Moving plane exchanges quantum keys with Earth: Flying at a height of 20 kilometres and a speed of nearly 300 km per hour, the challenge was to tightly align the infrared laser pulses transmitted by the aircraft with the receiving station on the ground. Any deviation would limit the number of photons reaching the target.
The researchers kept the laser on track using moving mirrors both in the aircraft and on the ground. Performing the experiment shortly after sunset avoided interference from sunlight. The transmission lasted for 10 minutes, amounting to a key long enough to encrypt 10 kilobytes of data.