Jian-Wei Pan nearly wept as he watched a Long March rocket lift off from China’s Jiuquan Satellite Launch Center in the Gobi Desert in August 2016. The rocket carried the Quantum Science Satellite, which Pan and coworkers at the University of Science and Technology of China (USTC) had spent five years building and refining. He had spent twice that long lobbying to get the $100 million project funded. To see it finally off the ground, he says, “was truly a relief.”

The satellite, nicknamed Micius after the ancient Chinese philosopher, was to be the hub of an experimental quantum communications network. To start, it would take turns beaming pairs of polarization-entangled photons from its low-Earth, Sun-synchronous orbit to four ground stations throughout China (see figure 1). Such transmissions would allow two stations to secretly exchange information: Quantum mechanics posits that a measurement on one of a pair of...

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