The network currently supports ten users, demonstrating a move beyond laboratory experiments toward practical, long-duration quantum communication. The work, detailed in a paper published September 29, 2026, reports continuous operation of a secure link for over 325 hours across 50 kilometers, and extends key distribution to a 100 kilometer link connecting university campuses.
Long-Duration QKD Deployment Over 50km Fiber Network
The design choice streamlines infrastructure while preserving key rate performance. Results indicate a wide portion of the spectrum is usable, confirming the broadband source’s suitability for real-world quantum network implementations, though key generation dips near wavelengths of 1561 nanometers and 1581/1541 nanometers. The network’s architecture incorporates three fiber segments measuring 19. 2 kilometers, 32. 7 kilometers, and 48. 2 kilometers, respectively. They further extend secure key distribution up to a 100 km operational link connecting several campuses of the University Côte d’Azur to an optical ground station.
This configuration allowed for testing of long-distance entanglement distribution and key generation under realistic network conditions. The team’s work demonstrates the potential for scaling quantum communication beyond laboratory settings.
Secret key rate performance is demonstrably affected by transmission losses and distance, as illustrated by the data presented. The research identifies a point beyond which the Cascade protocol, used for key generation, no longer yields optimal results. This understanding helps optimize network parameters and maximize the secure communication range. The team’s analysis of secret key rate as a function of distance provides a valuable benchmark for future QKD network designs.
The system’s ability to simultaneously generate raw keys and evaluate synchronization, without impacting the secret key rate or increasing the quantum bit error rate, represents an advancement. This concurrent operation streamlines the key exchange process and improves overall network efficiency. The team’s approach addresses a key challenge in long-distance QKD: maintaining synchronization between distant nodes without interrupting quantum communication. This resilience is essential for ensuring continuous operation in a real-world environment.
Data collected during the deployment demonstrates the system’s ability to recover from these events and maintain a stable key rate over time. The ability to withstand and recover from common operational issues is a critical step toward practical QKD deployments.
Extending QKD Range to 100km for Inter-City Links
Achieving this range required addressing significant optical losses, providing valuable data for future long-distance architectures intended for intercontinental implementation via satellite. This is accomplished through an actively unbalanced Mach-Zehnder interferometer, operating in a Franson configuration, which measures superposed states locally without requiring a dedicated clock signal fiber. Exploiting the full spectrum of photon pairs generated by the source further enhances the network’s capacity, enabling the creation of up to 18 independent QKD links.
This capability allows for investigation of various quantum network topologies within a metropolitan area, facilitating simultaneous key sharing between all connected users without relying on trusted nodes. The resulting stable network now is a platform for testing more advanced quantum communication protocols, marking a progression toward large-scale quantum networks.
BBM92 Protocol and Time-Energy Observables for Secure Keys
The deployed network uses the BBM92 protocol alongside time-energy observables to facilitate secure key exchange in a multi-user environment, extending the reach of quantum-secured communication. This approach allows for the simultaneous generation of raw keys and evaluation of synchronization, a critical advancement for practical implementation. Custom-built post-processing software continuously monitors key parameters like quantum bit error rates, transmission losses and time drift, linking these metrics to the system’s physical components.
This software performs real-time sifting and privacy amplification, converting raw keys into secure keys stored locally on each user’s computer. The ability to continuously correct instabilities in both the source and analysis modules is central to the network’s sustained performance, enabling autonomous operation without human intervention. The software also tracks the number of photons detected and the time drift between local clocks, allowing for optimization of the secure key rate.
The network demonstrated 325 hours of continuous key generation, achieving an average secure key rate of 7. 069 kbps. Adaptability to varying communication distances, up to 100 kilometers with 56 decibels of total transmission loss, further highlights the system’s resilience. Polarization drift, primarily induced by the 50 kilometer fiber deployment, is actively compensated for by the software, ensuring stable key distribution. The system’s architecture also employs dense wavelength-division demultiplexing, separating paired photons to enable QKD across multiple standard channels.
This allows for secure key exchange among the ten users currently supported by the network. The demonstration moves beyond proof-of-concept, representing a transition toward concrete applications of quantum technologies. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal, as is standard practice for academic publications.
