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.
We report on entanglement-based quantum key distribution between a low-Earth-orbit satellite equipped with a space borne entangled-photon source and a ground observatory. One of the entangled photons is measured locally at the satellite, and the other one is sent via a down link to the receiver in the Delingha ground station. The link attenuation is measured to vary from 29 dB at 530 km to 36 dB at 1000 km. We observe that the two-photon entanglement survives after being distributed between the satellite and the ground, with a measured state fidelity of ≥0.86. We then perform the entanglement-based quantum key distribution protocol and obtain an average final key rate of 3.5 bits/s at the distance range of 530-1000 km.
Geosynchronous Earth orbit (GEO) satellites can extend the transmission distance of the quantum states and the intercrossing time between the satellites and the ground, which will greatly promote the application spatial scale of quantum communications. In this work we report the first experimental test of transmission of photonic polarization states from a GEO SJ-20 satellite at more than 36,000 km distance to the Li-jiang ground station at Yun-nan Observatory. The test equipment carried on the satellite can send four kinds of photonic polarization states at 50 MHz pulse frequency. A 1.76 m telescope and related optical system in the ground station are used to receive the photonic polarization states, and then these polarization states are probed by the superconducting nanowire single photon detector. The test results show that the optical attenuation between the GEO satellite and the ground station is 100 ˜ 110 dB and the channel bit error rate is 2%–8%, which meet the security criteria of quantum key distribution and quantum secure direct communication.