Quantum networks could share fiber with conventional telecom traffic instead of requiring operators to provide separate dark fiber, according to a new experiment by researchers at Duke University and the University of Maryland.

The team distributed ion-photon entanglement over a 1.7 mile (2.8 kilometer) deployed fiber loop carrying Ethernet traffic at 400 gigabits per second (Gb/s) and an analog 5G radio-over-fiber signal. The classical control signals needed to coordinate and stabilize the quantum link also traveled over the same fiber.

The researchers described the experiment as the first demonstration of memory-photon entanglement coexisting with conventional telecom traffic on a single fiber.

That distinction matters because memory-based quantum networks could connect quantum processors, memories, and sensors using entanglement between stationary qubits and photons. Previous long-distance demonstrations have used dark fiber to prevent conventional optical signals from overwhelming the much weaker quantum signals.

Requiring dedicated fiber would restrict where these links could be deployed, particularly in cities where much of the available infrastructure already carries traffic.

The experiment instead used wavelength-division multiplexing to combine quantum and classical signals on the same underground fiber loop at Duke’s campus.

The researchers entangled a trapped strontium ion with a photon transmitted at a wavelength of 1,092 nanometers. This sits outside the telecom bands used by the Ethernet and 5G signals, helping the researchers filter unwanted light before it reached the single-photon detectors.

The Ethernet equipment transmitted data at 400 Gb/s using coherent optics representative of a data center interconnect. The second classical channel carried a 5G New Radio waveform over fiber in a setup designed to emulate the connection between a central office and a base station.

Over almost 14 hours, the system recorded 32,500 photon detection events, equivalent to an entanglement generation rate of 39.3 per minute. The researchers reported average entanglement fidelity bounds of 92.4% to 96.1%.

They found no measurable reduction in fidelity when the Ethernet and 5G transmitters operated at maximum power. The Ethernet link remained error-free after forward error correction, while the 5G signal stayed within the error threshold specified by 3GPP.

The system also monitored changes in the fiber’s polarization and automatically paused quantum transmission to correct drift. It spent 99.3% of the experiment distributing entanglement.

However, the test did not use a live carrier network. Both quantum endpoints were housed in the Duke Quantum Center, with the deployed fiber forming an underground campus loop. The researchers said aerial fiber could experience faster and more severe polarization changes.

The photons’ 1,092-nanometer wavelength also experiences greater fiber loss than wavelengths in the conventional telecom band, limiting the current approach to several kilometers.

The results were published as an arXiv preprint and have not yet been peer-reviewed. Even so, they indicate that memory-based quantum links may be able to reuse active telecom fiber, potentially reducing one of the infrastructure barriers to city-scale quantum networks.