A quantum internet signal is extraordinarily fragile. A single photon carries the information, and ordinary internet traffic runs millions of particles of light through the same cables at once. Scientists at Northwestern University have now shown those two things can share the same wire without destroying each other.
In a study published July 20 in the journal Optica Quantum, researchers sent entangled photons through a 24.4-kilometer fiber-optic cable connecting Evanston and downtown Chicago. The same cable was simultaneously carrying high-capacity commercial internet traffic. The quantum signals survived with more than 94% fidelity, according to Phys.org.
The experiment marks the first demonstration of entanglement distribution between remote nodes over a fiber simultaneously carrying modern commercial telecommunications traffic. That distinction matters because it means future quantum networks may not require entirely new infrastructure built from scratch.
Senior author Prem Kumar, a professor of electrical and computer engineering at Northwestern's McCormick School of Engineering, described the challenge in stark terms. "Quantum signals are very, very tiny compared to classical signals," Kumar said. "It's like an ant traveling through a path filled with elephants. Our results show that photons can survive the journey and remain entangled."
The reason quantum signals are so vulnerable comes down to how information travels. Today's internet encodes data as bits, either 0 or 1, and those signals are robust enough to handle noise. Quantum networks instead rely on quantum states, including entanglement, a property in which two particles remain linked regardless of the distance between them. Even a tiny amount of optical noise can overwhelm a single photon.
Kumar explained the scale of the difference. "In optical communications, all signals are converted to light," he said. "While conventional signals for communications typically comprise millions of particles of light, quantum information uses single photons."
The team's solution came from earlier research. In a 2024 study, Kumar and collaborators discovered a less crowded wavelength of light inside fiber-optic cables by studying how light scatters within them. Placing the quantum photons in that quieter wavelength gave them a kind of protected lane inside the cable while conventional data traffic moved through the rest of the fiber.
Graduate student Gina Talcott, a member of Kumar's research group, is the study's first author. Kumar directs Northwestern's Center for Photonic Communication and Computing.
The practical implication is significant. One of the major obstacles to building a quantum internet has been the assumption that quantum signals would need their own dedicated cables, separate from the existing global network. This experiment suggests that may not be necessary, at least over distances of roughly 15 miles. Whether the approach scales to longer distances or denser traffic conditions remains to be tested.
