In-Field Quantum-Protected Control-Based Key Distribution with a Lossy Urban Fiber Link
Vladlen Statiev, Abdufattokh Ashurov, Vladimir M. Semenov, Dmitrii Kozliuk, V. V. Zemlyanov, Aleksei Kodukhov, Valeria A. Pastushenko, V. M. Vinokur, Markus Pflitsch
Quantum cryptography protocols offering unconditional protection open great rout to full information security in quantum era. Yet, implementing these protocols using the existing fiber networks remains challenging due to high signal losses reducing the efficiency of these protocols to zero. The recently proposed Quantum-protected Control-based Key Distribution (QCKD) addresses this issue by physically controlling interceptable losses and ensuring that leaked quantum states remain non-orthogonal. Here, we present the first in-field development and demonstration of the QCKD over an urban fiber link characterized by substantial losses. Using information-theoretic considerations, we configure the system ensuring security and investigate the interplay between line losses and secret key rates. Our results backed by the statistical analysis of the secret key, confirm QCKD’s robustness under real-world conditions, and establish it as a practical solution for quantum-safe communications over existing fiber infrastructures.
Quantum cryptography protocols offering unconditional protection open great rout to full information security in quantum era. Yet, implementing these protocols using the existing fiber networks remains challenging due to high signal losses reducing the efficiency of these protocols to zero. The recently proposed Quantum-protected Control-based Key Distribution (QCKD) addresses this issue by physically controlling interceptable losses and ensuring that leaked quantum states remain non-orthogonal. Here, we present the first in-field development and demonstration of the QCKD over an urban fiber link characterized by substantial losses. Using information-theoretic considerations, we configure the system ensuring security and investigate the interplay between line losses and secret key rates. Our results backed by the statistical analysis of the secret key, confirm QCKD’s robustness under real-world conditions, and establish it as a practical solution for quantum-safe communications over existing fiber infrastructures.
Quantum cryptography revolutionizes secure information transfer, providing defense against both quantum and classical computational attacks. The primary challenge in extending the reach of quantum communication comes from the exponential decay of signals over long distances. We meet this challenge by experimentally realizing the Quantum-Protected Control-Based Key Distribution (QCKD) protocol, utilizing physical control over signal losses. By ensuring significant non-orthogonality of the leaked quantum states, this control severely constrains eavesdroppers' capacities. We demonstrate the performance and scale of our protocol by experiments over a 1707 km long fiber line. The scalability of the QCKD opens the route for globally secure quantum-resistant communication.
Quantum key distribution (QKD) is a cryptographic technique that uses quantum mechanical principles to enable secure key distribution, offering information-theoretic security guaranteed by physical laws. Practical deployment of QKD requires robust, cost-effective systems that can operate in challenging field environments. A major challenge is achieving reliable clock synchronization without adding hardware complexity. Conventional approaches often use separate classical light signals, which increase costs and introduce noise that degrades quantum channel performance. To address this limitation, we demonstrate a QKD system incorporating a recently proposed qubit-based distributed frame synchronization method, deployed over a metropolitan fiber network in Nanning, China. Using the polarization-encoded one-decoy-state BB84 protocol and the recently proposed qubit-based distributed frame synchronization method, our system achieves synchronization directly from the quantum signal, eliminating the need for dedicated synchronization hardware. Furthermore, to counteract dynamic polarization disturbances in urban fibers, the system integrates qubit-based polarization feedback control, enabling real-time polarization compensation through an automated polarization controller using data recovered from the qubit-based synchronization signals. During 12 hours of continuous operation, the system maintained a low average quantum bit error rate of 1.12 ± 0.48%, achieving a secure key rate of 26.6 kbit/s under 18 dB channel loss. Even under a high channel loss of 40 dB, a finite-key secure rate of 115 bit/s was achieved. This study represents a successful long-term validation of a frame-synchronization-based QKD scheme in a real urban environment, demonstrating exceptional stability and high-loss tolerance, and offering an alternative for building practical, scalable, and cost-efficient quantum-secure communication networks.
Information security faces unprecedented challenges in the digital age, particularly in the face of cyber threats and the emergence of quantum computing. This research examines QKD as a solution for secure communications in urban fiber networks. The study analyzes QKD protocols, including BB84 and Twin-Field QKD, implementing a 46-node metropolitan quantum network and key distribution over distances greater than 1002 kilometers. Various transmission media, such as multicore optical fibers and free-space communication, were evaluated, demonstrating high-capacity quantum communication with advanced noise mitigation techniques. The results reveal significant advances in quantum communication security, highlighting the potential of QKD to provide theoretically secure key distribution based on quantum mechanical principles. The research addresses challenges such as transmission loss, environmental disturbances, and network scalability, laying the groundwork for future implementations in metropolitan networks. This work contributes significantly to the standardization of evaluation protocols and metrics, which is crucial for the global adoption of QKD in a world increasingly dependent on cybersecurity, especially in developing smart cities.
Quantum key distribution (QKD) is a secure communication scheme for sharing symmetric cryptographic keys based on the laws of quantum physics, and is considered a key player in the realm of cyber-security. A critical challenge for QKD systems comes from the fact that the ever-increasing rates at which digital data are transmitted require more and more performing sources of quantum keys, primarily in terms of secret key generation rate. High-dimensional QKD based on path encoding has been proposed as a candidate approach to address this challenge. However, while proof-of-principle demonstrations based on lab experiments have been reported in the literature, demonstrations in realistic environments are still missing. Here we report the generation of secret keys in a 4-dimensional hybrid time-path-encoded QKD system over a 52-km deployed multicore fiber link forming by looping back two cores of a 26-km 4-core optical fiber. Our results indicate that robust high-dimensional QKD can be implemented in a realistic environment by combining standard telecom equipment with emerging multicore fiber technology.
In-field demonstrations in real-world scenarios boost the development of a rising technology towards its integration in existing infrastructures. Although quantum key distribution (QKD) devices are already adopted outside the laboratories, current field implementations still suffer from high costs and low performances, preventing this emerging technology from a large-scale deployment in telecommunication networks. Here we present a simple, practical and efficient QKD scheme with finite-key analysis, performed over a 21 dB-losses fiber link installed in the metropolitan area of Florence (Italy). Coexistence of quantum and weak classical communication is also demonstrated by transmitting an optical synchronization signal through the same fiber link.
Quantum Key Distribution (QKD) is a promising tool for secure communication in the near future. In combination with one-time-pad technique, it provides an unconditionally secure communication channel (meaning that it is secure against an adversary, even with unlimited computational power), at least in principle. However, a major implementation challenge for some schemes is the reliable creation, transportation, and measurement of entangled photon pairs over long-distance fiber networks. Our project aims to explore the possibilities for distributing quantum information on an existing network infrastructure while measuring the effects of real-world conditions. We characterized a commercial source of entangled photons. We measured its spectrum, brightness (1.6±0.3)×104 pairs/s/μW, and performed quantum state tomography (QST) to reconstruct the density matrix of the quantum state. Our implementation focuses on an all-fiber solution, which would enable a simplified QKD implementation. In laboratory conditions, we achieved the visibility equal to (0.957 ± 0.004) as a mean in both bases with a coincidence rate of (275 ± 4) counts/s and successfully ran QKD protocol with secret key rate of (86 ± 1) bits/s and average quantum bit error rate (QBER) of (4.8 ± 0.7) %.