24 May 2009 ICFO in Nature Physics

Density profile and velocity field of a one-vortex state.

A paper co-authored by Prof. Maciej Lewenstein describes for the first time the quantum state that gives place to the formation of a vortex in a rotating superfluid. When a superfluid is put in rotation it slips within its container due to the absence of viscosity in this state of matter. But if the velocity of rotation is brought above a certain threshold, an abrupt transition gives place to a vortex, like common rotating fluids. Then, two or more vortices can be formed. An article published in Nature Physics has described for the first time the complicated quantum state of the superfluid at the transition: a complex superposition of the presence and the absence of the vortex. The article is published by a group of researchers working at the University of Barcelona, ICFO Barcelona/ICREA, and ENS Paris/CNRS.

According to the authors, the result sheds light on the large class of dramatic structural changes in mesoscopic systems, ranging from superconductors to ultracold atoms. The system studied in this work is a droplet of cold bosons in a superfluid state due to Bose-Einstein condensation. When the droplet is rotated, a vortex appears above a specific angular velocity. This abrupt change can be explained in terms of a change of symmetry, like in the case of standard phase transition of macroscopic systems.

It is easy to describe both states: with and without the presence of vortices, since bosons are weakly interacting systems. However, understanding what happens during the change is not so straightforward. The authors have managed to develop a theory that describes this intermediate state. While the interactions remain weak, symmetry breaking generates a highly correlated state.

This state is a superposition of the presence and absence of vortices, but it is more complicated than a standard “Schrödinger’s cat”. The researchers highlight that their model provides the theoretical instruments to measure the features of this state in an experiment. Moreover, they sustain that this result can be generalized to a large class of weakly interacting quantum systems.