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arXiv:2205.12862 · npj Quantum Information

Towards metropolitan free-space quantum networks

Andrej Kržič, Sakshi Sharma, Christopher Spiess, Uday Chandrashekara, Sebastian Töpfer, Gregor Sauer, Luis Javier González-Martín del Campo, Teresa Kopf, Stefan Petscharnig, Thomas Alexander Grafenauer, Roland Lieger, Bernhard Ömer, Christoph Pacher, René Berlich, Thomas Peschel, Christoph Damm, Stefan Risse, Matthias Goy, Daniel Rieländer, Andreas Tünnermann, Fabian Steinlechner

Published Sep 27, 2023• 54 citations

Abstract

Abstract Quantum communication has seen rapid progress towards practical large-scale networks, with quantum key distribution (QKD) spearheading this development. While fibre-based systems have been shown to be well suited for metropolitan scales, suitable fibre infrastructure may not always be in place. Here, we make the case for an entanglement-based free-space quantum network as a practical and efficient alternative for metropolitan applications. We developed a deployable free-space QKD system and demonstrated its use in realistic scenarios. For a representative 1.7-km free-space link, we showcase its ad hoc deployability and achieve secure key rates of up to 5.7 kbps, with 2.5 kbps in direct noon sunlight. By extrapolating experimental data, we show that kbps key rates are achievable even for 10-km distances and multi-user scenarios. We anticipate that our work will establish free-space networks as a viable solution for metropolitan applications and an indispensable complementary building block in the future global quantum internet.

DOI: 10.1038/s41534-023-00754-0

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3 \times 10^{4}

s

^{-1}

between any pair of nodes on the network, for sifted key rate of over 400 kbps. Our approach overcomes the active switching hurdle that has hindered the free-space deployment of quantum multiplexing, is fully passive, easily scalable to more nodes and compatible with fibre-based integration, thus opening a new path to scalable and resource-efficient quantum networks that utilise free-space links.

Quantum key distribution (QKD) together with one time pad encoding can provide information-theoretical security for communication. Currently, though QKD has been widely deployed in many metropolitan fiber networks, its implementation in a large scale remains experimentally challenging. This letter provides a brief review on the experimental efforts towards the goal of global QKD, including the security of practical QKD with imperfect devices, QKD metropolitan and backbone networks over optical fiber and satellite-based QKD over free space.

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