Microcanonical and canonical fluctuations in atomic Bose-Einstein condensates – Fock state sampling approach

• Published in: SciPost physics Vol. 14; no. 3; p. 036
• Main Authors: Kruk, Maciej Bartłomiej, Hryniuk, Dawid, Kristensen, Mick, Vibel, Toke, Pawłowski, Krzysztof, Arlt, Jan, Rzążewski, Kazimierz
• Format: Journal Article
• Language: English
• Published: SciPost 01-03-2023
• ISSN: 2542-4653; 2542-4653
• DOI: 10.21468/SciPostPhys.14.3.036

The fluctuations of the atom number between a Bose-Einstein
condensate and the surrounding thermal gas have been the subject of a
long standing theoretical debate. This discussion is centered around the
appropriate thermodynamic ensemble to be used for theoretical
predictions and the effect of interactions on the observed fluctuations.
Here we introduce the so-called Fock state sampling method to solve this
classic problem of current experimental interest for weakly interacting
gases. A suppression of the predicted peak fluctuations is observed when
using a microcanonical with respect to a canonical ensemble. Moreover,
interactions lead to a shift of the temperature of peak fluctuations for
harmonically trapped gases. The absolute size of the fluctuations
furthermore depends on the total number of atoms and the aspect ratio of
the trapping potential. Due to the interplay of these effect, there is
no universal suppression or enhancement of fluctuations.