From Scientific American:

By Karen Kwon

The launch of the Chinese satellite Micius in 2016 could have been viewed as merely a single addition to the 2,700-odd instruments already orbiting Earth. But Micius, which is solely dedicated to quantum information science, arguably represents the nation’s lead in an emerging contest among great powers at the frontiers of physics. The brainchild of physicist Jian-Wei Pan of the University of Science and Technology of China, the satellite has helped him and his colleagues achieve several groundbreaking results that are bringing the once esoteric field of quantum cryptography into the mainstream. Pan’s team presented a secure method of quantum messaging using Micius in a new paper, published on June 15 in Nature. The achievement brings the world—or China, at least—one step closer to realizing truly unhackable global communications.

In 2017 the team, along with a group of researchers in Austria, was able to employ the satellite to perform the world’s first quantum-encrypted virtual teleconference between Beijing and Vienna. Despite being a huge milestone, this method was not bulletproof against hacking. Micius itself was the weak point: The satellite “knew” the sequences of photons, or keys, for each location, as well as a combined key for decryption. If, somehow, a spy had carefully eavesdropped on its activity, the integrity of the teleconference could have been compromised.

To overcome this problem, the new demonstration by Pan and his colleagues ensured that Micius would not “know” anything. The trick was to avoid using the satellite as a communications relay. Instead the team relied on it solely for simultaneously transmitting a pair of secret keys to allow two ground stations in China, located more than 1,120 kilometers apart, to establish a direct link. “We don’t need to trust the satellite,” Pan says. “So the satellite can be made by anyone—even by your enemy.” Each secret key is one of two strings of entangled photon pairs. The laws of quantum physics dictate that any attempt to spy on such a transmission will unavoidably leave an errorlike footprint that can be easily detected by recipients at either station.

This is the first time the technique—called entanglement-based quantum-key distribution—has been demonstrated using a satellite.

Even though this is the first time that entanglement-based quantum-key distribution has been performed via satellite, there have been successful ground-based experiments. In ground-based quantum communications, however, the optical fibers that connect two locations absorb transmitted photons, and the rate of absorption increases over distance. “Trusted nodes” placed along the fibers decrypt and reencrypt keys to extend the key-transfer distance. But like Micius in the 2017 demonstration, each of these intermediaries possesses all the quantum keys and is thus vulnerable to hacking. Although prototype devices called quantum repeaters offer better security, the technology is not yet advanced enough to be practical. In comparison, because signals from a satellite travel through empty space most of the time, photon loss is less of a concern—allowing secure transmissions across arbitrarily large distances.

That situation does not mean that the satellite-based system is inherently better than the ground-based one. “It’s kind of apples and oranges,” says Paul Kwiat, a physicist at the University of Illinois at Urbana-Champaign, who was also not involved in the study. “The satellite has a couple of problems. One is there aren’t many [quantum research] satellites that are flying at the moment. Two, those satellites are not always parked over your own telescopes that you want.”

Read more at Scientific American.

Photo taken on Nov. 26, 2016 shows a satellite-to-earth link established between quantum satellite “Micius” and the quantum communication ground station in Xinglong, north China’s Hebei Province. Credit: Jin Liwang Alamy