  Quantum key distribution (QKD) together with one time pad encoding can
provide information-theoretical security for communication. Currently, though
QKD has been widely deployed in many metropolitan fiber networks, its
implementation in a large scale remains experimentally challenging. This letter
provides a brief review on the experimental efforts towards the goal of global
QKD, including the security of practical QKD with imperfect devices, QKD
metropolitan and backbone networks over optical fiber and satellite-based QKD
over free space.
