Cloud parameter estimation for interacting BEC after time-of-flight

TL;DR

This paper investigates how interactions between condensed and thermal atoms in Bose-Einstein condensates affect the expansion profiles after time-of-flight, improving parameter estimation methods through simulation and fitting techniques.

Contribution

It introduces a simulation-based approach to account for interactions in BEC expansion, enhancing the accuracy of extracted system parameters compared to traditional models.

Findings

Simulated expansion profiles reveal interaction-induced deformation effects.

Fitting with a Bose-enhanced distribution introduces estimation errors.

Using simulation as a fitting function yields better agreement with semi-ideal models.

Abstract

Experiments on Bose-Einstein condensates at finite temperature typically extract the system parameters, such as temperature, atom number, and condensed fraction from time-of-flight images taken after a free expansion time. This paper systematically examines the effect of repulsive interactions between the condensed and thermal atoms in partially condensed clouds on the expansion profile of the thermal cloud. An analytical expression for the expansion can be obtained only if the interactions between the Bose-Einstein condensate and thermal atoms are neglected, resulting in a Bose-enhanced distribution for the thermal component. Here, the deformation of the cloud due to interactions and the effects on estimated parameters are investigated by simulating the expansion using a ballistic approximation. By fitting the simulated expansion profiles with a Bose-enhanced distribution, the errors of using such a fit are estimated, and the results are explained phenomenologically. The simulation was also used as a fitting function for experimental data, showing better agreement of the extracted condensed fraction with the semi-ideal model than results from a Bose-enhanced fit.