On September 29, 2017, a Chinese satellite known as Micius made possible an unhackable videoconference between Vienna and Beijing, two cities half a world apart. As it whisked across the night sky at 18,000 miles (29,000 kilometers) per hour, the satellite beamed down a small data packet to a ground station in Xinglong, a couple of hours’ drive to the northeast of Beijing. Less than an hour later, the satellite passed over Austria and dispatched another data packet to a station near the city of Graz.
The packets were encryption keys for securing data transmissions. What made this event so special was that the keys distributed by the satellite were encoded in photons in a delicate quantum state. Any attempt to intercept them would have collapsed that state, destroying the information and signaling the presence of a hacker. This means they were far more secure than keys sent as classical bits—a stream of electrical or optical pulses representing 1s and 0s that can be read and copied.