The same team that tied the first "quantum knots" in a superfluous number of years ago has now discovered that the knots are decaying, or "loosening", pretty soon after formation, before turning into a vortex. The researchers also produced the first "film" of the decay process in action, and they described their work in a new article in Physical Review Letters.
A mathematician would probably define a real knot as a kind of kringle shape, or a knotted circle. A quantum knot is a little different. It consists of particle-like rings or loops that connect to each other exactly once. A quantum knot is topologically stable, similar to a soliton – that is, it is a quantum object that acts as a moving wave that continues to roll forward at constant speed without losing its shape.
Physicists had long thought that it should be possible for such knotted structures to be formed in quantum fields, but it proved challenging to produce them in the laboratory. So there was great excitement in early 2016 when researchers at Aalto University of Finland and Amherst College in the United States announced that they had achieved the urgency of natural physics. The knots created by Aalto's Mikko Möttönen and Amherst's David Hall were similar to smoking rings.
Hall and Möttönen used a quantum state of matter known as a Bose-Einstein Condensate (BEC) as their medium – technically an excess liquid. Then they "tie" the knots by manipulating magnetic fields. If you think of the quantum field as points in space that each has an orientation – like arrows that everyone points up, for example – the core of a quantum knot would be a circle where the arrows all point downwards, resembling the yarn pattern of a god's eye. "If you followed the magnetic field line, it would move toward the center, but at the last minute it would scale away in a perpendicular direction," Hall told Gizmodo 2016. "It's a special way to rotate these arrows that gives you this linked configuration."
Eventually they became so good at making quantum knots that they could make small films of the exotic structures, yet it was still not clear what would happen to the quantum knots over time. Sure, they were topologically stable. thought that the knots should shrink over time as a way to minimize their energy, just as a bubble assumes a spherical shape, or a ball "wants" to roll down a hill, thus minimizing its potential energy. In other words, quantum knots may not are dynamically stable, and blink out of existence before their superfluous medium lapses. If they can surpass their superfluous medium, they would be effectively stable.
The group has since its got even better control over the BEC medium, which enabled 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 "disturbed" BEC by removing the field and mapping what happened next. The experiment showed two distinct steps in decay. First, the knot remained stable, while several "ferromagnetic islands" developed in (non-magnetic) BEC. But then the knot disintegrated after a few hundred milliseconds, and the ferromagnetic islands migrated to the BEC's edges leaving a non-magnetic core at its center. Finally, a vortex of atomic spin was formed between the two magnetic regions of the BEC.
"The fact that the knot decays is surprising, since topological structures like quantum knots are usually exceptionally stable," said co-author Tuomas Ollikainen. "It is also exciting for the field because our observation that a three-dimensional quantum defect falls into a one-dimensional defect has not seen before in these quantum gas systems."
At least quantum knots remain at the laboratory's curiosity, but research may be of importance to ongoing research into building topological quantum computers. Such a device would braid pieces into various topologically stable structures, making the computer more robust to errors. This latest finding indicates that time can be an important factor, given the knots' decay rate.
"It would be great to see this technology used one day in a practical application, which may well happen," said Möttönen. "Our latest results show that even though quantum knots in atomic gases are exciting, you need to be quick to use them before they unload. Therefore, the first applications can probably be found in other systems."
DOI: Physical Review Letters 2019. 10.1103 / PhysRevLett.123.163003 (About DOIs).