On 14/04/2026, at 1:15 PM, in room 2011 of the Faculty of Physics, University of Białystok, Krzysztof Pawłowski, from Center for Theoretical Physics, Polish Academy of Sciences, Warsaw, Poland, will give a lecture entitled:
„Fluctuations and correlations in ultracold gas at equilibrium”
Tomasz Karpiuk
Andrzej Maziewski
„Fluctuations and correlations in ultracold gas at equilibrium”
Krzysztof Pawłowski
Center for Theoretical Physics, Polish Academy of Sciences, Warsaw, Poland
Fluctuations and correlations play a central role in characterizing quantum many-body systems. While they effectively capture intrinsic many-body features, they also serve as sensitive indicators of thermal effects and the specific physical constraints of the experimental setup. In standard statistical physics curricula and foundational textbooks, the analysis of fluctuations is typically conducted within the Grand Canonical Ensemble, following the original approach used by A. Einstein to derive Bose-Einstein condensation. However, this theoretical framework fails significantly when applied to a quantity as fundamental as the condensate atom number fluctuations in an isolated ideal gas - systems that closely resemble current experimental realizations. These discrepancies between ensemble predictions and physical reality are now being explored with increasing precision. I will present the efforts of many groups over the past decades. They were finalized with the first observation of condensate atom number fluctuations [1], subsequently providing the first experimental evidence of their microcanonical nature [2]. To further the study of these phenomena, we have recently derived analytically full quantum statistics, based on saddle-point approximations extended by higher-order cumulants (1D and 3D), and advanced numerical techniques. Some of the results are summarized in our comprehensive review [3]. I will present these results alongside our current theoretical predictions regarding phase and density-density correlations.
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