An object made of hundreds of atoms exhibits a quantum property normally only associated with very small objects

Quantum entanglement. Conceptual artwork of a pair of entangled quantum particles or events (left and right) interacting at a distance. Quantum entanglement is one of the consequences of quantum theory. Two particles will appear to be linked across space and time, with changes to one of the particles (such as an observation or measurement) affecting the other one. This instantaneous effect appears to be independent of both space and time, meaning that, in the quantum realm, effect may precede cause.

A concept image of a pair of entangled particles

A quantum property associated with tiny objects has been found to persist in an experiment with more than a thousand atoms. This could help us understand where the boundary between the quantum world and the macroscopic world lies – if such a boundary exists.

The property for an object in question is two-fold: firstly, that its physical characteristics depend on whether it is being measured and secondly, that it can be influenced over long distances by another object without exchanging any known signals. Experiments with quantum particles have repeatedly shown this to be true for very small objects. However, it is far from our usual experience of reality. This contradiction between our experience and the experience of very small objects is known as the EPR paradox, named after physicists Albert Einstein, Boris Podolsky and Nathan Rosen.

The three physicists studied this paradox in 1935, but researchers still don’t know how large an object must be before it loses its quantum properties and starts behaving like the objects we use every day. Paolo Colciaghi at the University of Basel in Switzerland and his colleagues wanted to see whether two quantum-entangled objects made of 700 rubidium atoms each might be big enough.

They started with a cloud of 1400 atoms chilled to nearly absolute zero and held in place by electromagnetic forces. They used these forces and pulses of microwaves to first entangle each atom with all the 1399 others, then to split the cloud in two and push the smaller clouds apart.

The researchers then used a test for whether an object’s properties are quantum that Einstein, Podolsky and Rosen had devised. The test required them to measure the atoms’ pseudospin, which is a physical attribute like the spin of an electron. The researchers made more than a thousand pseudospin measurements and found the test showed that the 700 rubidium atoms in each cloud were still acting in a quantum way.

Colciaghi says that the two atom clouds were larger than objects in previous tests but not as large as everyday objects, so they were a good candidate for a size where quantum properties may vanish. You always see quantum behaviour for a couple of particles, but there was no guarantee that would persist at this size, he says.

Anthony Leggett at the University of Illinois Urbana-Champaign says that even though the new experiment tested the quantumness of unprecedentedly many atoms, the question of whether it implies that any object smaller than 700 atoms will act in counterintuitive quantum ways is still unsettled. He says that making the atoms extremely cold substantially changed their properties and may have made them more susceptible to quantum effects than a room temperature object of the same size.

“In my view, to more clearly test where quantum behaviour stops, we should use not just bigger and bigger objects, but objects that are also more and more like those from our everyday,” says Leggett.

Journal reference: Physical Review X, forthcoming