LEO-to-Ground Gaussian Modulated Continuous Variable Quantum Key Distribution

Australia and New Zealand are currently developing an optical communications ground station network to test technologies for optical and quantum communications that will enable faster data transfers and security during information exchange. A goal of the network is to implement Gaussian modulated continuous variable quantum key distribution (GM-CVQKD) between LEO satellites and optical ground stations. This protocol offers quantum cryptography, providing information-theoretic security through quantum mechanics and is compatible with hardware needed for coherent optical communications that satellites are starting to use to combat the crowded spectrum from increased traffic in radio-frequency communications. GM-CVQKD avoids the need for the specialised and complex equipment required by discrete variable QKD to achieve secret key exchange, as it can use cost-effective commercially available off-the-shelf hardware. We present a study to determine the feasibility of a proposed implementation of GM-CVQKD. As a first step, a model of the quantum signal downlink propagation from a LEO satellite to an optical ground station (OGS) has been created. This was used to explore adverse effects from varying atmospheric conditions, the Doppler shift owing to the relative motion between a LEO satellite and an OGS, and the presence of an attack/hack from an eavesdropper. The results show that suitable atmospheric conditions (negligible turbulence and higher visibility) produce a larger positive secret key rate (SKR) while less favourable conditions (high turbulence and lower visibility) significantly decrease the achievable SKRs. The effects of Doppler shift are negligible and still permit a positive SKR. The protocol allows the presence of an eavesdropper to be detected so that communications can be terminated. The results indicate that GM-CVQKD is a viable protocol for space-based QKD and is a stepping-stone towards a global quantum network.