Abstract:Terrestrial QKD channels can connect two links with a maximum distance of few
hundred kilometres. In the case of fibre links, this is due to the signal attenuation in
the fibre; in the case of free-space link the losses are due to atmospheric turbulence
and absorption. Free-space optical terminals exploiting satellite-based relays are the
only resource that can enable global scale quantum key distribution, since single photon
propagation is for the main part in vacuum with no turbulence or absorption, and
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