Northwestern Proves Quantum Internet Can Share Existing Telecom Fiber Over 15 Miles at 94 % Fidelity

Quantum networking has long faced a deployment problem that had nothing to do with physics: the fiber it needs already belongs to someone else. Commercial fiber-optic cables carry millions of internet users' traffic, and the conventional assumption was that a quantum signal — fragile, individual-photon-scale, instantly destroyed by a single stray photon of noise — could never share that environment. Building a quantum internet, therefore, meant leasing or laying dedicated "dark fiber" that carries nothing else, a requirement that has proven cost-prohibitive and impractical and has stalled real-world deployment for years. A team at Northwestern University has just erased that assumption — in the real world, on a live city network. Researchers at Northwestern's McCormick School of Engineering transmitted entangled photons 24.4 kilometers (15.2 miles) through a deployed commercial fiber-optic link between the university's Evanston campus and downtown Chicago, while that same cable simultaneously carried 1.6 terabits per second of live commercial internet traffic. The entangled signals survived the journey with a Bell state fidelity of 94.2% — a level that is physically impossible for any classical communications system to replicate, and statistically identical to what the team measured on the same link with no commercial traffic at all. That…