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Physicists create bizarre quantum Alice rings for the first time

Physicists have peered through the proverbial looking glass, and the atoms on the other side belong to a world of opposites. For the first time, researchers have made an exotic quantum object called an Alice ring, which changes the properties of other quantum objects when they pass through it – or when they are simply viewed through it.

Quantum systems – such as collections of very cold atoms, or even our whole universe – should theoretically contain odd objects called topological defects. Some are like long strings, and others are stranger still: Zero-dimensional dots, at the centre of which things like magnetic fields become mathematically impossible to describe.

Such defects are difficult to create and observe. But Mikko Möttönen at Aalto University in Finland and his colleagues have worked out how to create a topological defect that quickly transforms into another.

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To do it, they first placed 250,000 rubidium atoms into a small chamber devoid of air, then hit them with lasers to slow their natural motion and push them to a temperature close to absolute zero. Under these conditions, all the atoms behaved as one large quantum object. Because of a quantum property called spin, that object was sensitive to magnetic fields.

Möttönen says the team used computer simulations and mathematical models to determine how to pattern the direction and strength of magnetic fields in order to twist the atoms until a topological defect appeared. This approach had previously been used to create defects called monopoles, particles analogous to a magnet with a single pole.

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Now, the researchers also observed a monopole’s fate: After a few milliseconds, each monopole they created expanded into an Alice ring with one very strange property.

“There is a peculiarity to this Alice ring. Depending on whether you look at some nearby monopole through the ring, or from the side of the ring, its charge looks different. So, the ring is inverting the charge of the objects that you look at,” says Möttönen. Computer simulations further showed that a monopole’s charge would fully flip – from positive to negative, say – if it moved through the Alice ring.

Möttönen and his colleagues have previously used this method to create topological defects in ultracold atoms, including knot-like structures and special swirls called skyrmions. The next challenge they are setting their sights on is to not only create a monopole and an Alice ring, but to make one pass through the other to directly test its looking-glass-like function, he says.

Janne Ruostekoski at Lancaster University in the UK says the method the researchers developed is unique, and could even make it possible to visualise abstract mathematical theorems. For example, it could give scientists a way to investigate the so-called “hairy ball theorem”, which dictates the texture of fields around topological defects.

This opens up “unprecedented opportunities” for probing theories in cosmology or high-energy physics “where there has been no experimental evidence before”, he says.

Journal reference:

Nature Communications DOI: 10.1038/s41467-023-40710-2

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