null How quantum knots untie

Particle densities related to the decay of the quantum knot (left), which surprised researchers by untying itself after a few microseconds and eventually turning into the spin vortex (right) Image credit: Tuomas Ollikainen/Aalto University

How quantum knots untie

After first reporting the existence of quantum knots in 2016, Aalto University and Amherst College (USA) researchers now report how the knots untie.

In this project the researchers studied how the knots behave over time. A quantum gas can be tied into knots using magnetic fields and in this new study the researchers could control the state of quantum gas more precise than before. The surprising result is that the knots untie themselves over a short period of time, before turning into a vortex.

The research was mainly carried out by Tuomas Ollikainen, a PhD student at Aalto university who split his time between carrying out experimental work in Amherst in Massachusetts, and analyzing the data and developing his theories at Aalto.

– We hadn't been able to study the dynamics of these sorts of three-dimensional structures experimentally before, so this is the first step to this direction. The fact that the knot decays is surprising, since topological structures like quantum knots are typically exceptionally stable. It's also exciting for the field because our observation that a three-dimensional quantum defect decays into a one-dimensional defect hasn't been seen before in these quantum gas systems, says Ollikainen.

The scientists from Aalto University and Amherst College managed both to observe and tie a quantum knot first time ever in the world in 2016. In practice, the knots exist in the quantum-mechanical field, also known as a Bose-Einstein condensate.

CSC's resources were utilized in the computational part of the study.

– We utilized CSC's Taito supercluster in modelling quantum gas dynamics. More precisely and technically, we computed the dynamics of quantum gas in a cubic 2003 grid, based on Gross-Pitaevski partial differential equations. The supercomputer is extremely important to minimize computation time, says Ollikainen.

– The simulations are very important for the credibility of the experimental results, since the analysis of the experiments is based on comparing the data with the computed data. We get close to one-on-one correspondence with experiments and modeling, and based on this we can determine what happens in quantum gas. In addition, over half of the image panels in the article are directly visualized from simulation data, demonstrating the importance of simulations for research.

The sources used are Aalto University news Researchers watch quantum knots untie (21.10.2019), Science Magazine article and CSC news on the first quantum nots (18.1.2016).

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Tommi Kutilainen