The QuNET project tested a quantum communication channel using a research aircraft connected to an optical ground receiving station
The latest key experiment of the QuNET initiative was successfully completed in early October with a flight experiment between Oberpfaffenhofen and Erlangen. The aircraft formed a mobile node in a quantum network and established a connection to a ground station. There, the photons were successfully received and measured.
The technologies from the demonstrated key experiment are groundbreaking for future secure quantum communication. It is not easy to send individual photons from an aircraft in a targeted manner, capture them in a ground station, and also detect them. Researchers have now succeeded in doing this: they have even measured various quantum channels between an aircraft and a ground station several times, sent photons to an ion trap, and tested technologies for quantum key distribution.
The flight experiment took place as part of the QuNET initiative, which develops technologies for quantum-secure communication. Photons, or particles of light, can be used to generate quantum cryptographic keys that make future communication practically tap-proof. The technologies are also groundbreaking for a future quantum internet that connects quantum computers with each other.
A collaborative research effort
Scientists from the German Aerospace Center (DLR), the Max Planck Institute for the Science of Light (MPL), Friedrich Alexander University Erlangen (FAU), as well as the Fraunhofer Institutes for Applied Optics and Precision Engineering (IOF) and Heinrich Hertz Institute (HHI) participated in the experiment. They presented the results to the Federal Ministry of Research, Technology, and Space (BMFTR), which funds the QuNET initiative.
Quantum key distribution is particularly important for communication between governments and authorities, but also in general for protecting infrastructure and data in everyday life in the future. «We are working on practical solutions for satellite-based quantum communication, which can be used to transmit quantum states over long distances and generate secure keys. In fiber optics, this is only possible over a few hundred kilometers. Quantum encryption via satellite, on the other hand, enables arbitrarily greater distances on Earth», says Florian Moll from the DLR Institute of Communications and Navigation, explaining the future technology.
The current experiment
To cover long distances, satellites, aircraft, or other mobile platforms are to become part of quantum networks in the future. The current experiment was flown using a DLR research aircraft from the Flight Experiments facility. The scientists installed an optical communication terminal in the Dornier 228. The aircraft formed a mobile node in a quantum network and established a connection to an optical receiving station on the ground. This ground station is a mobile container with an integrated receiving terminal, known as the QuBUS, provided by Fraunhofer IOF in Jena.
| What they achieved Researchers successfully established a quantum communication link between an aircraft and a ground station, transmitting and detecting single photons in flight — a key step toward secure, long-distance quantum networks. |
Researchers integrated a laser!optical communication system into the Dornier 228 aircraft, which
served as a mobile node in a quantum network connecting to a ground station. The experiment was part of the QuNET initiative for quantum!secure communications. Credit: © DLR. All rights reserved.
Fraunhofer IOF responsible for tracking and fiber coupling
Research into modern systems for highly secure quantum communication has been a research focus at Fraunhofer IOF for many years. IOF researchers contributed their expertise to the latest flight campaign of the QuNET initiative on several levels. A module developed in Jena with an integrated photon pair source for generating quantum-entangled light particles flew on board the DLR research aircraft.
These particles were sent from the aircraft to the QuBUS. There, a special tracking system ensures that the ground station’s receiving terminal tracks the aircraft’s movements and maintains the connection. When the particles are exchanged through the air, atmospheric turbulence and interference inevitably occur. Adaptive optics, developed specifically in Jena, correct these distortions and ensure a stable connection.
For the current experiment, several research flights have taken place over Erlangen in recent months. The ion trap for measuring the received light particles is set up in the laboratory of the local MPL. From the QuBUS, the signals captured by the free beam were fed into a fiber optic cable. They were then forwarded to the experimental setups in the QuBUS and the MPL laboratories. The Fraunhofer researchers were also responsible for coupling the signals into the fiber optic cable. «The tracking and fiber coupling provided by Fraunhofer IOF thus offered the necessary environment for the actual experiments», explains Christopher Spiess from the Fraunhofer Institute in Jena.
Technically highly complex
Individual photons are difficult to handle. For quantum communication, researchers must generate them with high quality and ensure they remain clearly detectable even under strong external interference. To achieve the best possible results, researchers must precisely adjust the wavelength of the photons. «We have shown in various experiments that this is possible. The approach we tested can be used not only from aircraft, but also from satellites», adds Florian Moll. The states of the “flying” particles were successfully verified in measurements at the MPL ion trap – which was one of the goals of the experiment. For example, future quantum networks can use this communication technology to connect quantum memories or quantum computers.
