Researchers achieved the first real-world demonstration of quantum entanglement over a busy telecommunications fiber network.
A single photon carrying quantum information had to travel more than 24 kilometers through an optical fiber already crowded with powerful internet signals. Despite the noise, it reached downtown Chicago still entangled with its partner at Northwestern University’s Evanston campus.
Researchers successfully distributed entangled photons through 24.4 kilometers of installed fiber while the same cable carried high-capacity telecommunications traffic. The quantum connection retained more than 94% fidelity, showing that delicate quantum signals can survive alongside conventional data streams in existing infrastructure.
The result points toward a practical way to develop quantum networks without laying entirely separate fiber systems.
Published in Optica Quantum, the study is the first to demonstrate entanglement distribution between distant nodes through fiber simultaneously carrying modern commercial telecommunications traffic.
“Quantum signals are very, very tiny compared to classical signals,” said Northwestern’s Prem Kumar, the study’s senior author. “It’s like an ant traveling through a path filled with elephants. Our results show that photons can survive the journey and remain entangled.”
Kumar is a professor of electrical and computer engineering at Northwestern’s McCormick School of Engineering and director of the Center for Photonic Communication and Computing. Gina Talcott, a graduate student in Kumar’s research group, is the study’s first author.
Quantum signals face overwhelming noise
Today’s internet represents information using bits with values of 0 or 1. Quantum networks instead use quantum states, including qubits in quantum computing, to process and exchange information.
One important resource is quantum entanglement, in which the states of two particles remain inseparably connected even when the particles are far apart. Entanglement can support quantum teleportation, a process that transfers quantum information over long distances without the original particle physically carrying that information to its destination.
Although quantum communication can offer exceptional speed and security, it is highly vulnerable to interference. Even faint optical noise can overwhelm the individual photons used to encode quantum information.
“In optical communications, all signals are converted to light,” Kumar said. “While conventional signals for communications typically comprise millions of particles of light, quantum information uses single photons.”
A quieter wavelength protects photons
Kumar and his collaborators began addressing that imbalance in a 2024 study. By closely examining how light scatters inside fiber-optic cables, they identified a less congested wavelength where quantum photons could avoid much of the noise generated by ordinary internet traffic.
The researchers also introduced specialized filters to suppress additional interference. In laboratory tests, they used the method to perform quantum teleportation across a 30-kilometer cable carrying high-speed internet data.
“In that experiment, we showed the art of the possible,” Kumar said. “We achieved actual quantum teleportation over 30 kilometers, but it was in the lab. We wanted to add more realism and bring the experiment into the real world.”
Chicago fiber carries both networks
For the latest test, Kumar produced entangled photon pairs in his laboratory on Northwestern’s Evanston campus. One photon from each pair stayed in Evanston, while its partner entered installed fiber and traveled 24.4 kilometers to the StarLight International/National Communications Exchange Facility on Northwestern’s downtown Chicago campus.
The photons shared that fiber with two data channels, each operating at 800 gigabits per second. The researchers also added optical power representative of a commercial fiber link operating at full capacity.
“The fiber carried enough power to potentially transmit 36 terabits-per-second of classical data,” Kumar said. “That’s roughly equivalent to 20 million YouTube videos streaming simultaneously through a single fiber.”
To shield the quantum signals from the far stronger conventional traffic, the researchers assigned them separate regions of the optical spectrum. The entangled photons traveled through the relatively quiet O-band, while the commercial communications remained in the C-band normally used by telecommunications systems.
Distance created another challenge. The equipment in Evanston and downtown Chicago had to remain synchronized closely enough for researchers to determine which arriving photons belonged to the same entangled pair.
Using an optical timing system called White Rabbit, they synchronized the two sites with picosecond precision, equivalent to trillionths of a second. This timing accuracy allowed matching photon pairs to be identified in real time despite the intense surrounding traffic.
Measurements showed entanglement fidelity above 94%. That level confirmed that the quantum connection survived the trip and exceeded what a classical communications system could reproduce.
Teleportation remains the next test
The experiment reflects a broader effort to integrate quantum technology with telecommunications equipment already in use.
The researchers now plan to attempt quantum teleportation between remote nodes through a real telecommunications network. Kumar has previously demonstrated teleportation in the laboratory, but the next objective is to repeat it across metropolitan fiber carrying commercial traffic.
“We crossed a major bridge, showing we can do entanglement distribution,” Kumar said. “But there are two steps in teleportation. First, you need to distribute entanglement. Then, you need to transfer information. Each step is progressively more complicated and difficult, but we are showing that it’s possible.”
Reference: “Quantum entanglement distribution coexisting with high-rate, broadband classical optical communications over a real-world fiber connecting remote, synchronized nodes” by 24 August 2026, Optica Quantum.
DOI: 10.1364/OPTICAQ.592786
Supported by the United States Department of Energy (DE-AC02-07CH11359) via Fermilab.
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