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Scientific Reports

Quantum key distribution over a three-user fiber network with passive optical circulator

Youn-Chang Jeong, Seok Kim, In-Cheol Baek, Changho Hong, Daesung Kwon, Jingak Jang, Se-Wan Ji

Published Oct 23, 2026• 0 citations
Publisher

Abstract

Abstract We report the implementation of a quantum key distribution (QKD) system over a three-user fiber-optic network in which each user employs an identical bidirectional transceiver, with a single asymmetric interferometer shared between the encoding and decoding paths to reduce duplicated optical resources at the node level. The three users-Alice, Bob, and Charlie-are interconnected through a passive circulator-based hub built from 3-port optical circulators, which enables secure key exchange between any pair of them without active switching or a trusted intermediary. Each user is connected to the central circulator via standard telecom fiber links of 10-15 km, resulting in effective inter-user distances of 20-25 km. We employed the BB84 protocol with a two-decoy-state method (weak and vacuum) for secure key generation. For each user pair, sifted keys of $$10^6$$ bits were processed with a composable security parameter $$\varepsilon \sim 10^{-8}$$ . Secret key lengths were evaluated using three approaches: (1) theoretical estimation based on measured quantum bit error rate (QBER) and gains, assuming an error correction efficiency of 1.16; (2) experimental post-processing using Winnow; and (3) experimental post-processing using Cascade. All links achieved low QBERs and yielded positive secret key lengths. Cascade consistently produced longer final keys due to its higher reconciliation efficiency. These results demonstrate the feasibility of a resource-sharing bidirectional node architecture for secure multi-user quantum communication using passive circulator-based routing.