Dense Wavelength-Division Multiplexing Enables Multi-User QKD
Employing dense wavelength-division multiplexing, the deployed network separates paired photons to facilitate quantum key distribution across multiple standard channels, using the BBM92 protocol and time-energy observables in a multi-user configuration. This approach allows for secure key exchange by routing short wavelengths to one party and long wavelengths to another, a strategy implemented with a coarse wavelength division multiplexing scheme centered at 1531 nm, 1551 nm, 1571 nm and 1591 nm.
The selection of wavelengths relies on low-loss passive components, avoiding active routing for simplicity and stability. At both Alice’s and Bob’s stations, dense wavelength division multiplexing modules demultiplex the incoming spectrum into up to 36 100 GHz channels, aligning with the standard ITU grid. Following separation, photons originating from a correlated pair undergo local measurement in either the Z or X basis, corresponding to time and energy observables.
This measurement process uses fiber-based beam splitters and actively stabilized Mach-Zehnder interferometers to project photons onto specific states, enabling the establishment of a secure key. The resulting secret keys are then stored locally on each user’s computer, completing the secure communication process. Beyond key generation, the software actively monitors critical QKD indicators, including quantum bit error rate in each basis, photon detection counts, and time drift between local clocks.
These parameters are linked to control devices, allowing for continuous optimization of the secure key rate. This capability is essential for correcting instabilities within the source and analysis modules, ensuring the long-term reliability of the quantum communication link. “Consequently, the present experiment demonstrates the spectral availability required for multi-user operation rather than the simultaneous deployment of 36 independent users,” the paper states. Real-field measurements, alongside laboratory tests simulating equivalent attenuation, confirm the viability of this approach.
The evolution of the secure key rate is directly linked to the wavelength used within the dense wavelength division multiplexing system, with orange dots representing real-field data and blue crosses indicating laboratory results. The background coloring illustrates the measured CWDM transmission used to split the source spectrum between Alice and Bob. This demonstration signifies a transition from proof-of-concept experiments to concrete applications of quantum technologies.
Automation of Entanglement-Based Quantum Link for Continuous Keys
Automated recovery from failures is now integral to the deployed quantum network, enabling continuous secret key generation over extended periods without manual intervention. The system sustained operation for over 325 hours across a 50 kilometer fiber link connecting remote locations, a demonstration of long-duration key production previously limited by the need for constant monitoring and adjustment. This achievement moves beyond controlled laboratory settings toward realistic and scalable implementations of quantum key distribution.
While these fluctuations can reduce detection efficiency by up to 6 decibels in cases of orthogonal polarization, the system maintains key generation capabilities. This resilience is essential for real-world deployments where maintaining precise environmental control is impractical, and it demonstrates a practical approach to mitigating signal degradation. The work demonstrates a significant step toward practical, automated quantum communication systems, capable of sustained operation in real-world conditions and opening the way to wider adoption of this secure communication technology.
Quantum Network Challenges: Synchronization and Analyzer Stabilization
Extending secure communication to 100 kilometers, the deployed network connects University Côte d’Azur campuses and an optical ground station, a configuration designed to test compatibility with future quantum satellite links. The network’s architecture uses existing fiber infrastructure, minimizing the need for dedicated quantum channels alongside classical communication lines. This approach contrasts with earlier demonstrations that often required temporary interruptions to resynchronize or stabilize analyzers, a process delegated to the classical network layer.
Stabilizing interferometers, critical for detecting entangled photons presents a unique challenge, demanding identical delays and consistently locked relative phases between the devices used by each user. The team addressed this by transferring synchronization and stabilization tasks from the classical to the quantum layer, using quantum signals themselves to maintain alignment.
This method allows for continuous key generation over extended periods without requiring a separate signal dedicated to synchronization or analyzer stabilization, a feat demonstrated during a 325-hour operational period over 50 kilometers of fiber. 2, 32. 7, and 48. Measurements in the X basis, which rely on non-local interference, require interferometers with identical delays and locked relative phases. “Our setup only uses quantum resources for stabilization and synchronization,” the researchers state, highlighting the efficiency of their approach.
Losses significantly impact the system’s effectiveness, and optimizing analyzers helps extend the range of long-distance quantum key distribution. The team’s work demonstrates a move toward fully automated quantum links capable of continuous, optimal secret key generation without reliance on classical signaling for synchronization or stabilization.
This automation is particularly significant as it addresses a major hurdle in scaling quantum networks beyond laboratory settings. Previous demonstrations often simplified remote synchronization and analyzer stabilization, limiting the number of connected users to eight in local tests.
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