Quantum Network Goes the Distance Using Existing Telecom Infrastructure

Insider Brief

• Researchers demonstrated secure quantum communication over 254 kilometers of existing telecom fiber using a coherence-based protocol, marking a milestone for practical quantum networks.
• The system maintained optical phase coherence without cryogenic cooling, using only standard semiconductor components in real-world data centers across Germany.
• The experiment validated twin-field quantum key distribution (TF-QKD) and opened pathways to future applications in quantum computing, sensor networks, and large-scale quantum internet infrastructure.
• Image: Nature

A team of scientists has successfully transmitted secure quantum information across 254 kilometers of commercial fiber-optic cable in Germany, marking a key milestone for practical quantum networks.

The study, published in Nature by researchers at Toshiba Europe, demonstrates that quantum communication based on optical coherence can be deployed over long distances using standard telecommunications equipment.

The team writes in a brief on the research that it used an approach called twin-field quantum key distribution (TF-QKD), which relies on the interference of dim light pulses to establish encryption keys between two parties. The result is a proof-of-concept for secure quantum communications that could one day operate alongside today’s internet infrastructure.

Avishek Nag is at University College Dublin, who did not take part in the research, said the work represents a significant technological leap.

“The manuscript presents a groundbreaking quantum key distribution (QKD) implementation over Germany’s 254-km commercial telecom network,” writes Nag in the research brief. “The work represents a substantial advance in QKD technology, doubling the practical communication distance compared with previous implementations. The integration of coherent quantum communications into existing telecommunications infrastructure is novel, and this research makes strides towards the real-world deployment of quantum networks, by aligning quantum-communication requirements with current telecom capabilities.”

Quantum communication promises unbreakable encryption, but implementing it at scale has long been hindered by the fragility of quantum signals over long distances. At the heart of this challenge is coherence — the ability of light waves to remain in phase as they travel, according to the researchers. Coherence is essential for techniques like quantum teleportation and entanglement swapping, which are central to building large-scale quantum networks. However, as light travels through fiber, its coherence typically degrades due to environmental disturbances like temperature shifts and mechanical vibrations.

Until now, preserving coherence over long distances has required complex setups confined to laboratory environments, often involving cryogenically cooled components. The new study overcomes these barriers by using only commercially available semiconductor components, enabling a portable and energy-efficient system that fits into standard data-center racks.

Using Telecom Fiber

The researchers deployed their quantum communication system across three data centers in Germany — located in Frankfurt, Kirchfeld, and Kehl — using standard telecom fiber. Their approach centered on maintaining the coherence of single-photon-level light pulses as they traveled from two endpoints to a central measuring station. By actively stabilizing the phase of the light during transmission, they overcame the disruptive effects of the long fiber links.

In the TF-QKD protocol, each sender—dubbed Alice and Bob in cryptographic literature—encodes information in the phase of dim light pulses and sends them to a third party, Charlie. Charlie performs interference measurements on the incoming pulses. The results, when combined with the private encoding choices of Alice and Bob, allow the two parties to independently generate identical secret keys without Charlie—or any potential eavesdropper—knowing them.

Secure and Reliable Operation

The study confirmed that the protocol could operate securely and reliably over the full 254-kilometer span. This sets a new benchmark for real-world quantum communication using coherence, a property that, until now, had only been demonstrated effectively in controlled lab conditions.

If refined further, this approach could also enable distributed quantum computing, in which different quantum processors exchange information over large distances, or quantum sensor networks that offer precision measurements at scales currently unattainable with classical systems.

Still, technical hurdles remain. Many quantum processors use atoms or ions that respond to light at near-visible wavelengths — wavelengths that are poorly suited to telecom fibers. Bridging that gap will require innovations in frequency conversion or new types of quantum memory that are compatible with telecom wavelengths.

Despite these challenges, the Toshiba team sees their work as a step toward integrating quantum communication into today’s global data infrastructure. Their system’s ability to operate in standard environments without exotic cooling or custom infrastructure could accelerate adoption across telecommunications networks.

The experiment was not without its share of drama, added Mirko Pittaluga, lead author of the study, of Toshiba Europe.

“After years of preparation and meticulous testing in our Cambridge laboratory, deploying the system in Germany was both thrilling and nerve-wracking,” writes Pittaluga. “No matter how much you prepare, the first real-world deployment of new technology always brings surprises. After installation, the equipment checks and preliminary tests went smoothly. But when we ran the protocol, one of the quantum links simply did not work. We spent hours troubleshooting, testing every possible failure mode, and confirmed that everything should have been working. We tested the last and, in our view, most unlikely hypothesis: that the problem was not with our system, but with the optical-fibre link. It had been checked just weeks before but, sure enough, since then, it had been cut. Relief set in, because it wasn’t a fundamental flaw in our approach. Once the fibre was repaired, everything clicked into place, and we were finally able to demonstrate the validity of our method.”