Self-induced density modulations in the free expansion of Bose-Einstein condensates

TL;DR

This paper numerically investigates the free expansion of Bose-Einstein condensates, revealing nonlinear phenomena such as shock waves and density oscillations that depend on the strength of atomic interactions.

Contribution

It demonstrates the emergence of shock waves and density modulations during free expansion, a novel observation in ultra-cold atomic gases.

Findings

Weak interactions lead to self-similar expansion.

Strong interactions cause shock wave formation and internal depletion.

Radial density waves with multiple minima and maxima are observed.

Abstract

We simulate numerically the free expansion of a repulsive Bose-Einstein condensate with an initially Gaussian density profile. We find a self-similar expansion only for weak inter-atomic repulsion. In contrast, for strong repulsion we observe the spontaneous formation of a shock wave at the surface followed by a significant depletion inside the cloud. In the expansion, contrary to the case of a classical viscous gas, the quantum fluid can generate radial rarefaction density waves with several minima and maxima. These intriguing nonlinear effects, never observed yet in free-expansion experiments with ultra-cold alkali-metal atoms, can be detected with the available setups.