Quantum key distribution 1,2 (QKD) is the first commercial application in the new field of quantum information with first routine applications in governmental and financial sectors 3 and with successful demonstrations of trusted node networks 4,5. Today, the grand goal is efficient long range key distribution either via quantum repeaters 6 or via satellites 7-9 in order to enable global secure communication. On the way to QKD via satellites a free-space demonstration of secure key distribution was performed over 144 km between two ground stations 10. This scenario is comparable to links between satellites in low earth orbits (LEO) and ground stations with respect to both attenuation and fluctuations. However, we still miss key exchange with rapidly moving platforms. Here we prove for the first time the feasibility of BB84 quantum key distribution between an airplane and a ground station. Establishing a stable and low noise quantum communication channel with the plane moving with 290 km/h at a distance of 20 km, i.e., 4 mrad/s, our results are representative for typical communication links to satellites 11 or to high altitude platforms. Quantum key distribution provides a whole new level of information security. Any information gained by eavesdropping on the quantum channel can be quantified by the observed transmission noise, the quantum bit error ratio (QBER) 12. Security proofs, solely based on the laws of quantum mechanics, show how to determine the necessary amount of privacy amplification (i.e., key shrinkage according to the QBER) to eliminate the knowledge of a possible adversary 13. Starting with a first quantum channel of 30 cm length in 1989 14 quantum key distribution quickly was enabled on successively longer distances. Two main branches for communicating qubits encoded with quantum states of light were established: either via telecom fiber channels or via free-space transmission. In both cases increasing attenuation and noise limit the maximum distance for a successful key distribution to typically 150−200 km 10,15,16. So far, long range free-space quantum communication experiments used a direct line of sight either between two Canary Islands (144 km) or across a lake in China (95 km) to demonstrate free-space QKD, entanglement distribution 10,17-19 , or quantum teleportation 20,21. In spite of this remarkable progress, all quantum communication so far was performed with stationary systems only. Contrary, for classical optical free-space communication high bandwidth links to aircrafts and satellites have been shown to be feasible in recent years 11,22,23. Here we report on an experiment combining recent advances in classical and in quantum optical technologies to demonstrate the feasibility of quantum key distribution from an airplane to ground (fig. 1). Major challenges in this experiment are the higher pointing requirements compared to classical free-space communication, the development of a precise compensation technique to account for the relative rotations of airborne and ground station qubit en...

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Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center (DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2° and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the experiment and highlights the results of the measurement trials.

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Real-time monitoring allows new possibilities in applications like disaster management or traffic observation and guidance. The German Aerospace Center is currently developing an aircraft based observation system. Among other sensors a high resolution camera platform together with an optical downlink terminal is an integral part of the system. The optical terminal was tested in the first stage of expansion in November and December 2008. At distances up to 85 km the achieved mean tracking offset with pure CPA tracking was 266 µrad. Initial communication tests have been successfully performed up to a distance of 40 km.