The same team that tied the first "quantum knots" to a superfluid a few years ago has now discovered that the knots decompose, or "untie" themselves, soon after formation, before turning into a vortex. Researchers also created the first "film" of the process of decomposition in action, and they described their work in a recent paper in Physical Review Letters.
The mathematician would probably define the real knot as a form of frying or a round. The quantum knot is a little different. It is made up of rings in the form of particles or loops that connect to each other exactly once. The quantum node is topologically stable, similar to soliton – that is, a quantum object that acts as a traveling wave that constantly rolls forward at constant speed without losing its shape.
Physicists have long thought that such wicker structures of quantum fields should be formed, but it has proved a challenge to produce them in a laboratory. So there was a lot of excitement in early 2016, when researchers from Alto University in Finland and Amherst College in the United States announced that they had achieved a feat in natural physics. The knots created by Majko Atono, Miko Metinen and David Hall of Amherst resembled smoke rings.
Hall and Methane used a quantum state of matter known as the Bose-Einstein Capacitor (BEF) as their medium – technically as superfluid. Then they tied the knots by manipulating magnetic fields. If you think of the quantum field as points in space that everyone is oriented to – like the arrows everyone points out, for example – the nucleus of the quantum knot will be a circle in which the arrows are lowered down, similar to the yarn god pattern. "If you followed the line of the magnetic field, it would move toward the center, but at the last moment it would pull in the normal direction," Hall told Gizmodo in 2016. "It's a special way of rotating these arrows that gives you this related configuration. "
In the end they are so good at making quantum knots that they could make little films on exotic structures. However, it was not yet clear what would happen to the quantum nodes over time. Of course, they were topologically stable. But Hall and Methane thought that the knots should shrink over time as a means of minimizing their energy, in the same way as a bubble would naturally present a spherical shape, or the ball "wants" to roll over the hill, thereby minimize its potential energy. In other words, quantum nodes may not be dynamically stable, shining out of existence before their superfluids decompose. If they could exceed their superconducting medium, they would be effectively stable.
Since then, the group has gained even better control over the BEH medium, enabling them to detect node breakdown and the formation of a new type of topological malfunction. After creating a node through a carefully structured magnetic field, they "disrupted" the BEF by removing the field and painting what happened next. The experiment showed two different stages of the decay process. At first, the knot remained stable, while several "ferromagnetic islands" developed into (non-magnetic) BEH. But then the node dissolved after several hundred milliseconds, and the ferromagnetic islands migrated to the edges of the BEF, leaving a non-magnetic core in the center. Finally, a vortex of atomic spins forms between the two magnetic fields of the BEF.
"The fact that the node breaks down is surprising, because topological structures like quantum nodes are usually extremely stable," co-author Tuomas Olaquinen said. "It's also exciting on the ground because our observation that a three-dimensional quantum defect breaks down into a one-dimensional defect was not seen before in these quantum gas systems."
For the time being, at least, quantum nodes remain a laboratory curiosity, but research may have an impact on current research on building topological quantum computers. Such a device would braid buckets into various topologically stable structures, making the computer more susceptible to errors. This latest finding shows that time can be an important factor given the rate of knot decay.
"It would be nice to see this technology used some day in practical application, which can happen," Metinen said. "Our latest results show that although quantum knots in atomic gases are exciting, you should be quick to use them before they unbutton. Thus, the first applications are likely to be found in other systems."
DO: Physical examination letters, 2019. 10.1103 / PhysRevLett.123.163003 (For DOIs).