Nobody has done that before: a quantum key exchange via free-jet connection via 19, 2 kilometers in the middle of a big city. With a quantum key, the exchange of which an eavesdropper cannot physically eavesdrop on, partners can then securely encrypt their communication on public lines.

In their experimental setup Researchers in a group led by Professor Jian-Wei Pan, University of Science and Technology in Hefei, sent key codes as polarized photons from two skyscrapers in the middle of Shanghai’s Pudong financial center to a skyscraper between them. A system there generated the interference signal for both codes and sent it back to the two transmitters; this method is called “measurement device independent quantum key distribution” (MDI-QKD).

According to BB 84 protocol, both partners can then determine the new quantum key from the interference signal in view of their own bit code. If your own bit code or the interference signal has been intercepted en route, they cannot be completely copied due to the no-cloning theorem of quantum mechanics. The recipient can statistically prove exchanged codes with certainty.

Today only six bits per second At the moment the Chinese test setup is good Researchers, however, only as a scientific study, because the key exchange rate achieved was still very low at just over six bits per second. This is also due to the suboptimal conditions above a large city, in which light reflections and air turbulence disrupt the line of sight. The researchers are working on compensating for optical errors caused by thermo-optic turbulent air layers using adaptive optics.

The technical problems of quantum-secure communication have been working since November 2019 also the German research initiative QuNET. Last December, the project partners wanted to demonstrate, among other things, the quantum key exchange between two federal authorities in Bonn using a free-jet connection. Unfortunately, this demonstration fell victim to the corona restrictions. The participating researchers at the Fraunhofer IOF (Institute for Applied Optics and Precision Mechanics) only wanted to overcome about 300 meters as the crow flies, but in their experiments they already had quantum key exchange rates of 5000 to 10.000 Kilobits per second reached. The researchers at the IOF have also gathered years of experience with the use of adaptive optics in Jena. In the first quarter of this year they are planning attempts to exchange quantum keys in the urban area, including the compensation of inner-city air turbulence.

In future, quantum-safe in public networks In the QuNET initiative, research institutes and industrial partners are working on technically consistent and robust solutions for the exchange of quantum keys, both via free-beam connections and fiber optic lines (up to 100 kilometers Distance) and later also over long distances with satellite support. The advantage of quantum communication lies in the quantum-secure keys, the exchange of which an eavesdropper cannot overhear due to quantum mechanical effects such as the no-cloning theorem. The partners can then use the quantum secure key themselves for secure communication via public communication networks.

(agr)