About the QuNET initiative
The German Federal Ministry of Research, Technology and Space (BMFTR) funds QuNET (Quantum Network), a research network focused on developing highly secure communication systems based on quantum communication technologies. The German government launched QuNET in the fall of 2019 and planned to fund it for a period of seven years. The BMFTR is funding QuNET with 125 million euros. In addition to the DLR Institute of Communications and Navigation, QuNET involves the Fraunhofer Institute for Applied Optics and Precision Engineering IOF, the Fraunhofer Heinrich Hertz Institute (HHI), the Max Planck Institute for the Physics of Light (MPL), and the Friedrich-Alexander University Erlangen-Nuremberg (FAU).
QuNET aims to lay the foundations for secure and robust IT networks that are already resistant to cyber attacks of tomorrow. The security of IT communication networks is currently based primarily on mathematical assumptions. These offer protection against future technologies, such as powerful quantum computers, for example.
| Why it matters for industry Quantum communication offers unprecedented data security. These advances pave the way for satellite-based quantum networks, enabling future-proof encryption for sectors like telecom, finance, and critical infrastructure. |
The partners
Fraunhofer Institute for Applied Optics and Precision Engineering IOF
The Fraunhofer Institute for Applied Optics and Precision Engineering IOF, based in Jena, Germany, conducts research on the development of light as a means of solving a wide range of problems and application scenarios. The work of the research institute, founded in 1992, therefore focuses on application-oriented research on light generation, light guidance and light measurement.
Fraunhofer Heinrich Hertz Institute
Innovations for the digital society of tomorrow are the focus of the research at the Fraunhofer Heinrich Hertz Institute (HHI) in Berlin. Together with international partners from research and industry, Fraunhofer HHI works across the entire spectrum of the digital infrastructure – from basic research to the development of prototypes and solutions. The institute contributes significantly to the standards for information and communication technologies and creates new applications as a partner of industry.
Max Planck Institute for the Science of Light
The Max Planck Institute for the Science of Light (MPL) covers a broad spectrum of research, including nonlinear optics, quantum optics, nanophotonics, photonic crystal fibers, optomechanics, quantum technologies, biophysics and – in collaboration with the Max Planck Center for Physics and Medicine – links between physics and medicine. Some of the researchers look back on decades of experience in the field of quantum communication. They also apply telecom technology to exchange quantum keys, enabling the rapid commercialization of these procedures. In addition, the researchers from Erlangen have spent more than ten years studying how to transmit keys on the ground with laser light over several kilometers (known as a free-beam connection) or by satellite over greater distances.
DLR Institute of Communications and Navigation
The DLR Institute of Communications and Navigation conducts mission-oriented research in selected areas of communications and navigation. Its work ranges from the theoretical foundations to the demonstration of new procedures and systems in real-world environments. The institute currently employs around 200 people, including 150 scientists, at its sites in Oberpfaffenhofen and Neustrelitz. The institute develops solutions for the global networking of man and machine. It also works on high-precision and reliable positioning for future navigation applications. Additionally, it develops methods for autonomous and cooperative systems in transport and exploration. In addition, the institute is concerned with cyber security. Focal points in this area include post-quantum cryptography and the transmission of quantum keys via satellite.
Friedrich-Alexander-Universität Erlangen-Nürnberg
The research-intensive Department of Physics of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) cooperates closely with the Max Planck Institute for the Science of Light (MPL). The Chair of Optical Quantum Technologies, headed by Prof. Dr. Christoph Marquardt, focuses on the implementation of quantum protocols. Global quantum communication requires the integration of QKD devices into existing infrastructures, consisting of fiber optic networks, encryption hardware and satellite links. The chair is one of the world’s leading groups in satellite-based quantum communication.
References
- Meister, J., Kleinpaß, P., & Orsucci, D. (2025). Simulation of satellite and optical link dynamics in a quantum repeater constellation. EPJ Quantum Technology, 12.
- Krause, J., Walenta, N., Hilt, J., & Freund, R. (2025). Clock-offset recovery with sublinear complexity enables synchronization on low-level hardware for quantum key distribution. Physical Review Applied, 23.
- Häusler, S., Orsucci, D., & Moll, F. (2024). Measurement-based characterization of atmospheric background light in satellite-to-ground quantum key distribution scenarios. Optical Engineering, 63(4)
