Finite and Asymptotic Key Analysis for CubeSat-Based BB84 QKD with Elliptical Beam Approximation

Satellite and CubeSat-based quantum key distribution (QKD) presents a promising solution for secure long-distance communication by transmitting quantum keys through free space, with CubeSats offering a compact, cost-effective, and scalable platform for deployment. This study investigates the performance of statistical techniques used to compute the finite-block and single-pass secret key lengths (SKL) for weak coherent pulse (WCP)-based efficient BB84 and standard decoy-state BB84 protocols in CubeSat-based systems. An asymptotic key rate analysis is also conducted for both protocols, providing deeper insights into their theoretical performance within the CubeSat context. The channel transmittance is modeled using an elliptical beam approximation, and the key rate performance is evaluated under varying weather conditions for the downlink scenario. The results demonstrate that the efficient BB84 protocol consistently outperforms the standard version across different atmospheric conditions. Furthermore, the probability distribution of key rates (PDR) for both implementations is analyzed, offering a comprehensive evaluation of their practical effectiveness in CubeSat-based QKD applications.