A team of researchers has successfully sent quantum-secured encryption keys over a record-breaking 120 kilometers of optical fiber that was simultaneously carrying conventional internet data traffic. The achievement is a critical milestone for integrating theoretically unbreakable quantum security into existing, real-world telecommunications networks without the need for costly, dedicated infrastructure.
The breakthrough addresses the primary obstacle to widespread adoption of quantum key distribution (QKD): the ability for fragile quantum signals to coexist with the powerful classical data streams that populate modern fiber-optic lines. By demonstrating a robust and high-performance system that operates on standard, “lit” fiber, the researchers have paved the way for a more practical and scalable path toward a quantum-secure internet. This moves the technology from controlled laboratory settings into an environment that mirrors the commercial networks used globally.
The Fundamental Challenge of Coexistence
Integrating QKD into classical networks is a formidable technical problem due to a massive disparity in signal strength. Classical data channels, which carry internet, voice, and video traffic, use signals with power measured in milliwatts. In contrast, quantum signals used for QKD rely on extremely faint pulses of light, sometimes as weak as a single photon, with power levels below a nanowatt. This makes the quantum information exceptionally vulnerable to being lost in the noise generated by its high-powered neighbors.
When these two types of signals share the same fiber, the powerful classical light can scatter and create background photons through physical processes like Raman scattering. This added noise effectively drowns out the delicate quantum states, corrupting the information being sent and severely limiting the distance over which a secret key can be generated. To overcome this, previous QKD implementations often required dedicated, unused “dark fiber” or complex and expensive filtering systems to isolate the quantum channel, making large-scale deployment economically unfeasible for most telecom operators.
A Novel Continuous-Variable Approach
The research team, comprised of scientists from Denmark and the Czech Republic, employed a specific method called continuous-variable QKD (CV-QKD) to overcome the noise limitations. Unlike other QKD protocols that may use the polarization of single photons, CV-QKD encodes information in the properties of weak laser pulses. This approach has proven more resilient in noisy environments and is better suited for coexistence with classical data.
How the New System Works
The system is designed as a “plug-and-play” solution, meaning it can be added to existing networks without significant modification. It uses a local-oscillator CV-QKD setup with Gaussian-modulated coherent states, a technical design that inherently minimizes interference. A key advantage of their method is that it does not require additional filtering equipment or reserving specific wavelength bands that are kept clear of classical traffic. The quantum signals were transmitted alongside a fully populated coarse wavelength-division multiplexing (CWDM) system, which involves multiple classical data channels operating at different wavelengths of light within the same fiber, mimicking a typical high-traffic network environment.
Record-Breaking Performance
In the experiment, the researchers successfully transmitted the quantum keys over a total fiber length of 120 kilometers, which corresponds to an optical signal loss of approximately 20.17 decibels. For the practical generation of a secure key, a process that accounts for real-world system limitations, the team achieved a distance of 100 kilometers. Remarkably, their measurements confirmed that the active classical channels introduced no significant excess noise or crosstalk into the quantum channel. By carefully optimizing the modulation of the quantum signals, they were able to preserve the integrity of the key generation process even when surrounded by high-power data traffic.
Implications for a Future Quantum Internet
This achievement represents a major step toward the realization of a quantum internet, where cities, countries, and eventually continents are linked by quantum-secured communication channels. The primary significance lies in demonstrating that quantum security can be an upgrade to existing infrastructure rather than requiring a complete and prohibitively expensive overhaul. By proving that QKD can operate effectively on standard telecom fiber crowded with data, the study closes the gap between academic research and practical, real-world deployment.
The ability to leverage the billions of miles of fiber-optic cable already in the ground is a crucial enabler for any new communication technology. This work shows that the transition to quantum-safe encryption can be incremental and cost-effective, allowing network operators to fortify their systems against future threats, including the potential for future quantum computers to break today’s standard encryption algorithms. It makes global communication systems more resilient and future-proof.
The Broader Landscape of QKD Research
The field of QKD is evolving rapidly, with research groups around the world pursuing different techniques to extend the distance and practicality of quantum communications. While this study sets a record for CV-QKD coexisting with classical channels, other methods are also pushing boundaries. For example, researchers at Toshiba Europe have used a different protocol known as twin-field QKD (TF-QKD) to send secure keys over 254 kilometers of commercial fiber in Germany. That experiment, however, focused on distance in a dark-fiber environment rather than coexistence with live data traffic. Another approach involves space-based infrastructure, using satellites to extend quantum channels over thousands of miles to overcome the signal loss inherent in terrestrial fiber.
Each protocol comes with its own set of advantages and challenges. For shorter distances under 100 kilometers in high-traffic networks, some research suggests a light wavelength of 1310 nanometers is ideal, while longer distances in lower-traffic networks may benefit from 1550 nanometers. For truly global-scale quantum networks, technologies like quantum repeaters will likely be necessary to amplify and extend quantum signals without destroying their delicate states.
Paving the Way for Commercial Adoption
The 120-kilometer demonstration is powerful evidence that QKD is maturing from an experimental science into a viable commercial technology. The robust performance and plug-and-play compatibility of the CV-QKD system make it an attractive candidate for early adoption by governments, financial institutions, and other organizations that require long-term data security. This work confirms that quantum encryption can be deployed in today’s networks, marking a pivotal moment in the quest to build a fundamentally secure global communication system for the 21st century.