  We perform decoy-state quantum key distribution between a low-Earth-orbit
satellite and multiple ground stations located in Xinglong, Nanshan, and Graz,
which establish satellite-to-ground secure keys with ~kHz rate per passage of
the satellite Micius over a ground station. The satellite thus establishes a
secure key between itself and, say, Xinglong, and another key between itself
and, say, Graz. Then, upon request from the ground command, Micius acts as a
trusted relay. It performs bitwise exclusive OR operations between the two keys
and relays the result to one of the ground stations. That way, a secret key is
created between China and Europe at locations separated by 7600 km on Earth.
These keys are then used for intercontinental quantum-secured communication.
This was on the one hand the transmission of images in a one-time pad
configuration from China to Austria as well as from Austria to China. Also, a
videoconference was performed between the Austrian Academy of Sciences and the
Chinese Academy of Sciences, which also included a 280 km optical ground
connection between Xinglong and Beijing. Our work points towards an efficient
solution for an ultralong-distance global quantum network, laying the
groundwork for a future quantum internet.
