Quantum key skilled satellite makes tap-proof video call

Orbital Quantum Encryption: For the first time, researchers have encoded an intercontinental video conference using quantum cryptography. The key photons were sent to the earth by the quantum satellite Micius. This enabled researchers in Beijing and Vienna to communicate with each other over two continents. Thus, the first orbital-planetary quantum network has passed its field test.

Ground stations in Austria and China have encrypted their communications via the Chinese quantum satellite "Micius". © Johannes Handsteiner / ÖAW

Quantum cryptography is considered to be an almost tap-proof encryption, because it makes use of the quantum-physical phenomenon of entanglement for the transmission of the keys. The states of photons are coupled to each other in such a way that each measurement or manipulation of the one light particle also changes the other instantaneously and even over vast distances.

Quantum satellite as a relay

So far, however, such quantum information could only be sent over a limited distance. In glass fibers the range is little more than 100 kilometers - too little for intercontinental communication.

However, this is intended to change with quantum satellites, which generate and send as relay stations the keys encoded with entangled photons. In August 2016, the first satellite of this type was launched from China to earth orbit. "Micius" circulates at an altitude of around 500 kilometers and has demonstrated that it can generate quantum messages and send them to two ground stations that are far away from each other, just a few months ago.

Field test passed

Now the next test of this orbital-planetary quantum network has been successful: For the first time researchers have conducted a video conference between Beijing and Vienna, which was secured with a quantum key of the satellite.

A worldwide and secure quantum internet is thus a decisive step closer, says Anton Zeilinger of the Austrian Academy of Sciences (ÖAW).

For this, "Micius" first produced a key in the form of entangled polarized photons and sent it to a ground station in Graz.

If someone tries to intercept the photons exchanged between the satellite and the ground station and measures the polarization, then it changes the quantum-physical state of the particles through the measurement - and gets busted immediately, explains Johannes Handsteiner from the Vienna Institute of Quantum Optics and quantum information.

Key exchange between Earth and orbit

In the ground station, researchers determine the polarization state of some of these photons, thereby also changing the state of the photon message remaining in the satellite. This creates the first quantum key. At the same time, Chinese physicists are doing the same thing, so there are now two quantum keys on the satellite.

Both keys are now combined in orbit and the result of the combination is transmitted back to the ground stations in Austria and China. With the respective "own" key on the one hand and the combined key on the other hand, both ground stations can now generate a common code, which can be used for the clear ciphering and deciphering of information - and therefore for the secure encryption of the "quantum telephone".

Great potential

On September 29, 2017, physicists in Beijing and Vienna used this method for the first time to encrypt a video conference between them. Although the video data ran over a normal Internet connection, only the two video partners could decrypt the data - thanks to their quantum key received from the orbit.

Through the quantum encryption, the security of the conversation was at least one million times higher than in conventional cryptography, as the physicists explain.

The successful exchange of quantum-encrypted information between two continents illustrates the enormous potential of this technology, which is made possible by basic research, emphasizes Zeilinger.

Source: Austrian Academy of Sciences