Perfectly secure cryptography requires no technology more exotic than pencil and paper: Choose a random string of letters and numbers to serve as the key for an enciphered message. Write the key down on a piece of paper, use it once and then burn the paper. The trick is to make sure no one intercepts or tampers with the key—and on the Internet, keys are stolen or tampered with all the time.
Quantum-key distribution (QKD) solves this problem by creating a single-use key out of entangled photons—particles of light whose quantum states are linked. Any disturbance of one particle instantaneously mirrors itself in the other one, no matter how far away. The problem with QKD is that no one has figured out how to transmit entangled photons over large distances. This past August, however, the Chinese Academy of Sciences took a big step toward solving that problem when it put the world's first quantum satellite into orbit.
The program, called Quantum Experiments at Space Scale (QUESS), is a collaboration with the Austrian Academy of Sciences. The idea is to use the satellite to transmit quantum keys to two observatories on opposite sides of China, about 1,200 kilometers apart—more than eight times farther than the current distance record. According to Scientific American editorial board adviser and physicist Anton Zeilinger, whose team set that record in 2012 (and who is currently collaborating with his former student Jian-Wei Pan, now QUESS's chief scientist), a space-based platform was the only option. “Tell me any location on Earth where you can look 1,000 kilometers,” he says.
If the Chinese researchers set a new distance record, future satellites could provide an orbiting platform for an unhackable “quantum Internet,” in which packets of encrypted data have their security vouchsafed by the laws of physics. “We hope to establish intercontinental quantum communication,” Zeilinger explains. “It's not just a science-fiction idea—it's how computers in the future will talk to each other.”