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...

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