Now researchers have shown that there is a grey area where Nature cannot tell the difference between the classical and the quantum. This opens the possibility of first performing quantum experiments with a type of classical light called “classically entangled” light.
By preparing and sending a so-called “classically entangled” beam the team could show that this was identical to sending a quantum state. This means that the observed quantum entanglement decay due to noise in the link can be reversed, paving the way for major advances in secure quantum links in fibre and free-space.
“We showed for the first time that classical light can be used to analyse a quantum link, acting as a direct equivalent to the behavior of the quantum state,” says Bienvenu Ndagano, lead author and PhD student at Wits University.
“Not similar, or mimicking, but equivalent. To show this, we exploited a particular type of laser beam, called vector beams, that have the property of being non-separable and sometimes called ‘classically entangled’.”
Ndagano explains that the quintessential property of quantum entanglement is the non-separability of the state, meaning that one part of the system cannot be separated from the other. “But non-separability is not unique to the quantum world: you can find it in weather maps where the locations on the map and the temperatures at those locations can’t be separated.”
More intriguingly, classical vector beams have this property too, which the team calls “classically entangled” light.
Says Forbes, “What we asked was: does this mean that classical light can be used in quantum systems – a grey area between the two worlds that we call classical entanglement?”.
This work allows for long distance quantum links to be established and tested with classically entangled light: as there is no shortage of photons in the classical light, all the measurements needed to fix the errors in the quantum state can be done in real-time without destroying the quantum information.
Thus, real-time error correction is possible as you can run experiments in the classical world that will tell you how to fix the error in the quantum world.