Quantum internet leaves the lab

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Quantum internet leaves the lab

First real-world demonstration of quantum entanglement over busy telecom fiber

• In 2024, scientists demonstrated quantum teleportation in a lab-based cable filled with high-speed internet traffic
• New study sent entangled photons through a busy cable connecting Evanston to downtown Chicago
• Quantum signals remained remarkably intact, preserving entanglement with more than 94% fidelity
• Demonstrates practical path to building quantum networks without needing new communications infrastructure

EVANSTON, Ill. - Quantum information is notoriously fragile. Internet traffic is anything but.

Yet Northwestern University scientists have demonstrated they can peacefully coexist inside the same fiber-optic cable.

In a new study, researchers successfully sent entangled photons through a 24.4-kilometer fiber-optic cable connecting Evanston and downtown Chicago while the same cable simultaneously carried high-capacity internet traffic. Even amid the torrent of conventional data, the quantum signals remained remarkably intact - preserving entanglement with more than 94% fidelity.

By allowing fragile quantum signals and powerful classical data streams to share the same optical fiber, the work demonstrates a practical path toward building future quantum networks without requiring entirely new communications infrastructure.

The study was published on Monday (July 20) in the journal Optica Quantum. It marks the first demonstration of entanglement distribution between remote nodes over a 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."

An expert in quantum communication, Kumar is a professor of electrical and computer engineering at Northwestern's McCormick School of Engineering, where he directs the Center for Photonic Communication and Computing. Gina Talcott, a graduate student in Kumar's research group, is the study's first author.

Unlike today's internet, which stores information as bits (0s and 1s), quantum networks rely on quantum states, such as qubits, for example, in the context of quantum computing. This includes quantum entanglement, a property in which two particles' quantum states are inseparably linked, regardless of the distance between them. Instead of traveling to deliver information, entangled particles can help exchange quantum information over great distances - without physically carrying it - through a process called teleportation. While this approach is ultrafast and secure, it's also incredibly sensitive to disturbances. Even the tiniest amount of optical noise can overwhelm the photons carrying 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."

In a 2024 study, Kumar and his collaborators discovered a new way to help delicate photons steer clear of busy traffic. After conducting in-depth studies of how light scatters within fiber-optic cables, the team found a less crowded wavelength of light to place their photons. Then, they added special filters to reduce noise from regular internet traffic. In laboratory experiments using a 30-kilometer-long cable filled with high-speed internet traffic, the approach worked.

"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."

For the new experiment, Kumar generated pairs of entangled photons in his laboratory, located on Northwestern's Evanston campus. One member of each entangled photon pair remained in Evanston, while their partners traveled 24.4 kilometers through installed fibers to the StarLight International/National Communications Exchange Facility on Northwestern's campus in downtown Chicago. The photons shared the same fiber with two 800-gigabit-per-second data channels as well as additional optical power representative of a fully loaded commercial link.

"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 prevent the intense traffic from overwhelming the quantum photons, Kumar and his team shifted the photons into a quieter portion of the optical spectrum, known as the O-band. The conventional communications remained confined to the C-band, where commercial systems typically operate.

Because the experiment took place between two physically separate locations, the researchers also needed to keep both ends of the network synchronized to within trillionths of a second. Using an optical timing system called White Rabbit, they accomplished picosecond-level synchronization. That precision allowed the team to identify matching pairs of entangled photons in real time despite heavy background traffic.

Kumar and his team measured entanglement fidelity above 94%, confirming it survived the journey at a level impossible for a classical communications system to reproduce.

The study is a part of a broader shift toward integrating quantum technologies with existing telecommunications systems. Kumar and coauthor Jordan Thomas recently explored that evolution in a feature article for Optics & Photonics News.

Next, the team plans to perform quantum teleportation between remote nodes across a real-world telecommunications network. While Kumar has already performed teleportation in his lab, he wants to demonstrate it over a 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."

The study, "Quantum entanglement distribution coexisting with high-rate, broadband classical optical communications over a real-world fiber connecting remote, synchronized nodes," was mainly supported by the United States Department of Energy (DE-AC02-07CH11359) via Fermilab.

Prem Kumar

Professor of Electrical and Computer Engineering and of Physics and Astronomy
Director of Center for Photonic Communication and Computing

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