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 $\sim \mathrm{kHz}$ 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.

Measurement-device-independent quantum key distribution (MDI QKD) removes all detector side channels and enables secure QKD with an untrusted relay. It is suitable for building a star-type quantum access network, where the complicated and expensive measurement devices are placed in the central untrusted relay and each user requires only a low-cost transmitter, such as an integrated photonic chip. Here, we experimentally demonstrate a 1.25-GHz silicon photonic chip-based MDI QKD system using polarization encoding. The photonic chip transmitters integrate the necessary encoding components for a standard QKD source. We implement random modulations of polarization states and decoy intensities, and demonstrate a finite-key secret rate of $31\mathrm{bit}/\mathrm{s}$ over 36-dB channel loss (or 180-km standard fiber). This key rate is higher than state-of-the-art MDI QKD experiments. The results show that silicon photonic chip-based MDI QKD, benefiting from miniaturization, low-cost manufacture, and compatibility with CMOS microelectronics, is a promising solution for future quantum secure networks.

Polarization modulation plays a key role in polarization-encoding quantum key distribution (QKD). Here, we report a new, to the best of our knowledge, polarization modulation scheme based on an inherently stable Sagnac interferometer. The presented scheme is free of polarization mode dispersion and calibration as well as insensitive to environmental influences. Successful experiments at a repetition frequency of 1.25 GHz have been conducted to demonstrate the feasibility and stability of the scheme. The measured average quantum bit-error rate of the four polarization states is as low as 0.27% for 80 consecutive minutes without any adjustment. This high-speed intrinsically stable polarization modulation can be widely applied to many polarization-encoding QKD systems, such as BB84, MDI, etc.