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arXiv:0901.2725 · New Journal of Physics

A fully automated entanglement-based quantum cryptography system for telecom fiber networks

Alexander Treiber, Andreas Poppe, Michael Hentschel, Daniele Ferrini, Thomas Lorünser, Edwin Querasser, Thomas Matyus, Hannes Hübel, Anton Zeilinger

Published Apr 30, 2009• 90 citations
arXivHTMLPDF

Abstract

We present in this paper a quantum key distribution (QKD) system based on polarization entanglement for use in telecom fibers. A QKD exchange up to 50 km was demonstrated in the laboratory with a secure key rate of 550 bits s −1 . The system is compact and portable with a fully automated start-up, and stabilization modules for polarization, synchronization and photon coupling allow hands-off operation. Stable and reliable key exchange in a deployed optical fiber of 16 km length was demonstrated. In this fiber network, we achieved over 2 weeks an automatic key generation with an average key rate of 2000 bits s −1 without manual intervention. During this period, the system had an average entanglement visibility of 93%, highlighting the technical level and stability achieved for entanglement-based quantum cryptography.

DOI
10.1088/1367-2630/11/4/045013

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We present in this paper a quantum key distribution (QKD) system based on polarization entanglement for use in telecom fibers. A QKD exchange up to 50 km was demonstrated in the laboratory with a secure key rate of 550 bits s −1 . The system is compact and portable with a fully automated start-up, and stabilization modules for polarization, synchronization and photon coupling allow hands-off operation. Stable and reliable key exchange in a deployed optical fiber of 16 km length was demonstrated. In this fiber network, we achieved over 2 weeks an automatic key generation with an average key rate of 2000 bits s −1 without manual intervention. During this period, the system had an average entanglement visibility of 93%, highlighting the technical level and stability achieved for entanglement-based quantum cryptography.
Quantum cryptography or more specifically quantum key distribution (QKD) is the most advanced quantum information protocol. The field has progressed enormously both in theoretical developments and experimental realisations. We present here an entanglement-based QKD system designed to work at 1550nm for optimal distribution in optical fibers[1] using the BBM92[2] protocol. Our QKD system is realised as a compact device. The start-up and alignment is fully automated and stabilisation routines guarantee a hands-off and long-term operation. We show a reliable and stable key generation during a two-week demonstration over a 16km fiber.
We demonstrate the feasibility of entanglement-based quantum key distribution (eQKD) in high-security financial infrastructure over a 22 km fiber link with 8 dB loss between two data centers using polarization entanglement. The fully automated system continuously generated secure keys for four months at an average rate of 63.8 kb/s, which were stored into a key management system and consumed to establish a VPN tunnel. The setup achieved 93.7% total up-time, with no downtime caused by the quantum optical components. Active polarization control kept the quantum bit error rate below 2% for 97.4% of the time and timing synchronization based on the entangled photon pairs' intrinsic temporal correlations achieved sub-300 ps precision. Our standalone system requires neither polarized guide lasers nor external high-precision time references. These results show practical integration of eQKD into operational financial infrastructure.
We demonstrate a quantum key distribution (QKD) implementation over deployed dark telecom fibers with polarization-entangled photons generated at the O-band. One of the photons in the pairs is propagated through 10 km of deployed fiber, while the others are detected locally. Polarization drifts experienced by the photons propagating through the fibers are compensated with liquid crystal variable retarders. This ensures continuous and stable QKD operation with an average quantum bit error rate of 6.4% and a final key rate of 109 bits/s.
The threat quantum computers pose to classical public-key cryptography motivates the deployment of quantum-safe communication for critical infrastructure such as healthcare, finance, and energy systems. Quantum key distribution (QKD) and post-quantum cryptography (PQC) offer complementary security guarantees, information-theoretic key exchange and quantum-resistant end-to-end authentication that can be combined in a layered architecture. Here, we demonstrate a field-deployed quantum-secure network integrating entanglement-based QKD with end-to-end PQC over 140 km of installed fiber in Thuringia, Germany, connecting a rural health kiosk to a university hospital via a trusted-node architecture comprising heterogeneous underground and aerial fiber links. Unlike conventional deployments that rely on a dedicated key management system to forward keys to applications, our architecture injects QKD keys directly into standard Linux-based VPN tunnels between adjacent nodes, while PQC secures the communication end-to-end, remaining fully compatible with existing infrastructure and software. Polarization-entangled photon pairs were generated at 810 nm and 1550 nm, with the telecom photon transmitted over deployed fiber. Active polarization stabilization and dispersion compensation preserve the entanglement and enable 22 days of continuous, fully autonomous operation, further underscoring the technological maturity of entanglement-based QKD approaches in a real-world fiber environment. Although the two deployed links were operated during separate rather than concurrent periods, the predominantly aerial link exhibited markedly greater instability, with QBER variations most strongly correlated with wind speed. The generated keys secured a telemedicine proof-of-concept without modifying existing medical systems, demonstrating a practical framework for quantum-safe critical infrastructures.
Abstract Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration. We ultimately generate secure keys with secure key rates of 0.22 bps and 0.06 bps in the asymptotic and finite-size regimes, respectively. It shows that entanglement-based DO-QKD is reliable for long-distance realization in the field if its high requirement on time synchronization is satisfied.
Quantum key distribution (QKD) is on the brink of being deployed on a large scale. For a successful launch it is important though, that QKD devices can be incorporated into or use existing telecom networks. Although not at telecom wavelengths, QKD based on polarization entanglement has already been shown over 1.4 km outside the laboratory. However, it was thought that dispersion effects, mainly polarization mode dispersion (PMD), in optical fibers will limit the use of polarization entanglement for longer distances. We report the distribution of quantum correlations over a record length of 100 km in telecom fibers with such a high quality that it would be possible to extract a secret key without background subtraction.