  We present a ground-to-space quantum key distribution (QKD) mission concept
and the accompanying feasibility study for the development of the low earth
orbit CubeSat payload. The quantum information is carried by single photons
with the binary codes represented by polarization states of the photons.
Distribution of entangled photons between the ground and the satellite can be
used to certify the quantum nature of the link: a guarantee that no
eavesdropping can take place. By placing the entangled photon source on the
ground, the space segments contains only the less complex detection system,
enabling its implementation in a compact enclosure, compatible with the 12U
CubeSat standard (12 dm3). This reduces the overall cost of the project, making
it an ideal choice as a pathfinder for future European quantum communication
satellite missions. The space segment is also more versatile than one that
contains the source since it is compatible with a multiple of QKD protocols
(not restricted to entangled photon schemes) and can be used in quantum physics
experiments, such as the investigation of entanglement decoherence. Other
possible experiments include atmospheric transmission/turbulence
characterization, dark area mapping, fine pointing and tracking, and accurate
clock synchronization; all crucial for future global scale quantum
communication efforts.
