  Satellite based quantum communication has been proven as a feasible way to
achieve global scale quantum communication network. Very recently, a
low-Earth-orbit (LEO) satellite has been launched for this purpose. However,
with a single satellite, it takes an inefficient 3-day period to provide the
worldwide connectivity. On the other hand, similar to how the Iridium system
functions in classic communication, satellite constellation (SC) composed of
many quantum satellites, could provide global real-time quantum communication.
In such a SC, most of the satellites will work in sunlight. Unfortunately, none
of previous ground testing experiments could be implemented at daytime. During
daytime, the bright sunlight background prohibits quantum communication in
transmission over long distances. In this letter, by choosing a working
wavelength of 1550 nm and developing free-space single-mode fibre coupling
technology and ultralow noise up-conversion single photon detectors, we
overcome the noise due to sunlight and demonstrate a 53-km free space quantum
key distribution (QKD) in the daytime through a 48-dB loss channel. Our system
not only shows the feasibility of satellite based quantum communication in
daylight, but also has the ability to naturally adapt to ground fibre optics,
representing an essential step towards a SC-based global quantum network.
