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arXiv:1310.1301 · Physical Review A

Correlation evolution in dilute Bose-Einstein condensates after quantum quenches

Joanna Pietraszewicz, Magdalena Stobińska, Piotr Deuar

Published Feb 15, 2019• 9 citations
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Abstract

The universal forms of quantum density and phase correlations after an interaction quench are found for dilute one-dimensional (1D), 2D, and 3D condensates. A Bogoliubov approach in a local density approximation is used. We obtain compact expressions for the most visible effects. Our results show how loss of phase coherence and antibunching are built up after the quench by quantum fluctuations. We demonstrate further that the density correlations can be observed even with imaging resolution much worse than healing length. This indicates that the direct measurement of counterpropagating atom pairs in situ in a continuum system is realistic. The conditions in contemporary 1D experiments are especially favorable for the correlation wave observations.

DOI
10.1103/physreva.99.023620

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The universal forms of quantum density and phase correlations after an interaction quench are found for dilute one-dimensional (1D), 2D, and 3D condensates. A Bogoliubov approach in a local density approximation is used. We obtain compact expressions for the most visible effects. Our results show how loss of phase coherence and antibunching are built up after the quench by quantum fluctuations. We demonstrate further that the density correlations can be observed even with imaging resolution much worse than healing length. This indicates that the direct measurement of counterpropagating atom pairs in situ in a continuum system is realistic. The conditions in contemporary 1D experiments are especially favorable for the correlation wave observations.
We obtain the universal form of spatial density and phase correlations after a quantum quench in dilute ultracold Bose gases, and give compact time-dependent expressions for the most visible effects. A Bogoliubov description in a local density approximation is used. It is demonstrated that density correlations can be observed even with imaging resolution well below healing length, and estimated that the conditions in existing 1D experiments allow observation of the long-range density correlation wave. This would allow one to directly measure counterpropagating atom pairs in situ.
Macroscopic coherence is an important feature of quantum many-body systems exhibiting collective behaviors, with examples ranging from atomic Bose-Einstein condensates, and quantum liquids to superconductors. Probing many-body coherence in a dynamically unstable regime, however, presents an intriguing and outstanding challenge in out-of-equilibrium quantum many-body physics. Here, we experimentally study the first- and second-order coherence of degenerate quasi-one-dimensional (1D) Bose gases quenched from repulsive to modulationally unstable attractive interaction regimes. The resulting dynamics, monitored by in situ density and matter-wave interference imaging, reveals phase-coherent density wave evolutions, clearly distinguished from iconic soliton trains previously observed in attractive gases. This arises from the interplay between noise-amplified density modulations and dispersive shock waves of broad interest in nonlinear physics, plasmas, granular systems, and beyond. At longer times, the gases become phase-scrambled, exhibiting a finite correlation length. Interestingly, following an interaction quench back to the repulsive regime, we observe that quasi-long-range coherence can be spontaneously re-established. This captivating rephasing dynamics can be attributed to the nucleation and annihilation of density defects in the quasi-1D geometry. These results shed light on out-of-equilibrium phase coherence in quantum many-body systems in a regime where beyond mean-field effects may arise and theoretical approaches have not been well-established. Attractive Bose gases are modulationally unstable yet exhibit rich nonlinear many-body behaviors, owing to the interplay between their coherence and unstable collective excitations. Here, the authors probe out-of-equilibrium dynamics in quasi-1D Bose-Einstein condensates subject to quench dynamics between repulsive and attractive interactions.
We study, within the framework of the classical-field approximation, the density correlations of a weakly interacting expanding Bose gas for the whole range of temperatures across the Bose-Einstein condensation threshold. We focus on elongated quasi-one-dimensional systems where there is a huge discrepancy between the existing theory and experimental results [A. Perrin et al., Nat. Phys. 8, 195 (2012)]. We find that the density correlation function is not reduced for temperatures below the critical one as it is predicted for the ideal gas or for a weakly interacting system within the Bogoliubov approximation. This behavior of the density correlations agrees with the above-mentioned experiment with the elongated system. Although the system was much larger than that studied here, we believe that the behavior of the density correlation function found there is quite generic. Our theoretical study indicates also large density fluctuations in the trap in the quasicondensate regime where only phase fluctuations were expected. We argue that the enhanced density fluctuations can originate in the presence of interactions in the system, or more precisely in the presence of spontaneous dark solitons in the elongated gas at thermal equilibrium.
Significance Understanding the combined effects of disorder and interactions in quantum many-body systems is a key topic of condensed matter physics. While most studies have considered a static disorder, here we investigate the relaxation dynamics of a three-dimensional Bose–Einstein condensate after switching on or off an optical disorder potential. We study, experimentally and numerically, the transient and steady-state atomic density distribution and the ability of the gas to hydrodynamically expand, directly reflecting long-range phase coherence. We find that, when the gas is exposed to disorder, long-range phase coherence breaks down one order of magnitude faster than the density distribution responds. In the opposite case, surprisingly, long-range coherence takes two orders of magnitude longer times to revive than the density relaxation.
The understanding of nonequilibrium dynamics in many-body quantum systems is a fundamental issue in statistical physics. Experiments that probe universal properties of these systems can address such foundational questions. In this study, we report the measurement of universal dynamics triggered by a quench from the superfluid to normal phase across the Berezinskii-Kosterlitz-Thouless transition in a two-dimensional (2D) Bose gas. We reduced the density by splitting the 2D gas in two, realizing a quench across the critical point. The subsequent relaxation dynamics were probed with matter-wave interferometry to measure the local phase fluctuations. We show that the time evolution of both the phase correlation function and vortex density obeys universal scaling laws. This conclusion is supported by classical-field simulations and interpreted by means of real-time renormalization group theory.
We demonstrate high-resolution in situ imaging of density-wave ordering induced by cavity-mediated interactions in a unitary Fermi gas. We observe long-range spatial correlations throughout the formation of density waves, both for adiabatic preparation and following a quench, with a pattern controlled by the cavity mode structure. Our single-shot microscopic images together with the real-time readout of the cavity photons provide access to atom-photon correlations. We use this capability to investigate order fluctuations as a function of time following a quench and to directly confirm the correspondence between optical and atomic observables. Our system opens rich perspectives, from local patterning to correlation measurements in long-range interacting quantum gases.