Physicists have finally observed a quantum interaction between a group of four entangled electrons. Until now, this interaction was purely theoretical, but now it's been caught in action by cleverly cooling a superconducting crystal and stressing it under high pressure.
The results of this exciting experiment are now opening doors to further refining our knowledge of the bizarre quantum world, and they also may have potential for use in future technologies.
Unlike the usual pairing of two entangled electrons, researchers observed groups of four electrons sharing the same state, a complex arrangement that was only speculated to exist in a superconducting crystal.
The study was led by researchers from École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, who used a shower of neutrons to analyse the behaviour of electrons within a lattice of strontium copper borate under intense pressure.
As a type of superconductor, strontium copper borate's electrons can slip through the material with zero resistance at low enough temperatures.
The electrons link in what's called Cooper pairing, a quantum effect that is caused in part by the electron's charge tugging on the positive ions in its surroundings.
Cooper pairs are entangled electrons, meaning even if they're separated by a distance, forcing one to adopt a specific property will instantly reveal the complimentary property in its partner.
Such interactions are what's known as a many-body problem in physics, and can be incredibly hard to model. Such problems become ridiculously complex when we are adding quantum weirdness to the already complex problem of many interacting particles.
But modelling how entangled electrons behave under complex conditions is exactly what was described back in 1981 by theoretical physicists B. Sriram Shantry and Bill Sutherland.
"Quantum many-body physics remains a challenge where theory has only scratched the surface of how to deal with it," says Henrik Rønnow, one of the researchers on this new experiment.
While Shastry and Sutherland's theory could be applied to this new research, their work actually made a number of predictions describing phase changes in the superconductive material, and until now we didn't know which would be correct.
Amongst a number of possibilities, one was that the electrons' spins would entangle them into groups of four rather than pairs, a condition called a 'plaquette singlet'.
And that singlet is exactly what the researchers have now found.
At a pressure of around 21,500 atmospheres, they saw electrons in the super-chilled material had formed groups of four.
"This is a new type of quantum phase transition, and while there have been a number of theoretical studies on it, it has never been investigated experimentally," says Rønnow.
Since many materials such as semiconductors are used for their unique phase changes, swapping between a conductor and insulator under different conditions, such a strange new phase shift could potentially have uses in future technologies. Time will tell.
But for physicists, the fact that such a complex physical theory now has an experimental result can help refine our understanding of the weird, weird world of the quantum.
This research was published in Nature Physics.