  Quantum key distribution (QKD) uses the fundamental principles of quantum
mechanics to share unconditionally secure keys between distant users. Previous
works based on the quantum science satellite "Micius" have initially
demonstrated the feasibility of a global QKD network. However, the practical
applications of space-based QKD still face many technical problems, such as the
huge size and weight of ground stations required to receive quantum signals.
Here, we report space-to-ground QKD demonstrations based on portable receiving
ground stations. The weight of the portable ground station is less than 100 kg,
the space required is less than 1 m$^{3}$ and the installation time requires no
more than 12 hours, all of the weight, required space and deployment time are
about two orders of magnitude lower than those for the previous systems.
Moreover, the equipment is easy to handle and can be placed on the roof of
buildings in a metropolis. Secure keys have been successfully generated from
the "Micius" satellite to these portable ground stations at six different
places in China, and an average final secure key length is around 50 kb can be
obtained during one passage. Our results pave the way for, and greatly
accelerate the practical application of, space-based QKD.
