Scalability bottlenecks in entanglement-based satellite quantum key distribution: downlink asymmetry, orbit dependence and finite-key effects

Space-based entanglement distribution, as the backbone of the quantum Internet, is poised to enable global quantum networking from infrastructure to application platforms. Satellite-based quantum key distribution (SatQKD), with the entanglement-based QKD protocol of Bennett, Brassard, and Mermin (BBM92), promises source-independent security, but faces a trinity of scalability bottlenecks in dual-downlink configurations. Specifically, asymmetric link characteristics are exacerbated by optical ground station (OGS) pair system parameter differences, while orbit-dependent metrics, e.g., transmission window duration, overpass interval, and trajectory repeatability, introduce finite secret key length (SKL) extraction considerations. Further, quantum satellites will be within the range of multiple OGS pairs, which requires efficient scheduling of SatQKD to prioritize links, mitigate cloud cover disturbance, and maximize finite SKL. Here, we develop a co-modeling framework of dual-downlink SatQKD to support orbital parameter selection and performance comparison for asymmetric system-optics (e.g., aperture diameter, extraneous counts) parameters. We clarify the distinct performance of the SKL extraction methods for Sun-synchronous orbits (SSO) and polar orbits. We further incorporate the finite BBM92 SKL analysis into downlink scheduling optimisation to truncate adverse data collection periods and explore the advantages in flexibility and efficiency for global networking. Our work provides actionable design insights for future entanglement-based global quantum communications.