The construction of a world-spanning quantum Internet for secure communication have made good progress. Researchers from the Chinese Academy of Sciences, it is now in a free country attempt succeeded, a secret quantum key between two approximately 1120 kilometres away from each other remote ground stations, to exchange, without the need for the Code in between to refresh. So you could increase the range of quantum cryptographic encryption methods are about ten times, write Jian-Wei Pan, of the Chinese University of science and technology in Hefei, and his colleagues in the journal “Nature”. As a relay station of the Chinese satellite “Micius” circled for the past four years, the earth’s in an altitude of around 500 kilometres, and for thought was used to test modern methods of quantum cryptography.
Manfred Lindinger
editor in the Department “nature and science”.
F. A. Z.
For the most recent experiments were generated on Board by Micius with lasers and special optics, pairs of entangled photons and sent to the ground stations in Shenzhen (Guangdong province) and Delhi (Qinghai province). Each Station is an infra-red photon pair shared. On-site measurements, the perfectly coordinated behavior of the entangled particles of light occupied. The polarization of a photon was measured, for example, in Shenzhen, the vibration direction of the entangled partner in Delhi was in a moment. By repeated measurement of the polarization of the incoming photons to the ground station was able to generate in Shenzhen, a binary Code that was used by the researchers as a secure quantum key. Due to the entanglement of light particles, the 1120 kilometres away, in Delhi, had automatically via an identical key.
it was in Delhi possible to decipher a in Shenzhen encrypted and a glass-fiber-transmitted message, and to read. An eavesdropper, the, would anzapften to get to the key, a light quantum of Micius, would have betrayed it immediately. Through his eavesdropping he would make a measurement, what would notice the two ground stations immediately. This is the reason why quantum cryptography is regarded among experts as safe.
Free in the air, the empty space
The distribution of entangled photons to multiple stations is, however, a technical challenge: light particles, which take their way through the atmosphere are scattered by the air molecules, or absorbed in, what the range is limited. On earth, the achieved maximum distance for the exchange of a quantum key on the Basis of entangled photons is approximately 144 kilometers. With the so-called quantum repeaters, which refresh the signals, let the distance while zooming in. Each Repeater constitutes a security risk for the encrypted communication. In addition, these systems are not technically advanced yet.
A way out of this satellite as “Micius offer”. The on-Board generated photons need to overcome only a few kilometers through the atmosphere to the ground stations. In space can propagate the light particle unhindered. The two approximately 1120 kilometres distant ground stations in Shenzhen, and Delhi are each 2000 or 3000 meters above the sea level. This enabled because of the thinner air better communication with the satellite, according to the researchers.
In the Experiment, which took place from September to October, 2018, sent Micius every day at two o’clock in the morning for five minutes with his infra-red light quanta at a Rate of approximately six Megahertz to the earth. Of these, only a fraction is reached, however, the two ground stations, there are enough photons, but, to each pre-flight a quantum key, despite the strong noise signals. These researchers are subjected to numerous Tests, with the aim of possible loopholes to uncover, which would have compromised the safety of their procedure in question.
