From Coherent Modes to Turbulence and Granulation of Trapped Gases

TL;DR

This paper explores how external perturbations can excite trapped Bose gases from a condensed, symmetry-broken state to a fully symmetric, turbulent, and granular fluid state, combining theoretical and experimental insights.

Contribution

It introduces a comprehensive description of the transition from coherent modes to turbulence and granulation in trapped gases under external excitation.

Findings

Progression from Bose-Einstein condensate to turbulent fluid observed

External perturbations restore gauge symmetry in trapped gases

Experimental and theoretical analysis of nonequilibrium states

Abstract

The process of exciting the gas of trapped bosons from an equilibrium initial state to strongly nonequilibrium states is described as a procedure of symmetry restoration caused by external perturbations. Initially, the trapped gas is cooled down to such low temperatures, when practically all atoms are in Bose-Einstein condensed state, which implies the broken global gauge symmetry. Excitations are realized either by imposing external alternating fields, modulating the trapping potential and shaking the cloud of trapped atoms, or it can be done by varying atomic interactions by means of Feshbach resonance techniques. Gradually increasing the amount of energy pumped into the system, which is realized either by strengthening the modulation amplitude or by increasing the excitation time, produces a series of nonequilibrium states, with the growing fraction of atoms for which the gauge symmetry is restored. In this way, the initial equilibrium system, with the broken gauge symmetry and all atoms condensed, can be excited to the state, where all atoms are in the normal state, with completely restored gauge symmetry. In this process, the system, starting from the regular superfluid state, passes through the states of vortex superfluid, turbulent superfluid, heterophase granular fluid, to the state of normal chaotic fluid in turbulent regime. Both theoretical and experimental studies are presented.