Whirlpools with symmetries of squares and tetrahedrons observed in exotic quantum superfluid

A team of physicists have created and observed an entirely new class of vortices – tiny and exotic whirlpools – in an ultracold gas of atoms which produce ‘beyond state-of-the-art’ symmetries.

The international collaboration of researchers is led by Professor David Hall (Amherst College, USA) and UK researchers Dr Magnus Borgh (University of East Anglia) and Professor Janne Ruostekoski (Lancaster University).

The discovery, announced in the journal Nature Communications details the first laboratory studies of these exotic whirlpools in an ultracold gas of atoms at temperatures as low as tens of billionths of a degree above absolute zero.

The team’s work may have exciting future implications in unconventional realizations of quantum information and computing.

Vortices are familiar objects in nature, from water down a bathtub drain to the airflow around a hurricane.

In quantum-mechanical systems, such as an atomic Bose-Einstein condensate, the vortices tend to be tiny and their circulation comes in discrete, quantized units.

Such vortices have long been objects of fascination for physicists and have helped to illuminate the unusual properties of superfluidity and superconductivity.

The exotic nature of the observed whirlpools here, however, is due to their symmetries.

One especially fascinating property of physical theories from cosmology to elementary particles is the appearance of asymmetric world despite perfect underlying symmetries. When water freezes to ice, disordered molecules in a liquid arrange themselves to a periodic array.

Although the vortex medium here is a fluid, it also possesses a set of hidden discrete symmetries. For example, one of the team’s creations had the fourfold symmetry of a square, and another had the symmetries of a four-sided die, familiar to players of fantasy games everywhere.

“No ordinary fluids behave like this, and it may be that comparable objects only exist deep inside neutron stars,” explains Professor Ruostekoski. “Indeed, the vortices created by the team go beyond the state-of-the-art.”

“It’s partly these connections to the stranger domains of physics that makes our work appealing,” says Professor Hall. “And partly it’s our human aesthetic appreciation of symmetry.”

“The whirpools and the underlying symmetry of the fluid interact with one another in interesting ways,” Dr Borgh explains.

“One consequence is that if the positions of two vortices are interchanged, they can leave a trace of the process lingering in the fluid. This trace links the interacting vortices together permanently, like a rung in a ladder.”

Observing these behaviours directly has become the focus of the team’s research, the experimental part of which is based at Amherst College.

“We’re fortunate to have extremely talented and dedicated students who can do these kinds of challenging experiments,” said Prof Hall, crediting in particular Arthur Xiao, the lead author on the study.

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