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A trusted-node-free eight-user metropolitan quantum communication network
Siddarth Koduru Joshi, Djeylan Aktas, Sören Wengerowsky, Martin Lončarić, Sebastian Philipp Neumann, Bo Liu, Thomas Scheidl, Guillermo Currás-Lorenzo, Željko Samec, Laurent Kling, Alex Qiu, Mohsen Razavi, Mario Stipčević, John G. Rarity, Rupert Ursin
TL;DR
Scaling quantum key distribution beyond two users is limited by trusted nodes, active switching, and constrained multiplexing approaches. The paper demonstrates a passive, fully connected eight-user metropolitan network using one shared entangled-photon source and minimal user hardware. It achieves simultaneous secure connectivity across all 28 links while offering linear physical-resource scaling and configurable network subgraphs.
Problem
Scaling quantum key distribution to many users remains difficult because existing networks rely on trusted nodes, active switching, or approaches with limited scalability.
Method
The network passively combines wavelength and beamsplitter multiplexing to distribute bipartite entanglement from one source through one fiber and a small measurement module per user.
Results
The eight-user metropolitan network forms a fully connected graph, enabling every user to exchange a secure key with every other user simultaneously across 28 links.
Takeaways & Limitations
The architecture supports software- or hardware-defined subgraphs, traffic management, and metropolitan links while requiring only one additional PAM and fiber per added user.
Takeaways & Limitations
The maximum user count is limited by QBER from accidental coincidences, which increase as users receive more wavelength channels.
Abstract
from arXiv · showhide
Quantum communication is rapidly gaining popularity due to its high security and technological maturity. However, most implementations are limited to just two communicating parties (users). Quantum communication networks aim to connect a multitude of users. Here we present a fully connected quantum communication network on a city wide scale without active switching or trusted nodes. We demonstrate simultaneous and secure connections between all 28 pairings of 8 users. Our novel network topology is easily scalable to many users, allows traffic management features and minimises the infrastructure as well as the user hardware needed.
INTRODUCTION
Existing quantum networks are difficult to scale because they rely on trusted nodes, active switching, or approaches with limited scalability. This work presents an eight-user, city-wide, fully connected network without trusted nodes, using passive multiplexing and minimal user hardware.
- 8 users share secure keys simultaneously across all 28 pairings in a fully connected network without trusted nodes.
- The topology uses one entanglement source, one fiber per user, and minimal user hardware comprising two detectors and a polarization analysis module.
- Passive wavelength-filter and beamsplitter multiplexing produces a quadratic reduction in channel resources compared with the earlier scheme.
- The architecture avoids trusted nodes and does not require service-provider trust or user adaptations when users are added or removed.
- 16 wavelength channels and 2 beamsplitter channels distribute 8 entangled states among the 28 user links.
RESULTS
The experiment demonstrates a fully connected eight-user quantum network using a single entanglement source, standard multiplexing equipment, and one deployed fiber per user. Laboratory and metropolitan tests showed stable secure-key generation across heterogeneous hardware and links ranging from approximately 10 m to 16.6 km, while experimental imperfections limited key rates.
- Network architecture: 16 wavelength channels interconnect all 8 users, compared with 56 channels in the earlier scheme, using DWDM and beamsplitter multiplexing from one entangled-photon source.The architecture combines standard 100 GHz DWDM channels with in-fiber beamsplitters.
- Network architecture: Every pair of users shares an entangled photon pair in the logical layer, although the physical layer uses a simple hub-and-spoke topology with centralized multiplexing.The eight users are organized into two four-user subnetworks, with beamsplitters duplicating the first subgroup’s wavelength channels to create the second interconnected subgroup.
- Performance and limitations: Heterogeneous detectors did not significantly impact key-generation rates, but QBER and secure-key rates were limited by fiber polarization control, HWP/PBS alignment, extinction ratio, and shared pump-power optimization.The pair-generation rate could not be adjusted independently for each wavelength-channel pair, making the optimum pump power dependent on differing user hardware.
- Experimental demonstrations: 28 metropolitan links spanned effective user separations from approximately 10 m to 16.6 km, demonstrating operation across short fibers, spools, and deployed city fiber.The metropolitan demonstration connected users across Bristol and measured secure keys over long-distance links while accounting for finite-size effects.
- Experimental demonstrations: 18.45 hours of laboratory testing produced stable QBER and positive secure-key generation, including finite-key effects and a security parameter of 10^-5.The laboratory setup connected eight users to the quantum network service provider and multiplexing unit using approximately 10 m fibers and 16 detectors.
