Generally speaking, there are mainly two types of quantum channels, fiber links and free-space links. Considering absorptive losses in the channel and dark counts in the detection, the distances of current fiber-based QKD experiments are limited to hundreds of kilometers. For free-space links, considering the photon loss and decoherence is almost negligible in the outer space, satellite-to-ground QKD is acknowledged as one of the most promising way to run large-scale quantum communication schemes. On the other hand, we can also implement experiments via free-space optical links to research some fundamental problems in quantum physics such as testing quantum non-locality, showing quantum teleportation.