In a significant advancement for quantum technology, researchers have successfully demonstrated long-distance quantum communications over existing commercial telecommunications infrastructure, potentially bringing the quantum internet one step closer to reality.
A team led by scientists from Toshiba Europe Limited has implemented a coherent quantum communication system spanning 254 kilometers between Frankfurt and Kehl in Germany, achieving encryption key distribution at a rate of 110 bits per second. Their findings, published in the April 24, 2025 issue of Nature, represent one of the longest distances ever achieved for practical quantum key distribution (QKD) in a real-world setting without requiring specialized cryogenic cooling equipment.
Breaking Distance Barriers with Twin-Field QKD
The breakthrough hinges on a technique called twin-field quantum key distribution (TF-QKD), which effectively doubles the maximum communication distance compared to standard quantum protocols.
“Previous implementations of coherence-based quantum protocols required highly specialized equipment like ultra-stable optical cavities and cryogenic photon detectors, making them impractical for deployment in standard telecommunications environments,” explained lead researcher Dr. Mirko Pittaluga of Toshiba Europe.
What makes this implementation remarkable is that it operates on existing commercial fiber optic networks rather than in a controlled laboratory setting. The system connected data centers in Germany via standard optical fibers with high losses and asymmetric link lengths—conditions typical of real-world telecommunications infrastructure.
Practical Technology for Real-World Applications
A key innovation enabling this achievement is a more practical and cost-effective approach to maintaining optical coherence—the crucial property that allows quantum information to be preserved across long distances.
The research team developed a system where the central node distributes optical frequency references to transmitting nodes through service fibers. This allows the transmitters to lock their lasers to a common frequency reference, eliminating laser phase noise without requiring the ultra-stable lasers and external cavities used in previous laboratory demonstrations.
Additionally, the system uses non-cryogenically cooled detectors (avalanche photodiodes or APDs) instead of the superconductive nanowire single-photon detectors (SNSPDs) typically employed in laboratory settings. While APDs have some technical disadvantages, they are approximately 10-100 times less expensive than SNSPDs and can operate at temperatures compatible with standard telecommunications equipment.
Implications for Quantum Networks
This demonstration represents a significant step toward practical quantum networks that could one day form the backbone of a quantum internet. Such networks would enable applications that are impossible with classical communications, including ultra-secure communications, distributed quantum computing, and quantum sensing networks.
“Our research aligns the requirements of coherence-based quantum communication with the capabilities of existing telecommunication infrastructure,” Pittaluga noted. “This compatibility is likely to be crucial for the future development of high-performance quantum networks.”
The experiment also resulted in what researchers believe is one of the largest QKD networks featuring measurement-device-independent properties, adding an important security dimension to the implementation.
Overcoming the Repeaterless Bound
A particularly notable aspect of this achievement is that it overcomes what scientists call the “repeaterless secret key capacity bound” (SKC0)—the fundamental distance limitation of quantum channels without quantum repeaters.
By successfully implementing the twin-field QKD protocol over such long distances on commercial infrastructure, the research demonstrates a practical path toward quantum communications that could eventually span national and international distances.
Industry Implications
The telecommunications industry is closely watching these developments, as quantum-secure communications become increasingly important in an era of advancing computational power and security threats.
The experiment was conducted on network infrastructure provided by GÉANT, one of Europe’s leading research and education networks. The system was deployed in standard telecom racks within colocation data centers, demonstrating that quantum technology can operate alongside existing telecommunications equipment.
This practical implementation approach, using modular components compatible with telecom infrastructure, suggests that quantum communication technology could be integrated into existing networks more quickly than previously thought.
The Road Ahead
While this breakthrough represents significant progress, researchers acknowledge that challenges remain before widespread deployment of quantum networks becomes feasible. Future work will focus on improving key generation rates, extending distances further, and developing more robust systems capable of operating in diverse network environments.
Nevertheless, this demonstration proves that coherent quantum communications can function within the constraints of real-world telecommunications infrastructure—an essential milestone on the path to a global quantum internet.
As quantum technologies continue to mature, this research provides a roadmap for how quantum and classical communications might coexist and complement each other in the networks of tomorrow.