DOI
10.1038/s41598-026-76538-1

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Abstract We report the implementation of a quantum key distribution (QKD) system over a three-user fiber-optic network in which each user employs an identical bidirectional transceiver, with a single asymmetric interferometer shared between the encoding and decoding paths to reduce duplicated optical resources at the node level. The three users-Alice, Bob, and Charlie-are interconnected through a passive circulator-based hub built from 3-port optical circulators, which enables secure key exchange between any pair of them without active switching or a trusted intermediary. Each user is connected to the central circulator via standard telecom fiber links of 10-15 km, resulting in effective inter-user distances of 20-25 km. We employed the BB84 protocol with a two-decoy-state method (weak and vacuum) for secure key generation. For each user pair, sifted keys of 10610^6106 bits were processed with a composable security parameter ε∼10−8\varepsilon \sim 10^{-8}ε∼10−8 . Secret key lengths were evaluated using three approaches: (1) theoretical estimation based on measured quantum bit error rate (QBER) and gains, assuming an error correction efficiency of 1.16; (2) experimental post-processing using Winnow; and (3) experimental post-processing using Cascade. All links achieved low QBERs and yielded positive secret key lengths. Cascade consistently produced longer final keys due to its higher reconciliation efficiency. These results demonstrate the feasibility of a resource-sharing bidirectional node architecture for secure multi-user quantum communication using passive circulator-based routing.
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Secure communication networks enabled by commercial quantum key distribution (QKD) are already available. However, their widespread deployment will require great efforts towards reducing the currently prohibitive cost of QKD systems. Here, we propose a compact and cost-effective alternative to the asymmetric Mach-Zehnder interferometer commonly used to implement phase encoding in the Bennett-Brassard 1984 (BB84) QKD protocol. Our solution consists of an all-fiber, in-line, highly birefringent interferometer (HBI). The HBI shows improved tolerance to length mismatches and a simpler assembly, making it particularly desirable for the fabrication of multi-user systems where several interferometers must have matched delays and where cost and space considerations can be most critical, such as quantum access networks. As a proof-of-principle, we demonstrate point-to-point QKD operation with HBIs over 15.5 km drop fiber and an 8-port passive optical network splitter. We achieve a secure key generation rate of 299.4 ± 16.4 kbit/s with a quantum bit error rate of 2.89 ± 0.31% for a continuous 25 h operation period.
Unconditionally secure multiuser quantum key distribution (QKD) over long distance switched telecom fiber networks will have a revolutionary impact on people's, companies', and governmental exchange of confidential information. We report five-user QKD over switched fiber networks in both star and tree configurations, using the BB84-protocol with phase encoding. Both setups implement polarization insensitive phase modulators, necessary for birefringent single mode fiber networks. In both configurations, for the mean photon number μ=0.1, we have achieved raw rates between 50Hz and 300Hz at distances between 25km to 50km with quantum bit error rates between 1.24% to 5.56%. The measurements have showed feasibility of multiuser QKD over switched telecom fiber networks, using standard fiber telecom components.
We previously demonstrated a high speed, point to point, quantum key distribution (QKD) system with polarization coding over a fiber link, in which the resulting cryptographic keys were used for one-time pad encryption of real time video signals. In this work, we extend the technology to a three-node active QKD network - one Alice and two Bobs. A QKD network allows multiple users to generate and share secure quantum keys. In comparison with a passive QKD network, nodes in an active network can actively select a destination as a communication partner and therefore, its sifted-key rate can remain at a speed almost as high as that in the point-to-point QKD. We demonstrate our three-node QKD network in the context of a QKD secured real-time video surveillance system. In principle, the technologies for the three-node network are extendable to multi-node networks easily. In this paper, we report our experiments, including the techniques for timing alignment and polarization recovery during switching, and discuss the network architecture and its expandability to multi-node networks.
Quantum cryptography with entangled photon pairs can be more powerful than protocols based onsingle photons or weak coherent pulses: they can tolerate higher losses and thus allow thedistribution of quantum secret keys (QKD) over longer distances [1], they also provide a waytowards device-independent quantum cryptography [2]. However, in order to enable a wide use ofentangled photon pairs in future quantum telecommunication systems, further developments areneeded to demonstrate performant sources that can be easily fabricated and integrated into Telecomfiber networks.Here we present a source consisting of an aluminium gallium arsenide waveguide generatingphoton pairs in the Telecom band by type II spontaneous parametric down-conversion [3]. Such adevice has already been proven to work under electrical pumping [4]. Thanks to the very smallbirefringence of the guided modes, the pairs are directly generated in a polarization-entangled Bellstate, without the need for any post-compensation. Moreover, as the photons are emitted over alarge bandwidth (about 100 nm) with a joint spectrum that exhibits frequency anticorrelation, thesame source can be used to simultaneously distribute keys among multiple pairs of users by usingstandard Telecom wavelength demultiplexers [5]. Here, we experimentally show the distribution ofquantum secret keys with the BBM92 QKD protocol [6] between four different pairs of users with acommercial 100 GHz demultiplexer (0.8 nm channel width and spacing). Under CW pumpingconditions, using free-running InGaAs single-photon detectors, we achieve a secret key rate of 0.21bits/s and a qubit error rate (QBER) of 6.9% over 50 km of standard optical fiber.Our results, obtained with a robust and simple experimental set-up, open the way towards theimplementation of practical device-independent quantum communication protocols.
Quantum key distribution (QKD) offers unconditional information security by allowing two distant users to establish a common encryption key resilient to hacking. Resultingly, QKD networks interconnecting critical infrastructure and enabling the secure exchange of classified information, can provide a solution to the increasing number of successful cyberattacks. To efficiently deploy quantum networks, the technology must be integrated over existing communication infrastructure, such as optical fibre links. Yet, QKD poses stringent requirements on the conditions of the network over which it is deployed. This work demonstrates the first quantum communication network in Cyprus via the deployment of a multi-node quantum network, exploiting existing commercial underground optical fibre. The network employs bidirectional occupation of fibres and wavelength multiplexing in a ring architecture to achieve, with minimal use of dark fibres, high-rate QKD. Results obtained reveal consistent key generation rates across all nodes, confirming reliable operation in a real-world environment. This deployment highlights the feasibility of leveraging existing telecom infrastructure for quantum-secured communication, marking a significant step toward scalable and cost-effective quantum networks suited for critical applications.