Thermalization in small quantum systems

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Science  19 Aug 2016:
Vol. 353, Issue 6301, pp. 752-753
DOI: 10.1126/science.aah5776

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Chaos and ergodicity are the cornerstones of statistical physics and thermodynamics. Although classically, even small systems such as a particle in a two-dimensional cavity can exhibit chaotic behavior and thereby relax to a microcanonical ensemble, quantum systems formally cannot. However, recent theoretical work and, in particular, the eigenstate thermalization hypothesis (ETH), indicate that quantum systems can also thermalize. Indeed, ETH provides a framework connecting microscopic models and macroscopic phenomena, based on the notion of highly entangled quantum states. On page 794 of this issue, Kaufman et al. (1) demonstrate such thermalization in the relaxation dynamics of a small lattice system of interacting bosonic particles. By directly measuring the entanglement entropy of subsystems, as well as other observables, they show that after the initial transient time, the system locally relaxes to a thermal ensemble while globally maintaining a zero-entropy pure state.