  Optical quantum communication utilizing satellite platforms has the potential
to extend the reach of quantum key distribution (QKD) from terrestrial limits
of ~200 km to global scales. We have developed a thorough numerical simulation
using realistic simulated orbits and incorporating the effects of pointing
error, diffraction, atmosphere and telescope design, to obtain estimates of the
loss and background noise which a satellite-based system would experience.
Combining with quantum optics simulations of sources and detection, we
determine the length of secure key for QKD, as well as entanglement visibility
and achievable distances for fundamental experiments. We analyze the
performance of a low Earth orbit (LEO) satellite for downlink and uplink
scenarios of the quantum optical signals. We argue that the advantages of
locating the quantum source on the ground justify a greater scientific interest
in an uplink as compared to a downlink. An uplink with a ground transmitter of
at least 25 cm diameter and a 30 cm receiver telescope on the satellite could
be used to successfully perform QKD multiple times per week with either an
entangled photon source or with a weak coherent pulse source, as well as
perform long-distance Bell tests and quantum teleportation. Our model helps to
resolve important design considerations such as operating wavelength, type and
specifications of sources and detectors, telescope designs, specific orbits and
ground station locations, in view of anticipated overall system performance.

Quantum Physics;Optics