Relatively few people in history can say that they have seen, first-hand, the mysterious and uncommon electrical phenomenon of “ball lightning”—glowing, spherical objects in the sky that mostly appear during thunderstorms and are not fully understood by scientists.

But Physics Professor David S. Hall ’91 and his students are in the rarest company of all: They and a handful of colleagues are the only people in the world to have made and observed a microscopic simile of ball lightning.

Hall, members of his student research team and his collaborators at Aalto University in Finland recently created a three-dimensional skyrmion—a quasiparticle consisting of a knotted configuration of atomic magnetic moments, or spins—in a quantum gas in Hall’s lab. Scientists predicted the existence of the skyrmion theoretically more than 40 years ago, but this is the first time such an object has been observed in an experiment.

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

Hall and his colleagues’ findings were published by Science Advances in a paper titled Synthetic Electromagnetic Knot in a Three-Dimensional Skyrmion. Featuring the thesis research of Andrei-Horia Gheorghe ’15 and Wonjae Lee ’16 and computational contributions of visiting scholar Tuomas Ollikainen, the work builds on the collaboration’s previous studies of Bose-Einstein condensates, monopoles and quantum knots.

“The experiment is conceptually simple, but the phenomenon is both beautiful and remarkably complex,” said Hall. “Our own 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.”

Hall and his team 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 his lab. “When supercooled, all atoms in the gas end up in the state of minimum energy,” explained Hall. “The state no longer behaves like an ordinary gas, 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, said Hall. 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.

“It is remarkable that we could create the synthetic electromagnetic knot—that is, quantum ball lightning—essentially with just two counter-circulating electric currents,” said Mikko Möttönen, leader of the theoretical effort at Aalto University. “[This shows that] it may be possible that a natural ball lighting could arise in a normal lightning strike.”

Hall said that while the hot plasma of ball lightning might be a million times hotter than the ultracold gases with which his team works, he nevertheless found it interesting that such disparate physical contexts share common themes. He also noted the fact “the physics studied at large fusion reactors might also be studied on the small optical table [upon which much of his research equipment is located] that will soon make its brief journey across campus to the new science center.”

Hall’s experiments are supported by the National Science Foundation (grant no. PHY-1519174), and Möttönen’s research by the Academy of Finland through its Centres of Excellence Program (grant nos. 251748, 284621, and 308071), by the European Research Council under Consolidator (grant no. 681311) (QUESS), by the Magnus Ehrnrooth Foundation, by the Education Network in Condensed Matter and Materials Physics, and by the KAUTE Foundation through its researchers abroad program.

Cutaway view of the 3D skyrmion spin structure
Figure 2. Cutaway view of the 3D skyrmion spin structure. Figure credit: David Hall