An international team of physicists from Finland and the United States has made the first experimental observations of the dynamics of isolated monopoles in quantum matter. The results are published in the journal Physical Review X.
This is an artistic view of the decay of a quantum-mechanical monopole into a Dirac monopole. Image credit: Heikka Valja.
Unlike usual magnets, magnetic monopoles are elementary particles that have only a south or a north magnetic pole, but not both.
They have been theoretically predicted to exist, but no convincing experimental observations have been reported.
“In 2014, we experimentally realized a Dirac monopole, that is, Paul Dirac’s 80-year-old theory where he originally considered charged quantum particles interacting with a magnetic monopole,” said co-lead author David Hall, professor of physics at Amherst College.
“And in 2015, we created real quantum monopoles,” added co-lead author Dr. Mikko Möttönen, professor of quantum computing at the University of Jyväskylä and senior scientist in the Department of Applied Physics at Aalto University.
“Whereas the Dirac monopole experiment simulates the motion of a charged particle in the vicinity of a monopolar magnetic field, the quantum monopole has a point-like structure in its own field resembling that of the magnetic monopole particle itself.”
Now the team has produced an observation of how one of these unique magnetic monopole analogues spontaneously turns into another in less than a second.
“Sounds easy but we actually had to improve the apparatus to make it happen,” said study first author Tuomas Ollikainen, a doctoral candidate at Aalto University.
Left: experimental side image of the quantum monopole. Right: after 0.2 seconds, the quantum monopole has decayed into the Dirac monopole. The different colors represent the direction of the internal magnetic state of the atoms and the brightness corresponds to particle density. Image credit: Tuomas Ollikainen.
“We start with an extremely dilute gas of rubidium atoms chilled near absolute zero, at which temperature it forms a Bose-Einstein condensate,” the researchers explained.
“Subsequently, we prepare the system in a non-magnetized state and ramp an external magnetic-field zero point into the condensate thus creating an isolated quantum monopole.”
“Then we hold the zero point still and wait for the system to gradually magnetize along the spatially varying magnetic field.”
“The resulting destruction of the quantum monopole gives birth to a Dirac monopole,” they said.
The quantum monopole is a so-called topological point defect, that is, a single point in space surrounded by a structure in the non-magnetized state of the condensate that cannot be removed by continuous reshaping.
Such structures are related to the 2016 Nobel Prize in Physics which was awarded in part for discoveries of topological phase transitions involving quantum whirlpools, or vortices.
“Vortex lines have been studied experimentally in superfluids for decades; monopoles, on the other hand, have been studied experimentally for just a few years,” Prof. Hall said.
Although its topology protects the quantum monopole, it can decay since the whole phase of matter changes from non-magnetized to magnetized.
“No matter how robust an ice sculpture you make, it all flows down the drain when the ice melts,” Ollikainen said.
“For the first time, we observed spontaneously appearing Dirac monopoles and the related vortex lines,” Dr. Möttönen said.
_____
T. Ollikainen et al. 2017. Experimental Realization of a Dirac Monopole through the Decay of an Isolated Monopole. Phys. Rev. X 7 (2): 021023; doi: 10.1103/PhysRevX.7.021023
This article is based on text provided by Aalto University.
