Local emergence of thermal correlations in an isolated quantum many-body system

The relaxation mechanisms of isolated quantum many-body systems are insufficiently understood, but a one-dimensional quantum gas experiment uncovers the local emergence of thermal correlations and their cone-like propagation through the system. Understanding the dynamics of isolated quantum many-body systems is a central open problem at the intersection between statistical physics and quantum physics. Despite important theoretical effort 1 , no generic framework exists yet to understand when and how an isolated quantum system relaxes to a steady state. Regarding the question of how, it has been conjectured 2 , 3 that equilibration must occur on a local scale in systems where correlations between distant points can establish only at a finite speed. Here, we provide the first experimental observation of this local equilibration hypothesis. In our experiment, we quench a one-dimensional Bose gas by coherently splitting it into two parts. By monitoring the phase coherence between the two parts we observe that the thermal correlations of a prethermalized state 4 , 5 emerge locally in their final form and propagate through the system in a light-cone-like evolution. Our results underline the close link between the propagation of correlations 2 , 3 , 6 , 7 and relaxation processes in quantum many-body systems.