China's quantum communication ground station in Lijiang, Southwest China's Yunnan Province.
New technology can be used in defense, communication
Chinese scientists have sent a hack-proof code from a satellite to the Earth, becoming the first in the world to realize quantum key distribution (QKD) from space to the ground.
The achievement, based on experiments conducted with the world's first quantum satellite, Quantum Experiments at Space Scale (QUESS), was published in the authoritative academic journal Nature on Thursday.
The Nature reviewers commented that the experiment was an impressive achievement, and constituted a milestone in the field.
Nicknamed "Micius," after a 5th Century BC Chinese philosopher and scientist who has been credited as the first person ever to conduct optical experiments, the 600-kilogram-plus satellite was sent into a sun-synchronous orbit at an altitude of 500 kilometers on August 16, 2016.
"We are very pleased with the achievements as we had originally scheduled to complete them in two years rather than only one," Pan Jianwei, lead scientist of QUESS and a Chinese Academy of Sciences academician, was quoted as saying by China Central Television (CCTV).
An assistant researcher on Pan's team, who requested anonymity, told the Global Times on Thursday that the technology will not be exclusively used in the fields of national defense and finance.
However, as the cost of quantum communication is still high, and due to the nature of areas like national defense where confidentiality is crucial, the application of the technology in these areas may come before other fields, the researcher said.
The satellite sent quantum keys to ground stations in Xinglong, in North China's Hebei Province, and Nanshan, near Urumqi, capital of northwest China's Xinjiang Uyghur Autonomous Region, the Xinhua News Agency reported.
The communication distance between the satellite and the ground station varies from 645 kilometers to 1,200 kilometers, and the quantum key transmission rate from the satellite to ground is up to 20 orders of magnitude more efficient than that expected using an optical fiber of the same length, Pan said.
When the satellite flies over China, it provides an experiment window of about 10 minutes. During that time, the 300 Kbit secure key can be generated and sent by the satellite, he said.
"That, for instance, can meet the demand of making an absolutely safe phone call or transmitting a large amount of bank data," Pan said.
In an e-mail sent to the Global Times on Thursday, Pan's team from the University of Science and Technology of China (USTC) wrote that in the QKD, the information is encoded so that any eavesdropper on the quantum channel that attempts to gain information about the key will inevitably introduce disturbances in the system, and this will be detected by the communicating users.
"Satellite-based quantum key distribution can be linked to metropolitan quantum networks where fibers are sufficient and it is convenient to connect numerous users within a city over 100 kilometers. We can thus envision a space-ground integrated quantum network, enabling quantum cryptography - most likely the first commercial application of quantum information, which is useful at the global scale," Pan said.
The establishment of a reliable and efficient space-to-ground link for faithful quantum state transmission paves the way for global-scale quantum networks, he added.
Theoretically, quantum communication using QKD is unbreakable, which has been proved mathematically and in physics, the assistant researcher explained.
Pan also revealed that Chinese scientists will work with their counterparts from European countries, including those from Italy and Germany, to explore the possibility of inter-continental QKD.
"We have begun to do the satellite-to-ground quantum key distribution between the Micius satellite and an Austrian ground station, and we will finish the tests this month," read the e-mail.
Apart from the research experiments, the practicality and application of the technology is another focus for the Chinese scientists.
"Talks have been held between the research team and potential buyers of the technology. We are trying to realize the use of QKD in mobile targets such as ships at sea and foreign-based institutes," Pan told CCTV.
Pan also noted that to enhance practicality, just like the Beidou (China's GPS system) satellites, there should be a constellation of such satellites to build a global hack-proof communications network.
"We plan to launch satellites to higher orbits and construct a satellite constellation. We've begun to develop new techniques to increase the link efficiency, including larger-size telescopes, better acquiring, pointing, and tracking systems, and wave-front correction through adaptive optics," read the e-mail.