Laying The Foundation For A Future Quantum Network – Quantum Channels Tested In Flight

• A research aircraft has served as a mobile node in a quantum network, establishing a connection to a ground station.
• Photons were received and measured on the ground.
• Technologies from the flight experiment are paving the way for future secure quantum communication.
• Focus: Aviation, space, security, quantum technology

It is not easy to transmit individual photons precisely from an aircraft, capture them in a ground station and detect them successfully. Researchers have, however, now managed to do exactly that. They have even measured, multiple times, various quantum channels between an aircraft and a ground station, sent photons to an ion trap and tested technologies for quantum key distribution (QKD). The flight experiment in question took place as part of the QuNET initiative, aimed at developing quantum-secure communication. Using photons – particles of light – it is possible to generate quantum encryption keys, which will make future communication eavesdrop-proof. The technologies behind this are also paving the way for a future quantum internet, connecting quantum computers with one another.

Researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), the Max Planck Institute for the Science of Light (MPL), the Friedrich-Alexander-Universität (FAU), the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) and the Fraunhofer Heinrich-Hertz-Institut (HHI) were involved in the experiment. Their findings have since been presented to Germany's Federal Ministry of Research, Technology and Space (BMFTR) – the QuNET initiative sponsor. Quantum key distribution is particularly important for secure communication between governments and public authorities, but also for protecting infrastructure and everyday data in the future.

"We are working on practical solutions for satellite-based quantum communication, thanks to which we can transmit quantum states over large distances and generate secure keys. In fibre optics, this is only possible over a few hundred kilometres. Quantum encryption via satellite, on the other hand, enables distances of any length on Earth," says Florian Moll from the DLR Institute of Communications and Navigation, explaining this pioneering technology. To bridge long distances, satellites, aircraft or other mobile platforms will become part of future quantum networks.

The current experiment used a DLR research aircraft from the Flight Experiments facility. The researchers installed an optical communication terminal into the Dornier 228 aircraft, which during the flights acted as a mobile node in a quantum network and established a connection with a ground station. The receiving station in this case was a mobile container with an integrated receiving terminal – the 'QuBUS', provided by the Fraunhofer IOF.

Technically highly complex
Individual photons are difficult to handle. For quantum communication, they must be generated with a high quality and clearly detected despite strong external disturbances. To achieve optimum results, the wavelength of the photons also needs to be precisely calibrated. "We have shown in the various tests conducted this is possible. The approach we have tested is not only feasible from within aircraft, but also from satellites," adds Florian Moll.

For the current experiment, multiple research flights took place over the German city of Erlangen, as the ion trap used is located in the laboratories of the MPL institute there. The photons transmitted from the aircraft were directed at the ground station and into a glass fibre-optic cable, which connected to the measuring equipment for the various experiments. The states of the 'flying' particles could be detected in measurements at the ion trap – one of the experiment's objectives. This communication technology can also be used to connect quantum memories or quantum computers within a future quantum network.

Secure communication for the future
Systems were installed both in the aircraft and the ground station at Erlangen in order to carry out quantum key distribution (QKD) experiments. These systems are revolutionising satellite-based quantum communication. For example, a system for clock-channel-free quantum key distribution was tested. What's more, the researchers also detected photons from an entanglement source on the ground. Channel measurements and component tests for QKD systems with innovative and flexibly configurable protocols also provided valuable insights for further evolving secure communication of the future.

About QuNET
QuNET (Quantum Network) is a pilot initiative funded by the German Federal Ministry of Research, Technology and Space (BMFTR) to explore ultra-secure communication systems based on quantum communication technologies. Launched in autumn 2019, QuNET is planned to run for seven years, with 125 million euros in funding from the BMFTR. Alongside the DLR Institute of Communications and Navigation, the participating organisations include the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF), the Fraunhofer Heinrich-Hertz-Institut (HHI), the Max Planck Institute for the Science of Light (MPL) and the Friedrich-Alexander-Universität (FAU).

QuNET seeks to create the foundation for secure and robust IT networks that are fully prepared for the cyber threats of tomorrow. The security of today’s IT communication networks is primarily founded on mathematical assumptions, which will not provide sufficient protection against future technologies such as powerful quantum computers.

The DLR Flight Experiments facility, which operates multiple DLR research aircraft, was involved in planning and conducting the tests with the mobile nodes. The Dornier DO 228 is used in the QuNET project.

Research on optical data transmission at the DLR Institute of Communications and Navigation
The DLR Institute of Communications and Navigation can look back on more than 20 years of research and pioneering results in the field of free-space optical data transmission. For QuNETx, its researchers contribute their expertise in satellite-based quantum communication and are responsible for the free-space communication channel, which transmits the quantum states between satellites and Earth. The demands placed on signal quality and transmission efficiency constitute some of the greatest challenges in this context, which is why the transmission and receiving optics on the satellite and ground must therefore be designed so that the quantum states are transmitted with minimal interference and the beam alignment can be configured with extreme precision. DLR's research also involves detailed channel characterisation to quantify its influence on state transmission.

Transferring the developed technologies and know-how into industry applications is one of the DLR's core objectives and the centre is firmly pursuing this goal. Initial flight campaigns with optical links to ground stations for telecommunications and quantum communication were already carried out in 2009. The DLR Institute of Communications and Navigation has sites in Oberpfaffenhofen and Neustrelitz.