This paper reports the first demonstration of entanglement swapping happening at the same time as normal internet‑style light runs through the same optical fibers. Entanglement swapping is a teleportation‑like trick that makes two photons become entangled even though they never met. The team showed this across a five‑node relay made of four long fiber links while each link also carried 10‑gigabit‑per‑second classical data signals.

The experiment used two independent sources of time‑bin entangled photon pairs. All four quantum photons sat near 1536.6 nm in the C‑band. The researchers combined these quantum signals with classical traffic at 1547.7 nm using dense‑wavelength division multiplexing (DWDM). The total fiber distance they tested included both a 20 km and a 40 km relay, implemented as four equal fiber segments. They then performed a central Bell‑state measurement at the midpoint node to ‘‘swap’’ entanglement so the two outer nodes became entangled.

To make the measurement work in lit fiber, the group used several noise‑mitigation tools. The photons were filtered with a narrow 50‑GHz DWDM passband to reduce stray light. They used superconducting nanowire single‑photon detectors (SNSPDs) with about 90% detection efficiency and fast timing to apply a short 300‑ps coincidence window. Automated polarization controllers and a variable optical delay compensated for slow drifts in polarization and arrival time so the two photons could interfere cleanly at the midpoint.

A key technical challenge is spontaneous Raman scattering (SpRS). Strong classical light in the fiber scatters and creates noise photons inside the quantum wavelengths. That noise scales with classical power, the wavelength layout, and fiber length. The authors measured and modeled this trade‑off between quantum fidelity and Raman noise. Despite the added noise of working in the C‑band, they succeeded in demonstrating entanglement swapping over a maximum of 40 km while all fibers carried 10‑Gbps classical data.