  Satellite-based quantum terminals are a feasible way to extend the reach of
quantum communication protocols such as quantum key distribution (QKD) to the
global scale. To that end, prior demonstrations have shown QKD transmissions
from airborne platforms to receivers on ground, but none have shown QKD
transmissions from ground to a moving aircraft, the latter scenario having
simplicity and flexibility advantages for a hypothetical satellite. Here, we
demonstrate QKD from a ground transmitter to a receiver prototype mounted on an
airplane in flight. We have specifically designed our receiver prototype to
consist of many components that are compatible with the environment and
resource constraints of a satellite. Coupled with our relocatable ground
station system, optical links with distances of 3-10 km were maintained and
quantum signals transmitted while traversing angular rates similar to those
observed of low-Earth-orbit satellites. For some passes of the aircraft over
the ground station, links were established within 10 s of position data
transmission, and with link times of a few minutes and received quantum bit
error rates typically 3-5%, we generated secure keys up to 868 kb in length. By
successfully generating secure keys over several different pass configurations,
we demonstrate the viability of technology that constitutes a quantum receiver
satellite payload and provide a blueprint for future satellite missions to
build upon.
