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arXiv:2511.00466

Free-space multi-user quantum network with high key rate

Ayan Kumar Nai, G. K. Samanta

Published Nov 1, 2025• 0 citations
arXivHTMLPDF

Abstract

Emergent quantum networks are the essential ingredient for securely connecting multiple users worldwide, extensively deployed in both fibre and free-space. An essential element is the multiplexing of entanglement to multiple users, overcoming the peer-to-peer restriction of quantum key distribution (QKD), so far successfully shown in fibre-based architectures. Here, we demonstrate a free-space quantum space division multiplexing architecture using just one entanglement source to realise a fully connected twelve-channel quantum network for seamless QKD connections between six users. The network achieves record coincidence rates exceeding $3 \times 10^{4}$ s$^{-1}$ between any pair of nodes on the network, for sifted key rate of over 400 kbps. Our approach overcomes the active switching hurdle that has hindered the free-space deployment of quantum multiplexing, is fully passive, easily scalable to more nodes and compatible with fibre-based integration, thus opening a new path to scalable and resource-efficient quantum networks that utilise free-space links.

DOI
10.48550/arxiv.2511.00466

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Quantum networks scale the advantages of quantum communication protocols to more than just two distant users. Here we present a fully connected quantum network architecture in which a single entangled photon source distributes quantum states to a multitude of users [1] and our work scaling the network to a complete end to end metropolitan quantum network with 8 users [2]. Our network architecture optimises the resources required by each user without sacrificing security or functionality. We established several long-distance loopback connections and demonstrated extended stable operation with high secure key rates. Unlike previous attempts at multi-user networks, which have been based on active components, and thus limited to some duty cycle, our implementation is fully passive. Further, we present our efforts towards coexistence of 100 Gbps classical communication and all quantum channels used in the network over the same deployed optical fibres. Most importantly, we have used our network to implement a wide variety of protocols. These protocols overcome major limitations of quantum networks and each of them represent the first implementation of that class of protocols on a large scale network.
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Quantum networks have been shown to connect users with full-mesh topologies without trusted nodes. We present advancements on our scalable polarisation entanglement-based quantum network testbed, which has the ability to perform protocols beyond simple quantum key distribution. Our approach utilises wavelength multiplexing, which is ideal for quantum networks across local metropolitan areas due to the ease of connecting additional users to the network without increasing the resource requirements per user. We show a 10 user fully connected quantum network with metropolitan scale deployed fibre links, demonstrating polarisation stability and the ability to generate secret keys over a period of 10.8 days with a network wide average-effective secret key rate of 3.38 bps.
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Abstract Quantum communication has seen rapid progress towards practical large-scale networks, with quantum key distribution (QKD) spearheading this development. While fibre-based systems have been shown to be well suited for metropolitan scales, suitable fibre infrastructure may not always be in place. Here, we make the case for an entanglement-based free-space quantum network as a practical and efficient alternative for metropolitan applications. We developed a deployable free-space QKD system and demonstrated its use in realistic scenarios. For a representative 1.7-km free-space link, we showcase its ad hoc deployability and achieve secure key rates of up to 5.7 kbps, with 2.5 kbps in direct noon sunlight. By extrapolating experimental data, we show that kbps key rates are achievable even for 10-km distances and multi-user scenarios. We anticipate that our work will establish free-space networks as a viable solution for metropolitan applications and an indispensable complementary building block in the future global quantum internet.