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Scientists observe a new quantum particle with properties of ball lightning

This knotted skyrmion may provide insight into a stable ball of plasma that could enhance future fusion reactors.

Figure 1. Artistic impression of a quantum ball lighting. Figure credit: Heikka Valja.

Scientists at Amherst College and Aalto University have created, for the first time a three-dimensional skyrmion in a quantum gas. The skyrmion was predicted theoretically over 40 years ago, but only now has it been observed experimentally.

In an extremely sparse and cold quantum gas, the physicists have created knots made of the magnetic moments, or spins, of the constituent atoms. The knots exhibit many of the characteristics of ball lightning, which some scientists believe to consist of tangled streams of electric currents. The persistence of such knots could be the reason why ball lightning, a ball of plasma, lives for a surprisingly long time in comparison to a lightning strike. The new results could inspire new ways of keeping plasma intact in a stable ball in fusion reactors.

– It is remarkable that we could create the synthetic electromagnetic knot, that is, quantum ball lightning, essentially with just two counter-circulating electric currents. Thus, it may be possible that a natural ball lighting could arise in a normal lightning strike, says Dr Mikko Möttönen, leader of the theoretical and computational effort at Aalto University.

Möttönen also recalls having witnessed a ball lightning briefly glaring in his grandparents' house. Observations of ball lightning have been reported throughout history, but physical evidence is rare.

The dynamics of the quantum gas matches that of a charged particle responding to the electromagnetic fields of a ball lightning.

– The quantum gas is cooled down to a very low temperature where it forms a Bose–Einstein condensate: all atoms in the gas end up in the state of minimum energy. The state does not behave like an ordinary gas anymore but like a single giant atom, explains Professor David Hall, leader of the experimental effort at Amherst College.

Figure  2. Aalto University members of the collaboration from left to right: Konstantin Tiurev, Mikko Möttönen, and Tuomas Ollikainen. Photo: Mikko Raskinen / Aalto University.

The skyrmion is created first by polarizing the spin of each atom to point upward along an applied natural magnetic field. Then, the applied field is suddenly changed in such a way that a point where the field vanishes appears in the middle of the condensate. Consequently, the spins of the atoms start to rotate in the new direction of the applied field at their respective locations. Since the magnetic field points in all possible directions near the field zero, the spins wind into a knot.

The knotted structure of the skyrmion consists of linked loops, at each of which all the spins point to a certain fixed direction. The knot can be loosened or moved, but not untied.

– What makes this a skyrmion rather than a quantum knot is that not only does the spin twist but the quantum phase of the condensate winds repeatedly, says Hall.

If the direction of the spin is changing in space, the velocity of the condensate responds just as would happen for a charged particle in a magnetic field. The knotted spin structure thus gives rise to a knotted artificial magnetic field that exactly matches the magnetic field in a model of ball lightning.

– Theoretical ideas and the precise modelling and analysis of the experiments were critical for the creation of the skyrmion. We are able to obtain images of a Bose-Einstein condensate, but a skyrmion may be difficult to identify without information on the modelling data, which we can use for comparison. In fact, we have very often wondered about the complicated features in the experimental results, but numerical modelling has explained the reason behind them. We primarily used CSC's supercomputer and computers in Aalto University's Science-IT project for our modelling, says Möttönen.

– When capturing an image of a skyrmion, we let the condensate expand. In some cases, the expansion has no significant effect on how the image of the condensate will look, but in others it plays a major role. The extent of the effect depends on what kinds of particle flows are found in the condensate. In a skyrmion, the flows are very complex and form a knot-like field. This is why a skyrmion changes shape when it expands. With modelling, however, we're able to sort of go back in time and, in a certain way, see what an expanded condensate looked like before it expanded. If we couldn't do this, it would be difficult to say with such a confidence that we have actually created a skyrmion. Now, thanks to modelling, we are sure that we created three-dimensional skyrmions.

– More research is needed to know whether or not it is also possible to create a real ball lightning with a method of this kind. Further studies could lead to finding a solution to keep plasma together efficiently and enable more stable fusion reactors than we have now, Möttönen explains.

Further information:

W. Lee, A.H. Gheorghe, K. Tiurev, T. Ollikainen, M. Möttönen and D.S. Hall: Synthetic Electromagnetic Knot in a Three-Dimensional Skyrmion, Science Advances 4, eaao3820 (2018).

PID http://dx.doi.org/10.1126/sciadv.aao3820

Mikko Möttönen, Aalto University
email: mikko.mottonen at aalto.fi

David S. Hall, Amherst College, USA
Email: dshall at amherst.edu