Entanglement Swapping Coexisting with Classical Communication in a Quantum Relay Configuration

To enable quantum networking over telecom fibers beyond metropolitan-scale distances, entanglement swapping across multiple shorter segments of longer-distance fiber will play a crucial role in overcoming the exponential loss of signal in direct photon transmission. Real-world conditions in existing deployed fiber may necessitate that multiple photons used for entanglement distribution and Bell state measurements are each transmitted alongside much higher power conventional Internet traffic or synchronization signals, which can scatter noise photons with the same frequency as the entangled photons. However, the transmission of more than two photons has not been experimentally demonstrated to date. Here, we conduct a quantum relay time-bin entanglement swapping experiment between two C-band (1536nm) degenerate entangled-pair sources in which all four photons traverse 5-km fibers (20-km total), with each simultaneously carrying 10-Gbps C-band classical data signals (1547nm). Leveraging the high-noise regime brought by both signals operating in the C-band, we study the impact of Raman scattering noise in all four detectors on midpoint Bell state measurement and entanglement swapping fidelity. We further discuss methods to mitigate noise, including filters and quantum-classical wavelength engineering. This configuration lays out the foundations for designing scalable next generation quantum networks capable of extending the reach of both distance and accessible infrastructure.