Communications engineers in China are claiming a major breakthrough in security after demonstrating that a property of quantum science can be used to protect encoding of messages over long distances. Using a communications satellite called Micius, the researchers, from the University of Science and Technology of China, have transmitted quantum-entangled photon pairs to sites separated by 1200km: much farther than has ever been demonstrated before, and the first time this technique has been successfully used with space-based communication.
Many types of communication today use sophisticated encryption to keep them secret; most notably, financial information and other commercially- or security- sensitive communications. This cryptography typically uses long numbers as a key to scrambling and unscrambling the data. But as cyber hacking becomes more sophisticated, there are concerns that criminals could intercept and decode communications based on even the most complex numerical cryptography.
Quantum cryptography is an even more sophisticated method of keeping communications private. It relies on "entangled photons": particles of light, which are created simultaneously and have identical properties no matter how far apart they are. Because at the quantum level, observation changes that which is being observed, the act of eavesdropping interferes with the communication itself, indicating that the messages are not secure.
Networks using optical fibres have been used to exchange quantum-entangled messages, but even optical fibres have limits and will not work over distances of much further than 200km are. This is the significance of the Chinese discovery, described in a paper in the journal Science.
The researchers, led Jian-Wei Pan of the Heifei National Laboratory for Pphysical Sciences at the Microscale, use the Micius satellite, which was launched last August and orbits 100km above the Earth, to send entangled photons between two mountain-top stations. Theoretically. The vacuum of space should make it easier to send such messages, but in practice, clouds, dust and atmospheric turbulence meant that only one of ten million photon pairs created by the satellite’s optics was transmitted successfully; however, this efficiency is orders of magnitude greater than the best achieved by fibre-based communication. Even transmission of only one pair with their quantum properties intact is sufficient to demonstrate that the concept is sound for quantum cryptography.
“When I had the idea of doing this in 2003, many people thought it was a crazy idea,,” Pan told the BBC World Service “Because it was very challenging already doing the sophisticated quantum optics experiments in a lab - so how can you do a similar experiment at a thousand-kilometre distance and with optical elements moving at a speed of 8km/s?”
Pan now plans to team up with is former PhD supervisor, Prof Anton Zeilinger of the University of Vienna, to determine whether Micius can send entangled photons between continents. The satellite will overfly Beijing, establish a quantum key with a ground station, then establish another key with another station when it overflies Austria. Combining these two keys would establish a cryptographic link between the two distant cities.