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An international team of physicists from Amherst College and Aalto University has created the Shankar skyrmion, a quasiparticle consisting of a knotted configuration of atomic magnetic moments, or spins. Theoretical physicists predicted the existence of the Shankar skyrmion more than four decades ago, but this is the first time such a quasiparticle has been observed in an experiment. Published in the journal Science Advances, the team’s results could inspire new ways of keeping plasma intact in a stable ball in fusion reactors.
Artistic impression of a quantum ball lighting. Image credit: Heikka Valja.
“Our understanding of these skyrmions has evolved over several years, and it has taken us almost as long again to find accessible ways to communicate our results to the wider scientific community,” said co-lead author Professor David Hall, from Amherst College.
“It is remarkable that we could create the synthetic electromagnetic knot — that is, quantum ball lightning — essentially with just two counter-circulating electric currents,” added co-lead author Dr. Mikko Möttönen, of Aalto University.
“Thus, it may be possible that a natural ball lighting could arise in a normal lightning strike.”
The physicists created the environment for the skyrmion after cooling a gas of rubidium atoms to tens of billionths of degrees above absolute zero in an atomic refrigerator in their lab.
“The experiment is conceptually simple, but the phenomenon is both beautiful and remarkably complex,” Professor Hall noted.
“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.”
To create the skyrmion, the physicists then applied a tailored magnetic field to the supercooled gas, which influenced the orientation of the magnetic moments of its constituent atoms.
The characteristic knotted structure of the skyrmion emerged after less than one thousandth of a second.
“Remarkably, the skyrmion is accompanied by a knotted synthetic magnetic field that strongly influences the quantum gas,” Professor Hall said.
Such a knotted magnetic field is a central feature of a topological theory of ball lightning, which describes a plasma of hot gas magnetically confined by the knotted field.
According to the theory, the ball lightning can last much longer than an ordinary lightning bolt because it is very difficult to untie the magnetic knot that confines the plasma.
“While the hot plasma of ball lightning might be a million times hotter than the ultracold gases with which our team works, we nevertheless found it interesting that such disparate physical contexts share common themes,” Professor Hall said.
“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,” Dr. Möttönen added.
“Further studies could lead to finding a solution to keep plasma together efficiently and enable more stable fusion reactors than we have now.”
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Wonjae Lee et al. 2018. Synthetic electromagnetic knot in a three-dimensional skyrmion. Science Advances 4 (3): eaao3820; doi: 10.1126/sciadv.aao3820
