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

TL;DR

This work demonstrates a scalable free-space multi-user quantum network by spatially dividing a single SPDC ring into three sources to support twelve links among six users. The passive beam-splitter multiplexing enables a fully connected 12-channel topology with high per-link coincidence rates and entanglement fidelity, achieving a total sifted key rate of 407 kbps and a secure key rate of 76 kbps across all user pairs. The approach maintains high entanglement quality (visibilities 80–88%, fidelities 91–95%, and Bell parameters exceeding 2) while avoiding active switching, making it compatible with fibre integration and potentially extendable via WDM or one-to-many splitting for larger networks. This architecture offers a practical, high-throughput pathway toward global quantum networks suitable for satellite-based free-space links and scalable multi-user QKD without centralized trusted nodes.

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.

Figures (4)

• Figure 1: Conceptual scheme and experimental setup of the multi-user quantum network architecture. On the left, we have illustrated the concept of using two layers of abstraction. The top layer shows the fully connected mesh of 12 channels connecting six users through quantum correlation. The bottom layer shows the physical representation of the network topology, with the implementation shown on the right. The topology of the quantum network is established through the central unit, which derives three entangled photon sources from a single source and distributes them between six users through multiplexing. The complete end-to-end implementation has three sections: central unit, free space channel, and users. Again, the central unit has two subsections: SPDC source and spatial division and multiplexing. The SPDC source is realized using a single-frequency continuous-wave diode laser at 405 nm, with output power controlled via a half-wave plate (HWP) and a polarization beam splitter (PBS), which pumps a temperature-stabilized periodically poled KTP (PPKTP) crystal placed in a polarization Sagnac interferometer. The interferometer consists of a dual-wavelength PBS and HWP (D-HWP) for 810 nm and 405 nm wavelengths. A plano-convex lens (focal length 150 mm) focuses the pump at the crystal center and collimates the photon pairs generated via spontaneous parametric down-conversion (SPDC). In the spatial division multiplexing section, the collimated SPDC emission ring, separated from the pump by a dichroic mirror (DCM), is divided into two halves using a prism mirror (PM), with each half further segmented into three subsections by D-shaped mirrors (DMs). Six diametrically opposite sections form three entangled photon sources, I, II, and III, with section pairs (1, 1'), (2, 2'), and (3, 3') as shown in the source division table. These subsections are multiplexed using three 50:50 beam splitters (BS), while mirrors (M) in the experiment direct the beams through the optical paths. The multiplexed outputs are transmitted via the communication channel to six users. Each user’s receiver consists of a projection and detection module comprising an HWP, quarter-wave plate (QWP), PBS, an interference filter (IF; bandwidth of 3 nm, centered at 810 nm), a fibre coupler, and a single-mode fibre (SMF) connected to a single-photon counting module (SPCM). The SPCM outputs are processed by a time-to-digital converter (TDC) and a computer. The user allocation table lists the allocated subsections for each user. The quantum correlation graph illustrates the realized quantum links within the network, which can be extrapolated to form a potential global quantum network based on satellite technology.
• Figure 2: Characterization of spatially separated entangled photon sources derived from a single SPDC source. (a) Coincidence counts between different subsections, showing strong correlations between diametrically opposite sections carrying pair photons generated via the SPDC process. (b) Singles counts of different subsections, along with the coincidence visibility between diametrically opposite sections, 1 & 1', 2 & 2', and 3 & 3'. (c) Quantum interference of the spatially separated entangled photon sources (here, we used source II comprises of subsections 2 & 2') measured in the horizontal (H, black dots), vertical (V, red dots), diagonal (D, blue dots), and anti-diagonal (A, green dots) polarization bases. (d,e) Graphical representation of the absolute values of the real (d) and imaginary (e) parts of the reconstructed density matrix of the polarization-entangled Bell state $\ket{\phi^-}$.
• Figure 3: Performance characteristics of quantum links within the network. (a) Coincidence counts for each link connecting two users in the 12-link quantum network. (b) Measured quantum parameters for each link, including entanglement visibility, Bell parameter ($S$), and quantum state fidelity. The network supports both $\ket{\phi^+}$ and $\ket{\phi^-}$ Bell states, with each state distributed across six of the twelve links owing to the network's topology.
• Figure 4: Performance parameters of the entangled-photon source for scalable quantum networking. (a) Variation of the Bell parameter $S$ and coincidence counts as a function of pump power for two spontaneous parametric down-conversion (SPDC) emission-ring diameters, corresponding to crystal temperatures of $T = 37.7^{\circ}$C and $T = 39.7^{\circ}$C. (b) Tabulated SPDC source characteristics at the two crystal temperatures, showing that an increased emission-ring diameter allows finer spatial sectioning. This capability supports network-topology scaling to accommodate more users while preserving the entanglement quality of the source.