Entanglement swapping across a five-node relay in a multiplexed quantum-classical network

As quantum networks scale, they must eventually operate over existing fiber-optic infrastructure that is already saturated with high-power classical internet traffic. A major technical hurdle in this integration is spontaneous Raman scattering (SpRS), where classical light generates noise photons that degrade quantum signals. This study reports the first experimental demonstration of entanglement swapping—a fundamental process for quantum repeaters and distributed quantum computing—coexisting with 10-Gbps classical data transmission across a five-node relay topology.

The researchers utilized a five-node quantum relay architecture connected by four 10-km fiber segments (40 km total). They employed time-bin entangled photon pairs generated via spontaneous parametric down-conversion (SPDC) at 1536.6 nm. To enable coexistence, they used dense-wavelength division multiplexing (DWDM) to combine these quantum signals with 10-Gbps classical data channels in the C-band. The team implemented narrow spectral filtering to suppress Raman noise and used automated polarization controllers and variable optical delays to maintain high-fidelity interference at the central Bell state measurement (BSM) node.

The experiment successfully demonstrated entanglement swapping over 40 km of fiber while classical data was simultaneously transmitted. The researchers characterized the trade-off between quantum fidelity and classical signal power, showing that the system maintains a visibility of approximately 75% in the X-basis, which exceeds the threshold for a Bell test. Their theoretical model confirms that while C-band/C-band multiplexing is viable for distances up to roughly 100 km, longer-distance networks will likely require shifting quantum signals to the O-band to further mitigate Raman noise.

This work provides a practical roadmap for deploying quantum technologies on real-world telecommunications infrastructure. By proving that complex quantum protocols like entanglement swapping can function in noisy, high-power classical environments, the study demonstrates that a scalable quantum internet does not necessarily require dedicated, dark fiber. These findings inform the design of future quantum repeaters and hybrid quantum-classical networks.