Quantum keys fiber-linked over 75 kilometers

It is a milestone for the research of high-security quantum communications in Germany: on a test route between Jena and Erfurt, quantum keys have been successfully exchanged for the first time over a distance of 75 kilometers via optical fiber. More than 300,000 quantum keys were sent between the Thuringian cities over a test period of ten days.

The test route, funded by the state, had already been completed in the spring of this year. It connects Fraunhofer IOF in Jena with the Fraunhofer Center for Microelectronic and Optical Systems for Biomedicine (MEOS) in Erfurt. It uses conventional optical fiber and therefore can act as the basis for a nationwide implementation of quantum systems in the future. About six kilometers of the test routes run above ground, allowing scenarios of already existing networks to be simulated.

The first company that has now successfully exchanged quantum keys on the fiber test route with the support of Fraunhofer IOF is Quantum Optics Jena GmbH. In 2020, the young start-up had spun off from the Fraunhofer Institute and has since been developing plug-in solutions for quantum communications. “We see that the system works differently in the real field than in the laboratory,” says CEO Kevin Füchsel.

“In the lab, we transmit at about 300 bits per second with comparable losses. That gives us one encryption key per second. In the field, we're at about 200 bits, so a little less. A key is 256 bits long and can therefore be renewed almost every second for cryptographic protection, i.e., encryption and decryption, of the transmitted information. It is precisely this fast, tap-proof and automated key renewal that distinguishes quantum key distribution from established methods,” Füchsel continues.

Quantum Optics Jena is planning a second test run in the near future to further deepen the results obtained from the first test run and to develop further solutions.