Passively stable distribution of polarisation entanglement over 192 km of deployed optical fibre
Sören Wengerowsky, Siddarth Koduru Joshi, Fabian Steinlechner, Julien Zichi, Bo Liu, Thomas Scheidl, Sergiy M. Dobrovolskiy, René van der Molen, Johannes W. N. Los, Val Zwiller, Marijn A. M. Versteegh, Alberto Mura, Davide Calonico, M. Inguscio, Anton Zeilinger, André Xuereb, Rupert Ursin
Abstract Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85 ± 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre.
Abstract Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85 ± 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre.
Abstract Reliable long-distance distribution of entanglement is a key technique for many quantum applications, most notably quantum key distribution. Here, we present a continuously working, trusted-node free international link between Austria and Slovakia, directly distributing polarization-entangled photon pairs via 248 km of deployed telecommunication fiber. Despite 79 dB loss, we observe stable detected pair rates of 9 s −1 over 110 h. We mitigate multi-pair detections with strict temporal filtering, enabled by nonlocal compensation of chromatic dispersion and superconducting nanowire detectors. Fully automatized active polarization stabilization keeps the entangled state’s visibility at 86% for altogether 82 h. In a quantum cryptography context, this corresponds to an asymptotic secure key rate of 1.4 bits/s and 258 kbit of total key, considering finite-key effects. Our work paves the way for low-maintenance, ultra-stable quantum communication over long distances, independent of weather conditions and time of day, thus constituting an important step towards the quantum internet.
Quantum entanglement is one of the most extraordinary effects in quantum physics, with many applications in the emerging field of quantum information science. In particular, it provides the foundation for quantum key distribution (QKD), which promises a conceptual leap in information security. Entanglement-based QKD holds great promise for future applications owing to the possibility of device-independent security and the potential of establishing global-scale quantum repeater networks. While other approaches to QKD have already reached the level of maturity required for operation in absence of typical laboratory infrastructure, comparable field demonstrations of entanglement-based QKD have not been performed so far. Here, we report on the successful distribution of polarization-entangled photon pairs between Malta and Sicily over 96 km of submarine optical telecommunications fiber. We observe around 257 photon pairs per second, with a polarization visibility above 90%. Our results show that QKD based on polarization entanglement is now indeed viable in long-distance fiber links. This field demonstration marks the longest-distance distribution of entanglement in a deployed telecommunications network and demonstrates an international submarine quantum communication channel. This opens up myriad possibilities for future experiments and technological applications using existing infrastructure.
Distributing entanglement through standard telecommunication fiber is particularly important for quantum key distribution protocols with low vulnerability over metropolitan distances. However, entanglement distribution over long distance through optical fiber so far could only be accomplished with moderate photon pair rates. In this work, we present entanglement distribution over 50 km of standard telecommunication fiber with a pair rate of more than 10 000 ${\mathrm{s}}^{\ensuremath{-}1}$ using a bright nondegenerate photon pair source. Signal and idler wavelengths of this source are optimized for low dispersion in optical fiber and high efficiency for single-photon avalanche diode detectors, respectively. The resulting modest hardware requirement and high rate of detected entangled photon pairs could significantly enhance practical entanglement-based quantum key distribution in existing metropolitan fiber networks.
Quantum Key Distribution (QKD) is an emerging technology that enables secure data encryption by basing key generation on the transmission of quantum states rather than computational complexity, thereby anchoring security in the laws of physics and offering resilience against quantum computing attacks. Current research efforts focus on transitioning QKD from laboratory-based demonstrations to real-world deployment, thereby establishing it as a practical communication technology. Quantum entanglement plays a central role in QKD by enabling robust security assurance and providing a path towards scalable quantum networks. This thesis presents a polarisation-entanglement system designed for operation in practical research scenarios, comprising state preparation from a commercial source, transmission, reception and data analysis. The system's capabilities are demonstrated through measurement of polarisation correlation visibility, violation of CHSH-type inequalities, and quantum state tomography, culminating in the generation of the first discrete-variable secret key generated within the Experimental Quantum Communication Group at the University of York. Using the developed system, the integration of quantum communication into existing information infrastructure is experimentally investigated in two distinct scenarios. First, the coexistence of quantum and classical signals in the shared optical fibre is explored in the context of a White Rabbit time-distribution system, recovering entanglement visibilities above 96\% over co-propagation distances of up to 1~km. Second, the transmission of photon pairs from the entanglement source is demonstrated over a 224~km deployed submarine fibre link, connecting the UK and Ireland used for classical data traffic. Together, these efforts provide a versatile, robust research tool for future experimental studies of polarisation-based QKD and demonstrate the viability of integrating quantum communication into existing optical infrastructure, paving the way for practical QKD systems.
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 networks have begun to connect many users together with Quantum Key Distribution links. We present a scalable, full mesh, polarisation entanglement-based quantum network without trusted nodes. We discuss our progress towards building a dynamic quantum network with more users, long distance (≈50 km) links and improved polarisation stability in the optical fibres. Lastly, minimising the resource overhead and optimising the network control based on end-user requirements are important features we are incorporating into our network.