# Far-From-Equilibrium Quantum Many-Body Systems

From Universal Dynamics to Statistical Mechanics

### Abstract

This thesis examines, along side a series of experiments, the far-from-equilibrium dynamics of isolated quantum many-body systems. As a model example we study the relaxation of a single and two linearly coupled one-dimensional Bose gases, brought out-o↵-equilibrium through a rapid change in the system parameters. For the single Bose gas this quench is a rapid cooling of the system to the one-dimensional regime, leading to a far-from-equilibrium state. In the subsequent relaxation towards thermal equilibrium we find direct experimental evidence for a scaling evolution in space and time, signaling the approach of a non-thermal fixed point and universality far-from-equilibrium. For the two linearly coupled gases we demonstrate how the analysis of higher-order correlations and their factorization properties can be used to determine the validity of effective field theories. In thermal equilibrium we find the system is described by the sine-Gordon model up to 10th-order correlations. Lastly, the relaxation of two independent condensates, initialized in a strongly phase correlated state, is studied in the context of prethermalization, generalized statistical ensembles, and quantum recurrences. Our work paves the way for future studies of universality far from equilibrium.