DISCUSSION
The demonstrated network combines fully connected, trusted-node-free metropolitan operation with flexible scaling, traffic management, and reduced infrastructure and user hardware. Its scalability remains bounded by resources, loss, and error-rate growth, while simulations extend the topology to larger user counts.
- Flexibility: The network supports software- or hardware-defined sub-graphs and can multiplex states through shared fibers to create complex network configurations.These features enable traffic management and network customization without changing the entanglement source.
- Scaling: The physical topology grows linearly with each additional user, requiring only one additional polarization analysis module and fiber.Additional wavelength channels minimally affect existing key rates up to a noise-count limit, whereas additional beamsplitters reduce key rates but support more users with fewer added channels.
- User hardware: Several channels can share one detector to lower per-user cost, at the expense of a slight increase in QBER.The increase can be mitigated by demultiplexing the signal across multiple detectors.
- Performance and reach: The network demonstrated approximately 17 km or more of range, and simulations indicate reasonable secure key rates for 32 or 49 users.The range can be extended using repeaters and detector or wavelength-management techniques.
- Boundaries: The number of connected users is limited by available resources, loss, and marginal error-rate increases for simultaneous connections using a given detector.The network scales linearly in user hardware and deployed fibers, while service-provider wavelength capacity can be increased with narrower-band WDMs and broader-band downconversion.
MATERIALS AND METHODS
The network distributes polarization-entangled photon pairs through DWDM filters and beamsplitters to eight users, whose passive modules perform measurements and detection. The experiments use short and metropolitan fiber links to evaluate network stability and connectivity.
- Wavelength multiplexing: One 32-channel DWDM provides 16 used wavelength channels, selected symmetrically around 1550.217 nm to form correlated channel pairs.Energy conservation during down-conversion provides polarization entanglement between paired channels.
- User connectivity: Each wavelength channel is split by a beamsplitter and combined through add-drop multiplexers so every user receives four channels in one single-mode fiber.The resulting arrangement gives each user eight polarization-entangled connections to other users.
- User hardware: Each user uses a passive polarization analysis module and two single-photon detectors to measure incoming photons in the HV or DA basis.A beamsplitter directs photons through short and long paths, with a half-wave plate providing the 45° rotation for DA measurements and a 3.7 ns time-bin delay.
- Experimental links: The experiments used single-mode 1550 nm fibers ranging from approximately 10 m laboratory links to deployed university and Bristol city links, including approximately 12.6 km and 4.3 km spools.The first run evaluated short links, while the second used varying metropolitan link lengths characterized by optical time-domain reflectometry.
- User hardware: The polarization analysis modules were designed to be compact, portable, inexpensive to mass-produce and align, while using fiber-coupled optical components.The implementation used commercial beamsplitters, polarizing beamsplitters, half-wave plates and mirrors.
SUPPLEMENTARY MATERIAL
The supplementary material describes how the network scales, manages key-rate limitations, supports anonymous access, and secures key extraction without trusted nodes.
- Scalability: At least a few hundred wavelength channels could be supported using broader-bandwidth entanglement sources or closer-spaced WDM channels.The demonstrated source provides approximately 60 nm bandwidth and the current WDM channels use 100 GHz spacing, limiting the present design to about 75 channels.
- Key-rate limitations: Accidental coincidences increase QBER as users receive more wavelength channels, limiting the maximum network size and reducing secure key rates.Propagation-delay correction, multiple detectors, or selective wavelength detection are described as strategies for increasing secure key rates.
- Scalability: The topology can create networks with any integer number of users by constructing a larger network and leaving selected users unconnected.This extends the subnet and beamsplitter construction beyond configurations satisfying the idealized integer conditions.
- Scalability: 32 users in 2 subnets and 49 users in 7 subnets can maintain reasonable secure key rates under the current experimental constraints.Beamsplitter multiplexing acts like time sharing of the key, while wavelength multiplexing adds key sources; filtering can avoid additional noise from more wavelength channels.
- Access networks: Users can control an anonymous access network themselves rather than requesting the QNSP to establish each connection.Conventional access networks can sacrifice anonymity when users identify their chosen communication partner to the provider.
- Security analysis: Secure extraction retains only events with zero transmitter time offset when the path delay greatly exceeds the coincidence-window duration.Events with transmitter offsets of ±∆ can be manipulated by an eavesdropper and are therefore tagged as potentially known to Eve; the security analysis bounds untagged bits and phase errors.