Coexistence of Entanglement-based Quantum Channels with DWDM Classical Channels over Hollow Core Fibre in a Four Node Quantum Communication Network

TL;DR

This work tackles integrating entanglement-based quantum channels with carrier-grade DWDM classical channels over hollow-core fibre to enable multi-user quantum networks. It employs a central entangled-photon source, q-ROADM, and 100 GHz DWDM on an $11.5$ km HC-NANF link to co-propagate four classical channels ($800$ Gbps) with three quantum channels, achieving Bell state fidelities up to $90.0 \pm 0.8$% and preserving SKR over $55$ hours. The hollow-core fibre’s ultra-low nonlinearity and reduced Raman scattering are key to enabling high-power classical transmission alongside quantum signals, outperforming conventional SMF in this coexistence scenario. The results validate scalable, entanglement-based quantum networks with practical coexistence and point toward longer-distance heterogeneous quantum-classical deployments.

Abstract

We experimentally demonstrate the coexistence of three entanglement-based quantum channels with carrier-grade classical optical channels over $11.5$km hollow core nested antiresonant nodeless fibre, in a four user quantum network. A transmission of $800$Gbps is achieved with four classical channels simultaneously with three quantum channels all operating in the C-band with a separation of $1.2$nm, with aggregated coexistence power of $-3$dBm. We established quantum key distribution in the four-node full-mesh quantum network with Bell state fidelity of up to $90.0\pm0.8$%. The secret key rate for all the links in the network are passively preserved over $55$hours of experimental time.

Figures (5)

• Figure 1: Experimental testbed for quantum-classical coexistence in hollow core fibre. a) shows the setup for the coexistence of four classical channels and three entanglement-based quantum channels over 11.5 km HCF in a four-node quantum network. Purple components contain only quantum light, blue components contain only classical light, and orange components contain both quantum and classical light. b) shows a scanning electron micrograph image of the Hollow Core fibre (HCF) cross-section nespola2021ultra. c) shows the classical communications system, consisting of a bandwidth-variable transponder (BVT) transmitter and receiver pair.
• Figure 2: The quantum-enabled reconfigurable optical add-drop multiplexer (q-ROADM) used in the testbed shown in \ref{['fig:testbed']}a). Here (De)Mux is a (de)multiplexer, OFS is an optical fibre switch, WSS is a wavelength selective switch, and FPC is a fibre polarisation controller. The FPCs are labelled $\lambda_{15}$ to $\lambda_{-15}$ which corresponds to the wavelength pairs, centred where $\lambda_0$ is at ITU channel 34, and $\pm i$ denotes the distance in $100$ GHz ITU channels from the central channel.
• Figure 3: The spectral distribution of channels in the coexisted HCF. Blue shows the distribution of Quantum Flux and Orange shows the distribution of Classical Communications channels. The black arrow shows the spectral gap between the 10dB bandwidth edge of the classical signal at $191.7$ THz and the quantum signal at $191.9$ THz, with a width of $142$ GHz. The y-axis gives a representative power, relative to the maximum of the given signal type. Classical light was measured using the given channel parameters Tab. \ref{['tab:parameters']}. Photon flux spectrum is measure using ASE noise through exemplar wavelength multiplexing technology.
• Figure 4: The 3-user quantum network with coexistence of classical light through the HCF. a) shows the physical network topology with fibre links in black lines, as shown in \ref{['fig:testbed']}a), and the logical entanglement connectivities between the users in coloured lines. Link losses include contribution from fibre, q-ROADM, and classical light filtering. b) shows the secret key rate (SKR) of each entanglement link over $2$ hours, where each point is a $10$ minute average. c) shows the correlation histograms of each link, where the lighter peak is the expected correlation results and the darker peak is the erroneous (noise) correlations.
• Figure 5: The logical topology of the entanglement links between users, a), and a sketch of the physical topology of the fibre network, b), as detailed in \ref{['fig:testbed']} where link losses include all optical components between the entanglement source and the quantum measurement module, including loss associated with the q-ROADM.