The fate of phonons in freely expanding Bose-Einstein condensates

TL;DR

This paper investigates how phonon-like excitations in a Bose-Einstein condensate evolve during free expansion, revealing mechanisms of energy transfer and providing a new spectroscopy method for Bogoliubov excitations.

Contribution

It introduces a theoretical framework to understand phonon evaporation in expanding condensates using Gross-Pitaevskii and Bogoliubov equations.

Findings

Different evaporation regimes depending on phonon wavelength

Signatures of evaporated phonons in density distribution

New spectroscopy method for Bogoliubov excitations

Abstract

Phonon-like excitations can be imprinted into a trapped Bose-Einstein condensate of cold atoms using light scattering. If the condensate is suddenly let to freely expand, the initial phonons lose their collective character by transferring their energy and momentum to the motion of individual atoms. The basic mechanisms of this evaporation process are investigated by using the Gross-Pitaevskii theory and dynamically rescaled Bogoliubov equations. Different regimes of evaporation are shown to occur depending on the phonon wavelength. Distinctive signatures of the evaporated phonons are visible in the density distribution of the expanded gas, thus providing a new type of spectroscopy of Bogoliubov excitations.