Bose-Einstein condensates are an ideal platform to explore dynamical phenomena emerging in the many-body limit. A cloud of individual atoms in this state collectively behaves as a single fluid. This quantum fluid can flow without resistance – it is superfluid.

Two superfluids can exist at the same time in ultracold atomic clouds. Until now, their coexistence could not be observed experimentally. Now, however, physicists from Heidelberg University have demonstrated such a magnetic quantum fluid – it is fluid in two ways – in an atomic gas.

Markus Oberthaler, a Kirchhoff Institute for Physics researcher, explains, “In recent decades atomic Bose-Einstein condensates were created from very different types of atoms such as sodium and rubidium, but more recently also from more “exotic” atoms like erbium and dysprosium.”

“Most of these atoms also exhibit internal degrees of freedom – they have a spin and behave like small magnets. This can, in principle, also give rise to the phenomenon of Bose-Einstein condensation, but this has not been experimentally observed yet. This demonstration is now possible with an ultracold cloud of rubidium atoms.”

The method called evaporative cooling is usually used to prepare a Bose-Einstein condensate. This work is similar to cooling coffee in a cup by blowing on it.

The fastest atoms at the surface of the coffee are blown away, and one waits until the remaining atoms come to rest at a cooler temperature. This is extremely difficult for a spin, so Heidelberg physicists chose another method.

Dr. Maximilian Prüfer said, “We initialized the system far from equilibrium and waited until the rubidium atoms reached a new state of equilibrium. What at first seemed less intuitive turned out to be extremely efficient.”

The scientists used particular detection and perturbation techniques created just for this state’s creation and tracing. They noticed that the spin also turned superfluid along with the motional degree of freedom. Thus, there are two ways that magnetic quantum fluids can become exceedingly fluid. 

Markus Oberthaler, head of the “Synthetic Quantum Systems” research group, which is also part of the STRUCTURES Cluster of Excellence of Heidelberg University, said, “Our new research methods allow us not only to characterize the condensate but will also allow us to better understand the path from non-equilibrium to that state.”

Journal Reference:

  1. Prüfer, M., Spitz, D., Lannig, S. et al. Condensation and thermalization of an easy-plane ferromagnet in a spinor Bose gas. Nat. Phys. (2022). DOI: 10.1038/s41567-022-01779-6