadtime and mean photon number using our theoretical model, we achieve enhanced key rates and security. Our findings highlight the superiority of our approach and lay the foundation for high throughput HD-QKD implementation, offering a secure and efficient method for transmitting sensitive information.