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arXiv:2208.13584 · EPJ Quantum Technology

A study of polarization compensation for quantum networks

Matej Peranić, M. J. Clark, Rui Wang, Sima Bahrani, Obada Alia, Sören Wengerowsky, Anton Radman, Martin Lončarić, Mario Stipčević, John Rarity, Reza Nejabati, Siddarth Koduru Joshi

Published Aug 17, 2023• 26 citations
arXivHTMLPDF

Abstract

Abstract The information-theoretic unconditional security offered by quantum key distribution has spurred the development of larger quantum communication networks. However, as these networks grow so does the strong need to reduce complexity and overheads. Polarization-based entanglement distribution networks are a promising approach due to their scalability and no need for trusted nodes. Nevertheless, they are only viable if the birefringence of all-optical distribution fibres in the network is compensated to preserve the polarization-based quantum state. The brute force approach would require a few hundred fibre polarization controllers for even a moderately sized network. Instead, we propose and investigate four different realizations of polarization compensation schemes that can be used in quantum networks. We compare them based on the type of reference signals, complexity, effort, level of disruption to network operations and performance on a four-user quantum network.

DOI
10.1140/epjqt/s40507-023-00187-w

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Abstract The information-theoretic unconditional security offered by quantum key distribution has spurred the development of larger quantum communication networks. However, as these networks grow so does the strong need to reduce complexity and overheads. Polarization-based entanglement distribution networks are a promising approach due to their scalability and no need for trusted nodes. Nevertheless, they are only viable if the birefringence of all-optical distribution fibres in the network is compensated to preserve the polarization-based quantum state. The brute force approach would require a few hundred fibre polarization controllers for even a moderately sized network. Instead, we propose and investigate four different realizations of polarization compensation schemes that can be used in quantum networks. We compare them based on the type of reference signals, complexity, effort, level of disruption to network operations and performance on a four-user quantum network.
A Quantum Key Distribution (QKD) network can allow multi-user communication via secure key. Moreover, by actively switching communication nodes, one can achieve high key transmission rate for the selected nodes. However, the polarization properties of different fiber path are different and these properties also randomly drift over time. Therefore, polarization recovery after the switching and auto-compensation during key transmission are critical for the QKD network. In this work, we use programmable polarization controllers to implement polarization recovery and auto-compensation in the QKD network. We will also discuss its time limitation and future improvement.
Quantum key distribution – a process that encodes digital information – often utilises fibre optic technologies for commercial applications. Fibre provides the benefit of a dark channel as well as the convenience of independence of a line-of-sight connection between the sender and receiver. In order to implement quantum key distribution protocols utilising polarisation encoding, the birefringence effects of fibre must be compensated for. Birefringence is caused by manufacturing impurities in the fibre or a change in environmental conditions and results in a rotation of the state of polarisation of light as it is propagated through the fibre. With dynamic environmental conditions, the birefringence effects should be monitored with a test signal at regular time intervals so that the polarisation of each photon can be appropriately compensated to its original state. Orthogonal states are compensated simultaneously, but most protocols, such as BB84 and B92, require non-orthogonal basis sets. Instead of using a compensator for each basis, the presented scheme fixes the polarisation controller onto the plane on the Poincaré that passes through both bases, compensating both non-orthogonal bases simultaneously.
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.
In quantum key distribution (QKD) systems using polarization entanglement, channel polarization perturbations significantly compromise both efficiency and security. To address the slow response and instability of current entanglement-based polarization compensation techniques for entanglement-based quantum key distribution, we propose a polarization compensation scheme based on an adaptive gradient descent algorithm. The system's performance is tested over a 10 km single-sided fiber channel. Results show that this scheme maintains long-term stability of the polarization state under both random and periodic perturbations. The average compensation time is 248 ms, with an entanglement source average visibility of 83.43% and an average quantum error rate of 8.28%. This method effectively mitigates polarization perturbation in entanglement-based quantum key distribution (QKD). These results are pivotal for the development of long-distance, multi-node quantum communication networks.
Quantum networks have been shown to connect users with full-mesh topologies without trusted nodes. We present advancements on our scalable polarisation entanglement-based quantum network testbed, which has the ability to perform protocols beyond simple quantum key distribution. Our approach utilises wavelength multiplexing, which is ideal for quantum networks across local metropolitan areas due to the ease of connecting additional users to the network without increasing the resource requirements per user. We show a 10 user fully connected quantum network with metropolitan scale deployed fibre links, demonstrating polarisation stability and the ability to generate secret keys over a period of 10.8 days with a network wide average-effective secret key rate of 3.38 bps.
We present a new concept for birefringence compensation in Sagnac single mode fiber (SMF) interferometer, which has opened the door to secure multiuser quantum communications. The concept has been tested in both quantum key distribution (QKD) and secret sharing applications at total fiber loop lengths up to 150 km. Now, we present a Jones matrix based model of the concept as well as the visibility measurements confirming its practical validity. Sagnac based circular SMF network architecture is an attractive solution for both commercial telecom Intranets and backbone networks as well as for secure quantum information channels exploring QKD and secret sharing applications. Due to the difference in birefringence between the clockwise and counterclockwise transmission directions, Sagnac has not been so much explored by the quantum information community as other network architectures, especially the "plug & play". Our SMF Sagnac birefringence compensation uses polarization maintaining, horizontally aligned components only, except for the SMF fiber link. It converts the arbitrary elliptically polarized clockwise and counterclockwise light signals, transmitted over the SMF fiber, into the horizontal polarization. Thus, both the sent and received light signals have the same horizontal polarization. In order to avoid unnecessary power losses in the birefringence compensator, we have included a simple proportional-integral-derivative controller into our setup. The compensator provides a perfect birefringence compensation for the destructive interference, which leads to a high system visibility. The presented SMF Sagnac birefringence compensation could also be use in Sagnac metrology, for instance in very low power level sensing, requiring high measurement accuracy.