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Laser Physics Letters

Optical system design for daytime free-space quantum key distribution

Veronika M. Vakhrusheva, Andrey N. Klimov, V V Tretiakov, S. P. Kulik

Published Apr 1, 2026• 1 citation
Publisher

Abstract

Abstract This paper analyzes the design features of free-space quantum key distribution systems operating over horizontal, near-ground propagation paths under daylight conditions. It presents methods for calculating key quantum channel characteristics, such as loss levels and the signal-to-noise ratio. Based on the obtained results, some recommendations are formulated for the design and selection of optical parameters for such systems.

DOI
10.1088/1612-202x/ae56fa

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Abstract This paper analyzes the design features of free-space quantum key distribution systems operating over horizontal, near-ground propagation paths under daylight conditions. It presents methods for calculating key quantum channel characteristics, such as loss levels and the signal-to-noise ratio. Based on the obtained results, some recommendations are formulated for the design and selection of optical parameters for such systems.
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Free-space quantum key distribution (QKD), more popularly know as quantum cryptography, uses single-photon free-space optical communications to distribute the secret keys required for secure communications. At Los Alamos National Laboratory we have demonstrated a fully automated system that is capable of operations at any time of day over a horizontal range of several kilometers. This has proven the technology is capable of operation from a spacecraft to the ground, opening up the possibility of QKD between any group of users anywhere on Earth. This system, the prototyping of a new system for use on a spacecraft, and the techniques required for world-wide quantum key distribution will be described. The operational parameters and performance of a system designed to operate between low earth orbit (LEO) and the ground will also be discussed.
This work presents a model to estimate the performance of space-based, optical-based, quantum communication protocols. This model consists of components to account for optical channel propagation effects based on orbit selection and atmospheric conditions. The model presented is general purpose and can be leveraged to evaluate the performance of a variety of quantum communication protocols, of which, Quantum Key Distribution (QKD) systems served as our motivating use case of particular interest. To verify correctness, the model is used to produce estimates for QKD system scenarios and compared to published results. The performance of QKD systems is of interest as distance limitations for terrestrial-based systems have hindered their practical use, and satellite-based designs that can generate a shared key between two distant geographic locations have been proposed. For this application domain, a review of space-based designs that illuminate the need for a free space downlink channel model is presented followed by its development to estimate the performance of quantum exchanges between a satellite and ground site.