Relatively few people in history can say that they have seen, first-hand, the mysterious and uncommon electrical phenomenon of 鈥渂all lightning鈥濃攇lowing, spherical objects in the sky that mostly appear during thunderstorms and are not fully understood by scientists.

But Physics Professor David S. Hall 鈥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鈥攁 quasiparticle consisting of a knotted configuration of atomic magnetic moments, or spins鈥攊n a quantum gas in Hall鈥檚 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鈥 findings were published by Science Advances in a paper titled . Featuring the thesis research of Andrei-Horia Gheorghe 鈥15 and Wonjae Lee 鈥16 and computational contributions of visiting scholar Tuomas Ollikainen, the work builds on the collaboration鈥檚 previous studies of Bose-Einstein condensates, monopoles and quantum knots.

鈥淭he experiment is conceptually simple, but the phenomenon is both beautiful and remarkably complex,鈥 said Hall. 鈥淥ur 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.鈥

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. 鈥淲hen supercooled, all atoms in the gas end up in the state of minimum energy,鈥 explained Hall. 鈥淭he state no longer behaves like an ordinary gas, but like a single giant atom.鈥

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.

鈥淚t is remarkable that we could create the synthetic electromagnetic knot鈥攖hat is, quantum ball lightning鈥攅ssentially with just two counter-circulating electric currents,鈥 said Mikko M枚tt枚nen, leader of the theoretical effort at Aalto University. 鈥淸This shows that] it may be possible that a natural ball lighting could arise in a normal lightning strike.鈥

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 鈥渢he 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.鈥

Hall鈥檚 experiments are supported by the National Science Foundation (grant no. PHY-1519174), and M枚tt枚nen鈥檚 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