Quantum key distribution (QKD) uses photons polarised in two different ways to encode the 0s and 1s of an encryption key. The laws of quantum mechanics ensure the transmission is secure, as any attempt to intercept the key disturbs the polarisation – a tip-off to the receiver that the key has been seen and should be discarded.
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. The team presented the work at the QCrypt conference in Singapore on 12 September.