Quantum cryptography takes flight

Quantum cryptography has entered the friendly skies. A precise beam of photons sent from an airplane allowed researchers on the ground to create a nearly unbreakable encryption key to protect information. The experiment, reported March 31 in Nature Photonics, is an important step toward creating a secure global communication network based on beaming photons to and from satellites.

“It is a technical tour de force,” says Seth Lloyd, a mechanical engineer at MIT who was not involved in the study.

The idea of exploiting quantum mechanics to secure communication lines is not new: Since the 1980s, scientists have created encryption keys with sets of photons whose individual spins are known only to the sender and receiver. If a third party eavesdrops on the conversation, the photon spins change, alerting the communicating parties to stop exchanging information.

The impediment to any quantum communication, however, is establishing a robust connection that allows sender and receiver to detect and manipulate photons one by one. That’s hard enough to accomplish between stationary points (SN: 6/30/12, p. 10), but physicist Sebastian Nauerth of Ludwig Maximilians University of Munich and his team beamed the photons from an airplane flying at nearly 300 kilometers per hour.

The researchers installed a laser at the bottom of the plane that sent a narrow beam of photons to a ground station 20 kilometers away. The signal was strong enough and the tracking precise enough that a sender on the plane and a receiver on the ground would be able to establish a quantum encryption key, the researchers report. Such a key could one day be used to decode messages sent from anywhere in the world and relayed via satellite.