When a pair of photons are quantum entangled, you can instantly deduce the state of one by measuring the other, regardless of the distance separating them. This phenomenon, which Albert Einstein dismissively called “spooky action at a distance”, is the basis of quantum encryption – using entangled particles to ensure communications are secret.
Quantum networks are far more secure than the existing internet because any attempt to eavesdrop changes the state of the photons, alerting the recipient to foul play.
Entangled photons have been transported more than 1000 kilometres in tests between a satellite and ground stations before, but now Zhenda Xie at Nanjing University in China and his colleagues have shown that links can be made over shorter distances with relatively inexpensive hardware. It is also the first time that photon entanglement has been transmitted from one moving device to another.
A laser on board one of the 35-kilogram drones created a pair of entangled photons by splitting a single photon with a crystal. One photon was sent directly to a ground station and the other to a second drone a kilometre away via a relay drone.
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