Quantum entanglement is one of the many non-intuitive features of quantum mechanics. If two photons of light are allowed to properly interact with one another, they can become entangled.
One can even directly create pairs of entangled photons using a non-linear process called Spontaneous Parametric Down Conversion.
According to the Austrian-German ESA team, these two entangled photons can then be separated but as soon as one of them interacts with a third particle, the other photon of the pair will change its quantum state instantaneously.
This happens according to the random outcome of the interaction, even though this photon did not interact with a third particle.
"Such behaviour has the potential to allow messages to be swapped with complete confidence," the scientists stated.
"This is because, if an eavesdropper listens into the message, the act of detecting the photons will change the entangled partner.
"These changes would be obvious to the legitimate receiving station and the presence of the eavesdropper would be instantly detected."
According to the researchers the quantum entanglement effect could be used to communicate with satellites as it remains intact over a distance of 144 kilometres.
"We were sending the single-photon beam on a 144 kilometres path through the atmosphere, so this horizontal quantum link can be considered a 'worst case scenario' for a space to ground link," said Josep Perdigues, ESA's study manager.
The researchers added that a quantum communications system would be a valuable way to transmit banking information, or military communications, or even to distribute feature films without the fear of piracy.
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