  We describe systems and methods for the deployment of global quantum key
distribution (QKD) networks covering transoceanic, long-haul, metro, and access
segments of the network. A comparative study of the state-of-the-art QKD
technologies is carried out, including both terrestrial QKD via optical fibers
and free-space optics, as well as spaceborne solutions via satellites. We
compare the pros and cons of various existing QKD technologies, including
channel loss, potential interference, distance, connection topology, deployment
cost and requirements, as well as application scenarios. Technical selection
criteria and deployment requirements are developed for various different QKD
solutions in each segment of networks. For example, optical fiber-based QKD is
suitable for access networks due to its limited distance and compatibility with
point-to-multipoint (P2MP) topology; with the help of trusted relays, it can be
extended to long-haul and metro networks. Spaceborne QKD on the other hand, has
much smaller channel loss and extended transmission distance, which can be used
for transoceanic and long-haul networks exploiting satellite-based trusted
relays.
