Physicists capture first footage of quantum knots unraveling in superfluid

Enlarge / Researchers captured the decay of a quantum knot (left), which untied itself after a few microseconds and eventually turned into a spin vortex (right). (credit: Tuomas Ollikainen/Aalto University)

The same team who tied the first "quantum knots" in a superfluid several years ago have now discovered that the knots decay, or "untie" themselves, fairly soon after forming, before turning into a vortex. The researchers also produced the first "movie" of the decay process in action, and they described their work in a recent paper in Physical Review Letters.

A mathematician likely would define a true knot as a kind of pretzel shape, or a knotted circle. A quantum knot is a little bit different. It's composed of particle-like rings or loops that connect to each other exactly once. A quantum knot is topologically stable, akin to a soliton-that is, it's a quantum object that acts like a traveling wave that keeps rolling forward at a constant speed without losing its shape.

Physicists had long thought it should be possible for such knotted structures to form in quantum fields, but it proved challenging to produce them in the laboratory. So there was considerable excitement early in 2016 when researchers at Aalto University in Finland and Amherst College in the US announced they had accomplished the feat in Nature Physics. The knots created by Aalto's Mikko Mittinen and Amherst's David Hall resembled smoke rings.

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