The call was part of the QUESS (Quantum Experiments at Space Scale) project at the University of Vienna and uses the Chinese Micius satellite which was launched into space last year to suport quantum physics experiments.
Chunli Bai, the President of the Chinese Academy of Sciences, talked with Anton Zeilinger, the President of the Austrian Academy of Sciences and Heinz W. Engl, the Rector of the University of Vienna, over the link.
The Micius satellite sends photons to ground stations in China and Europe, including the Satellite Laser Ranging Station used by the Austrian Academy of Sciences’ Institute for Space Research in Graz. Using Micius as an orbital relay station avoids the technical limitations in quantum communication on the surface of the earth which are caused by the earth’s curvature and the signal loss in long fibre optic cables.
To produce the quantum key, Micius generated photons with random polarisation that were tehn transmitted as a sequence of ones and zeros to the ground station near Graz. There, the polarization states were measured and compared randomly with the sequence sent by the satellite.
"If somebody attempts to intercept the photons exchanged between the satellite and the ground station and to measure their polarization, the quantum state of the photons will be changed by this measurement attempt, immediately exposing the hackers," said researcher Johannes Handsteiner at the Vienna Institute for Quantum Optics and Quantum Information. The site hosts the Hedy Lamarr Quantum Communication Telescope which was used for the demonstration.
Any deviation of data measured at transmitter and receiver therefore enables immediate discovery of any eavesdropping attack. If the measured data are consistent, then sender and receiver have a first quantum key.
Then the Chinese ground station performed the same procedure, so that Micius ultimately had two quantum keys. These were then combined in orbit and the result of the combination was transferred back to each of the ground stations in Austria and China.