- In an experiment, physicists created a 3D skyrmion – a theoretical quasiparticle.
- They used experimental approaches originally designed for creating topological defects.
- The outcomes could inspire new methods to keep plasma intact in fusion reactors.
Researchers at Aalto University and Amherst College have built a 3D skyrmion in a quantum gas. For those who don’t know, skyrmion was theorized more than 4 decades ago. And now, it has been actually seen in an experiment.
The experiment seems simple, but the phenomenon is extremely complex. Our knowledge on skyrmions has evolved over the past few couple of years, and this study has made our understanding even better.
Scientists created a suitable surrounding for the skyrmion by cooling rubidium atoms’ gas to near absolute zero in an atomic refrigerator. The purpose of cooling down the atoms in gas is to bring its state to minimum energy level. Now, instead of behaving like a usual gas, it acts like a one big atom.
In this extremely cold and sparse quantum gas, they built magnetic knots of the constituent atoms. These knots share some common properties with ball lightning. Some researchers believe that they contain twisted streams electric currents.
The ball lightning exists for a long time as compared to usual lightning strikes, probably because of persistence of knots. Anyway, these outcomes could inspire new methods to keep plasma together efficiently in fusion reactors.
Quantum ball lighting | Artistic impression | Heikka Valja
Ball lightning is a less-understood event. As we’ve mentioned above, a topological theory ascribes this astonishing persistence to electromagnetic knots, with magnetic field lines comprising of linked, closed rings. These rings have tangled electric current in a plasma of ionized air.
Since the magnetic field lines cannot be unlinked by continuous deformations, The structure of electromagnetic field is topologically protected.
How Exactly Skyrmion is Created?
Physicists have created the very first instance of 3D skyrmion using experimental approaches originally designed for creating topological defects like monopoles and knot solitons in superfluid Bose-Einstein condensates.
3D skyrmion spin structure | Image credit: David Hall
They applied magnetic field to polarize the spin of all atoms to point them in the upward direction. Then they instantly altered the magnetic field in such a manner that it creates a field-vanishing point at the center of condensate.
As a result, atoms’ spins begin rotating in the new direction of applied magnetic field. The spins wind into a knot, because applied field equally points in all directions. The skyrmion’s knotted-structure has linked loops, where all spins point to a particular direction. One can lose the knot, or make it move, but it couldn’t be untied.
The spin of the atom twists, and condensate’s quantum phase winds regularly, which make it a skyrmion instead of just a quantum knot.
When spin’s direction changes in space, the condensate’s velocity responds similar to what charged particle does in a magnetic field. Therefore, it produces a knotted artificial magnetic field that is same as the magnetic field in ball lightning model.
However, according to the researchers, we need more detailed studies to find out whether it’s possible to generate a real ball lightning using similar methods. This could allow us to keep plasma intact and create more stable fusion reactors in future.