  Despite being an integral part of the vision of quantum Internet,
Earth-to-satellite (uplink) quantum communications have been considered more
challenging than their satellite-to-Earth (downlink) counterparts due to the
severe channel-loss fluctuations (fading) induced by atmospheric turbulence.
The question of how to address the negative impact of fading on
Earth-to-satellite quantum communications remains largely an open issue. In
this work, we explore the feasibility of exploiting spatial diversity as a
means of fading mitigation in Earth-to-satellite Continuous-Variable (CV)
quantum-classical optical communications. We demonstrate, via both our
theoretical analyses of quantum-state evolution and our detailed numerical
simulations of uplink optical channels, that the use of spatial diversity can
improve the effectiveness of entanglement distribution through the use of
multiple transmitting ground stations and a single satellite with multiple
receiving apertures. We further show that the transfer of both large
(classically-encoded) and small (quantum-modulated) coherent states can benefit
from the use of diversity over fading channels. Our work represents the first
quantitative investigation into the use of spatial diversity for
satellite-based quantum communications in the uplink direction, showing under
what circumstances this fading-mitigation paradigm, which has been widely
adopted in classical communications, can be helpful within the context of
Earth-to-satellite CV quantum communications.
