Equilibration of an isolated quantum many-body system

Despite extensive study, many gaps remain in our understanding of the statistical mechanics of quantum many-body systems. Among them is how an isolated quantum many-body system achieves thermal equilibrium. A new report by Jörg Schmiedmayer and colleagues at the Vienna University of Technology provides insights. A cigar-shaped, effectively one-dimensional ultracold gas of rubidium atoms provided a strongly isolated test system, intrinsically rife with fluctuations, that evolved on experimentally observable time scales. To probe its relaxation dynamics, the researchers split a trapped atom cloud longitudinally into two parts, let the resulting nonequilibrium state evolve for a certain time, then used matter-wave interferometry to measure the phase differences between the two parts along the length of the system. By conducting some 150 iterations for each wait interval, the team determined how the phase correlations between the two clouds evolved over time. Although the split clouds started off strongly correlated (symbolized by gray atoms in this plot), the short-range coherence immediately began to decay exponentially with distance—a signature of thermal correlations (indicated by mixed red and gray atoms)—but only up to a certain length, beyond which long-range coherence persisted. Notably, the position of the crossover moved: The thermal correlations propagated through the system at the speed of sound. Technically, the phases relaxed to a so-called prethermalized state. Nonetheless, the results suggest a general route through which classical properties emerge in isolated quantum many-body systems. (T. Langen et al., Nat. Phys. 9, 640, 2013. Image courtesy of the Vienna University of Technology.)