Introduction to One-Dimensional Bose Gases -- Experimental Realization of One-Dimensional Bose Gases -- Isolated Quantum Systems out of Equilibrium -- Relaxation and Prethermalization in One-Dimensional Bose Gases -- Local Emergence of Thermal Correlations -- Experimental Observation of a Generalized Gibbs Ensemble -- Relaxation Dynamics in An Imbalanced Pair of One-Dimensional Bose Gases -- Conclusion and Outlook.

This work presents a series of experiments with ultracold one-dimensional Bose gases, which establish said gases as an ideal model system for exploring a wide range of non-equilibrium phenomena. With the help of newly developed tools, like full distributions functions and phase correlation functions, the book reveals the emergence of thermal-like transient states, the light-cone-like emergence of thermal correlations and the observation of generalized thermodynamic ensembles. This points to a natural emergence of classical statistical properties from the microscopic unitary quantum evolution, and lays the groundwork for a universal framework of non-equilibrium physics. The thesis investigates a central question that is highly contested in quantum physics: how and to which extent does an isolated quantum many-body system relax? This question arises in many diverse areas of physics, and many of the open problems appear at vastly different energy, time and length scales, ranging from high-energy physics and cosmology to condensed matter and quantum information. A key challenge in attempting to answer this question is the scarcity of quantum many-body systems that are both well isolated from the environment and accessible for experimental study.