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 video conference 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 clearly confirms the Micius satellite as a robust platform for quantum key distribution with different ground stations on Earth, and points towards an efficient solution for an ultralong-distance global quantum network.
Quantum key distribution (QKD) can in principle offer unconditional security by making use of the fundamental laws of quantum mechanics. In practice, this is typically achieved by preparing individual photons in quantum superposition states and sending them to a remote receiver. To date, most commercially available QKD systems rely on the transmission of the photons via optical fibers, which, due to channel loss and detector noise, limits the distance over which QKD is feasible to a few hundred kilometers. Alternatively, satellite-based QKD facilitates low photon loss and negligible signal disturbance and offers a viable solution for establishing a global scale quantum network. Recently, a quantum science mission of the Chinese Academy of Sciences (CAS) in collaboration with the Austrian Academy of Sciences (AAS) and the University of Vienna aimed at a satellite-based intercontinental quantum-key relay. Using the Chinese Quantum Science Satellite “Micius” as a trusted relay, a quantum network consisting of three optical ground stations located in China (Xinglong, Nanshan) and Austria (Graz-Lustbühel) has been demonstrated successfully. Here we report on the development of a quantum receiving module, installed at the Satellite Laser Ranging Station in Graz (Austria) capable of implementing the so-called decoy-state QKD protocol in a downlink scenario from the LEO satellite “Micius”. Furthermore, we will present the experimental results obtained during several downlinks from the Chinese satellite focusing on the performance of the Austrian receiving station.