Quantum Fiber
Northwestern University researchers sent entangled photon pairs through a 24.4 km fiber-optic cable connecting Evanston and downtown Chicago — while the same cable simultaneously carried high-capacity internet traffic. Quantum signals rode the O-band (1310 nm) while classical data filled the C-band (1550 nm), preserving entanglement at >94% fidelity using picosecond-level synchronization. July 2026.
SPDC source — A nonlinear crystal in Evanston splits one 810 nm pump photon into two entangled "signal" photons. One photon is kept locally as a herald; the other travels into the shared fiber.
Band coexistence — A DWDM multiplexer routes quantum photons to the O-band (1270–1370 nm) and classical traffic to the C-band (1530–1565 nm), keeping them separated by >150 nm despite sharing the same glass.
SNSPD detector — At the Chicago end, a superconducting nanowire single-photon detector operates at 4 K, resolving photon arrivals to tens of picoseconds to match them with their Evanston herald via coincidence logic.
Why it matters — Previous experiments required a dedicated dark fiber. Running quantum signals over actively-used telecom infrastructure proves the quantum internet can piggyback on existing networks without new fiber deployments.