Boffins send encrypted quantum messages to spaaaace – and back

It may be possible to send quantum-encrypted messages through space, after physicists showed a beam of light sent to a satellite could return to Earth with its quantum properties intact, according to new research published in Physical Review Letters.

Quantum cryptography relies on the properties of quantum mechanics to encode and decode information.

Researchers from the University of Padua in Italy studied a property known as quantum interference. Just like sound waves can interfere with one another causing the noise level to increase or decrease, photons – the particle form of light – can interfere with themselves.

Quantum interference is a measure of the likelihood that the particle will appear in a certain place.

First, the scientists split a photon into two and shone the two halves through an optical apparatus that would allow them to recombine later on. The photons travelled different paths, where one is longer than the other, so when a photon emerged from the other side, it splits into two different wave packets – also known as 'temporal superposition'. One wave packet trails behind the other at a few billionths of a second.

Next, using a telescope, the photons were beamed upward to a moving satellite. Reflectors on the satellite reflected the photons back through the optical apparatus to reach a detector. Due to variations caused by quantum interference and the velocity of the satellite, the photons arrived at the detector at different times.

The quantum interference measured matched up with the physicists' predictions, which showed that quantum communication could work despite the signals in the experiment travelling more than 5,000 km (3,107 miles) through space.

Professor Alexander Sergienko told Science News: "Whether this could survive such long distances and harsh experimental conditions, that was a big question," who works in the Quantum Communication & Measurement Laboratory at Boston University.

"Everybody else is doing this either in the lab or kind of in a quiet environment somewhere."

In 2014, Villoresi and his team conducted a similar experiment by encoding the photons with different polarisation states. The photons also returned to earth with their polarisation states intact which showed that quantum information could be stored in its polarisation state. The new research, however, shows that quantum communications making use of quantum interference – a different property – are more effective.

Sending encrypted messages through altering photons' temporal superposition is more secure than using polarisation states, as research to date has concentrated on. The polarisation state might change when the photon bounces off the satellite's reflectors, which causes the photon to lose information on its previous polarisation state. In contrast, temporal superposition states are more stable.

"It's important for the sake of secure communication and advancement of physics," said Professor Paolo Villoresi, who led the study. But that wasn't the only reason for the study, Prof Villoresi admitted.

"I can more honestly say that it's cool." ®