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Physicists capture the first frames of quantum nodes tangled in superfluidity



  The researchers filmed a collapse of a quantum knot (left), which detached after a few microseconds and eventually turned into a spinning vortex (right).
Click to enlarge / The researchers filmed a breakdown of a quantum node (left) that unraveled after a few microseconds and eventually turned into a spinning vortex (right).

Tuomas Ollikainen / Aalto University

The same team that tied the first "quantum nodes" in a super-liquid several years ago had already discovered that the nodes were disintegrating or "unbundling" shortly after their formation before becoming a whirlwind. Researchers also produce the first "film" of the disintegration process, and they describe their work in a recent book in Physical Letter Review.

A mathematician would probably define a true knot as a kind of pretzel, or knit circle. A quantum node is a little different. It consists of rings or loops, similar to particles, which connect to each other exactly once. The quantum node is a soliton-like topologically stable, that is, it is a quantum object that acts as a traveling wave, which continues to roll at a constant speed without losing its shape.

Physicists have long believed that it should be possible for such nodal structures to be formed in quantum fields, but it has been a challenge to produce them in the laboratory. So there was considerable excitement in early 2016 when researchers at the University of Aalto in Finland and Amherst College in the US announced that they had done a feat in natural physics. The knots created by Mikko Möttönen of Aalto and David & # 39; s Hall of Amherst looked like smoke rings.

  Quantum nodes in superfluidity 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 nodes in superfluid resemble smoke rings.

David Hall, Amherst College

Hall and Möttönen used the quantum state of a substance known as Bose-Einstein Condensate (BEC) as their medium – technical superfluidity. Then they "tied" the nodes, manipulating magnetic fields. If you think of a quantum field as points in space, each of which has an orientation – like arrows, all pointed up, for example – the nucleus of a quantum node will be a circle where all the arrows point down, similar to the image of a god yarn, "If you follow the line on the magnetic field, it will go to the center, but at the last moment it will peel off in a perpendicular direction, "Hall tells Gizmodo in 2016." It's one particular way of turning those arrows that gives you that related configuration. "

They eventually became so e in the creation of quantum nodes that they were able to make small films of exotic structures, but it was not yet clear what would happen to the quantum nodes over time. Of course, they were topologically stable. But Hall and Möttönen believed that the nodes they have to shrink over time as a means of minimizing their energy, in the same way that a balloon naturally takes on a spherical shape or a ball "wants" to roll over a hill, thereby minimizing its potential energy. In other words, quantum nodes may not be dynamically stable, escaping from their existence before the collapse of their superfluid medium. If they manage to outlive their super-liquid environment, they would be effectively stable.

Since then, the group has gained even more control over the BEC environment, enabling them to detect node breakdown and the formation of a new type of topological defect (vortex). After creating a node through a carefully structured magnetic field, they "disturb" the BEC by removing the field and depicting what happens next. The experiment showed two distinct steps in the decay process. At first, the node remained stable until several (ferromagnetic islands) evolved into the (non-magnetic) BEC. But then the knot dissolved after a few hundred milliseconds and the ferromagnetic islands migrated to the ends of the BEC, leaving a nonmagnetic nucleus in the center. Finally, a vortex of atomic rotation formed between the two magnetic regions of BEC.

  The experimental setup 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 2x
Increase / The experimental setup at the Amherst College where quantum gases are produced.

David Hall / Amherst College

"The fact that a node is falling apart is surprising, since topological structures such as quantum nodes are usually extremely stable," said co-author Tuomas Ollikainen. "This is exciting for the field as well, because our observation that a three-dimensional quantum defect breaks down into a one-dimensional defect has not been observed so far in these quantum gas systems." influence on current research on the construction of topological quantum computers. Such a device would be interwoven into various topologically stable structures, making the computer more robust against errors. This last finding shows that time can be an important consideration given the speed of node breakdown.

"It would be great to see this technology being used someday in a practical application that could happen," Mötönen said. "Our recent results show that although quantum nodes in atomic gases are exciting, you have to be quick to use them before decoupling. Thus, the first applications are likely to be found in other systems. "

DOI: Physical examination letters 2019. 10.1103 / PhysRevLett.123.163003 (Per DOI).


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