  Quantum key distribution (QKD) takes advantage of fundamental properties of
quantum physics to allow two distant parties to share a secret key; however,
QKD is hampered by a distance limitation of a few hundred kilometers on earth.
The most immediate solution for global coverage is to use a satellite, which
can receive separate QKD transmissions from two or more ground stations and act
as a trusted node to link these ground stations. In this article, we report a
system capable of performing QKD in the high loss regime expected in an uplink
to a satellite using weak coherent pulses and decoy states. Such a scenario
profits from the simplicity of its receiver payload, but has so far considered
to be infeasible due to very high transmission losses (40 - 50 dB). The high
loss is overcome by implementing an innovative photon source and advanced
timing analysis. Our system handles up to 57 dB photon loss in the infinite key
limit, confirming the viability of the satellite uplink scenario. We emphasize
that while this system was designed with a satellite uplink in mind, it could
just as easily overcome high losses on any free space QKD link.
