Northwestern University’s Kumar Lab Sends Entangled Photons Through Chicago

Northwestern University researchers have successfully transmitted entangled photons through a 24.4-kilometer fiber-optic cable connecting Evanston and downtown Chicago while simultaneously carrying high-capacity internet traffic; this marks the first demonstration of its kind over an extended, real-world distance. Preserving entanglement, a crucial quantum phenomenon, with over 94 percent fidelity despite the presence of conventional data represents a significant step toward practical quantum networks. “Quantum signals are very, very tiny compared to classical signals,” explains Northwestern Engineering’s Prem Kumar, the study’s senior author. “It’s like an ant traveling through a path filled with elephants.” By demonstrating compatibility with existing fiber infrastructure, Professor Kumar and graduate student Gina Talcott’s work suggests a future where quantum communication doesn’t require entirely new cabling.

Entanglement Distribution via Fiber with Existing Telecom Traffic

The ability to transmit quantum entanglement alongside conventional internet traffic represents a significant leap toward practical quantum networks, circumventing the need for entirely new infrastructure. Researchers recently demonstrated successful entanglement distribution through a 24.4-kilometer fiber-optic cable. Preserving entanglement, a fragile quantum phenomenon, amidst the noise of commercial data streams required careful engineering. The team, led by Professor Prem Kumar of Northwestern University and graduate student Gina Talcott, utilized a quieter wavelength of the optical spectrum for the entangled photons. They also implemented advanced filtering and precise synchronization to shield the quantum signals from the intense optical noise generated by conventional data. The fiber itself carried enough power to potentially transmit 36 terabits per second of classical data. To further isolate the quantum signals, the researchers confined conventional communications to the C-band while shifting the photons to the O-band.

Maintaining synchronization between the Evanston and Chicago locations, to within trillionths of a second using an optical timing system called White Rabbit, was also critical for identifying entangled photon pairs in real time. Kumar emphasizes the significance of this step, stating that accurate timing is essential for successful entanglement distribution. The team now intends to demonstrate quantum teleportation across a metropolitan network, building upon this successful entanglement distribution.

The pursuit of a quantum internet faces a significant hurdle: the extreme fragility of quantum signals when sharing infrastructure with conventional data transmission. To shield the delicate quantum information, the team strategically positioned the photons within the O-band of the optical spectrum. Conventional communications traffic remained confined to the more congested C-band, minimizing interference. However, simply isolating the wavelengths wasn’t enough. Maintaining synchronization between the entangled photon source in Evanston and the receiver in downtown Chicago required picosecond-level precision. The researchers employed an optical timing system known as White Rabbit to achieve this, ensuring accurate identification of entangled photon pairs amidst the substantial background noise. “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.” This meticulous synchronization, combined with the O-band isolation, resulted in an impressive entanglement fidelity exceeding 94 percent, a level previously unattainable in a real-world, shared-fiber environment. The team confirmed this fidelity level would be impossible for a classical communications system to reproduce.

Professor Prem Kumar and graduate student Gina Talcott at Northwestern University have demonstrated a significant advancement in quantum networking by achieving 94 percent fidelity in preserving entanglement over a 24.4-kilometer fiber-optic cable. The next step for Kumar’s team is to demonstrate full quantum teleportation, the transfer of information itself, across a metropolitan fiber carrying commercial traffic building on this foundation of successful entanglement distribution.