Understanding the non-equilibrium dynamics of isolated quantum many-body systems is an unsolved problem in many areas of physics. I will present a series of experiments with ultracold one-dimensional Bose gases, which establish these gases as an ideal model system to explore a wide range of non-equilibrium phenomena.
In the experiments a single gas is coherently split into two parts. This creates a well-defined non-equilibrium situation that can be probed in great detail using matter-wave interferometry. The subsequent dynamics reveal the emergence of a prethermalized steady state, which differs strongly from thermal equilibrium [1,2]. Such thermal-like states had previously been predicted for a large variety of systems, but never been observed directly. A further detailed study of the relaxation process shows that the thermal correlations of the prethermalized state emerge locally in their final form and propagate through the system in a light-cone-like evolution . This provides first experimental evidence for the local relaxation conjecture, which links relaxation processes in quantum many-body systems to the propagation of correlations. Moreover, engineering the initial state of the evolution enables the first direct observation of a generalized thermodynamical ensemble . This points to a natural emergence of classical statistical properties from the microscopic unitary quantum evolution, and forms a cornerstone for a universal framework of non-equilibrium physics.
Finally, I will comment on recent advances in the tomography of quantum many-body states  and the study of tunnel-coupled one-dimensional Bose gases.
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