Person: Hofferberth, Sebastian
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Publication Two-point density correlations of quasicondensates in free expansion
(American Physical Society (APS), 2010) Manz, S.; Bücker, R.; Betz, T.; Koller, Ch.; Hofferberth, Sebastian; Mazets, I. E.; Imambekov, A.; Demler, Eugene; Perrin, A.; Schmiedmayer, J.; Schumm, T.We measure the two-point density correlation function of freely expanding quasicondensates in the weakly interacting quasi-one-dimensional (1D) regime. While initially suppressed in the trap, density fluctuations emerge gradually during expansion as a result of initial phase fluctuations present in the trapped quasicondensate. Asymptotically, they are governed by the thermal coherence length of the system. Our measurements take place in an intermediate regime where density correlations are related to near-field diffraction effects and anomalous correlations play an important role. Comparison with a recent theoretical approach described by Imambekov et al. yields good agreement with our experimental results and shows that density correlations can be used for thermometry of quasicondensates.
Publication Density ripples in expanding low-dimensional gases as a probe of correlations
(American Physical Society (APS), 2009) Imambekov, A.; Mazets, I. E.; Petrov, D. S.; Gritsev, V.; Manz, S.; Hofferberth, Sebastian; Schumm, T.; Demler, Eugene; Schmiedmayer, J.We investigate theoretically the evolution of the two-point density correlation function of a low-dimensional ultracold Bose gas after release from a tight transverse confinement. In the course of expansion thermal and quantum fluctuations present in the trapped systems transform into density fluctuations. For the case of free ballistic expansion relevant to current experiments, we present simple analytical relations between the spectrum of “density ripples” and the correlation functions of the original confined systems. We analyze several physical regimes, including weakly and strongly interacting one-dimensional (1D) Bose gases and two-dimensional (2D) Bose gases below the Berezinskii-Kosterlitz-Thouless (BKT) transition. For weakly interacting 1D Bose gases, we obtain an explicit analytical expression for the spectrum of density ripples which can be used for thermometry. For 2D Bose gases below the BKT transition, we show that for sufficiently long expansion times the spectrum of the density ripples has a self-similar shape controlled only by the exponent of the first-order correlation function. This exponent can be extracted by analyzing the evolution of the spectrum of density ripples as a function of the expansion time.