Abstract: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.With the growth of internet use and electronic commerce, a secure global network for data protection is necessary. A drawback of traditional public key cryptography is that it is not possible to guarantee it is information theoretically secure. It has been witnessed in history that every advance of encryption has been defeated by advances in hacking. In particular, with the advent of Shor's factoring algorithm [1], most of the currently used cryptographic infrastructure will be defeated by quantum computers.On the contrary, quantum key distribution (QKD) [2] offers unconditional security ensured by the law of physics. QKD uses the fundamental unit of light, single photons, encoded in quantum superposition states which are sent to a distant location. By proper encoding and decoding, two distant parties share strings of random bits called secret keys. However, due to photon loss in the channel, the secure QKD distance by direct transmission of the single photons in optical fibers or terrestrial free space was hitherto limited to a few hundred kilometers [3][4][5][6][7]. Unlike classical bits, the quantum signal in the QKD cannot be noiselessly amplified owing to the quantum no-cloning theorem [8], already contained at the core of Wiesner's proposal of uncopiable quantum money [9], where the security of the QKD is rooted.The main challenge for a practical QKD is to extend the communication range to long distances, ultimately on a global scale. A promising solution to this problem is exploiting satellite and space-based links [10,11]. That way, one can conveniently connect two remote points on Earth with greatly reduced channel loss because most of the photons' propagation path is in empty space with negligible loss and decoherence. In this work, QKD is performed in a ...
