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Physicists capture first footage of quantum knots unraveling in superfluid

 Researchers captured the decay of a quantum knot (left), which remained after a few microseconds and subsequently turned into a spin vortex (right).
Enlarge / Researchers captured the decay of a quantum knot (left), which after a few microseconds and finally turns into a spin vortex (right).

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 thus 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 child 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 to be challenging to produce them in the laboratory. So there was something excitement early in 2016 when researchers at Aalto University in Finland and Amherst College in the United States announced they had made the feat in Nature Physics. The knots created by Aalto's Mikko Möttönen and Amherst's David Hall resembled smoke rings.

 Quantum knots in a superfluid resemble smoke rings. "src =" https://cdn.arstechnica.net/wp-content/uploads/2019/11/knotB-640x180.jpg "width =" 640 "height =" 180 "srcset =" https: //cdn.arstechnica .net / wp-content / uploads / 2019/11 / knotB.jpg 2x
Enlarge / Quantum knots in a super fluid resemble smoke rings.

David Hall, Amherst College

Hall and Mottos uses a quantum state of matter known as a Bose-Einstein Condensate (BEC) as their medium-technically a superfluid. Then they "tied" the knots by manipulating magnetic fields. If you think of the quantum field as points in space, that is the point of orientation, the point of view is the point of view , Hall told Gizmodo in 2016. "It's a special way of doing this linked configuration. "

Eventually they got so good at making quantum knots that they were not good at making movies. Yet it was not clear what would happen to the quantum knots over time. Sure, they were topologically stable. But Hall and Möttönen thought the knots should shrink over time, as a means of minimizing their energy, the same way a bubble naturally takes a spherical shape, or a ball "minimizes its potential energy. In other words, quantum knots might not be dynamically stable, winking out of existence before their superfluid medium decays.

The group has since gained even better control over the BEC medium, enabling them to detect the decay of the knots and the formation of a new type of topological defect. a vortex). After creating a knot via a carefully structured magnetic field, they "perturbed" the BEC by removing the field and imaging what happened next. The experiment showed two distinct stages of the decay process. At first, the knot remained stable, while several "ferromagnetic islands" developed in the (nonmagnetic) BEC. But then the knot dissolved after a few milliseconds, and the ferromagnetic islands migrated to the edges of the BEC, leaving a nonmagnetic core at the center. Finally, a vortex of atomic spins formed between the two magnetic regions of the BEC.

 The experimental set-up at Amherst College where quantum gases are made. "Src =" https://cdn.arstechnica.net/wp- content / uploads / 2019/11 / knot2-640x438.jpg "width =" 640 "height =" 438 "srcset =" https://cdn.arstechnica.net/wp-content/uploads/2019/11/knot2.jpg
Enlarge / The experimental set-up at Amherst College where quantum gases are made.

David Hall / Amherst College

"The fact that the knot decays is surprising, since topological structures." like quantum knots are typically exceptionally stable, "said co-author Tuomas Ollikainen.

For now, at least, quantum knots remain a "It's also exciting for the field because our three-dimensional quantum defect decays into a one-dimensional defect has not been seen before." laboratory curiosity, but the research might have bearing. Such a device would make braid qubits in different topologically stable structures, making the computer more robust against errors.

"It would take a while to apply this technology today," said Mottönen. "Our latest results show that while quantum knots in atomic gases are exciting, you need to be quick to use them before they fall."

DOI: Physical Review Letters 2019. 10.1103 / PhysRevLett.123.163003 (About DOIs).

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