For the first time outside a laboratory, researchers have successfully transmitted entangled quantum photons through a busy, commercial telecommunications fiber network. The breakthrough, achieved by a team at Northwestern University, proves that delicate quantum signals can survive alongside massive streams of conventional internet traffic. This development is critical for telecommunications engineers and cybersecurity professionals, as it demonstrates that the foundation for an unhackable quantum internet can be built on existing city infrastructure, eliminating the need to lay billions of dollars worth of dedicated cables.
The experiment sent a single photon carrying quantum information across 24.4 kilometers of installed optical fiber, traveling from Northwestern’s Evanston campus to the StarLight facility in downtown Chicago. Despite sharing the cable with powerful commercial data streams, the photon reached its destination still entangled with its partner, retaining a remarkable fidelity of over 94 percent.
Quantum signals are very, very tiny compared to classical signals. It’s like an ant traveling through a path filled with elephants. Our results show that photons can survive the journey and remain entangled.
- Prem Kumar, Northwestern University
To protect the fragile quantum states from overwhelming optical noise, the team utilized a strategic wavelength separation. The entangled photons were routed through the relatively quiet O-band of the optical spectrum, while the heavy commercial communications remained in the standard C-band. The surrounding traffic was immense; the fiber carried optical power representative of a commercial link operating at full capacity, equivalent to 36 terabits-per-second, or roughly 20 million YouTube videos streaming simultaneously.
Distance introduced severe timing challenges. To determine which arriving photons belonged to the original entangled pair, the equipment at both locations had to be perfectly aligned. The researchers deployed an optical timing system known as White Rabbit, which synchronized the two sites with picosecond precision - equivalent to trillionths of a second. This allowed the team to identify matching photon pairs in real time despite the intense surrounding noise.
Published in the journal Optica Quantum, the study marks a crucial stepping stone toward real-world quantum teleportation. While senior author Prem Kumar previously demonstrated teleportation in a controlled lab setting, the team’s next objective is to execute the transfer of quantum information across this same metropolitan fiber network.
The End of the Dark Fiber Prerequisite
The most significant implication of this Northwestern study is not just the physics - it is the economics of future internet infrastructure. Until now, a major bottleneck for the quantum internet was the assumption that it would require "dark fiber," entirely new, unused cables dedicated solely to quantum states to prevent interference. By proving that quantum entanglement can coexist with 800-gigabit-per-second commercial data channels, this research effectively accelerates the timeline for quantum network deployment by a decade.
Telecommunications giants can now look toward upgrading endpoint hardware rather than digging up city streets to lay new fiber. This drastically lowers the barrier to entry for ultra-secure quantum communications, shifting the challenge from massive civil engineering projects to advanced photonic filtering and synchronization.
