September 1, 2016 | A. M. Kaufman, M. E. Tai, A. Lukin, M. Rispoli, R. Schittko, P. M. Preiss, and M. Greiner*
The paper explores the emergence of statistical mechanics in a quantum state, focusing on the role of quantum entanglement in facilitating thermalization. The authors experimentally study a Bose-Einstein condensate of ${ }^{87} \mathrm{Rb}$ atoms in a two-dimensional optical lattice, observing thermalization on a local scale while maintaining a globally pure quantum state. They measure entanglement entropy and observe how it assumes the role of thermal entropy in thermalization. Despite the full state remaining pure, entanglement creates local entropy, validating the use of statistical physics for local observables. The measurements agree with the Eigenstate Thermalization Hypothesis (ETH) and the presence of a near-volume law in entanglement entropy. The study highlights the equivalence between entanglement entropy and thermal entropy, providing insights into the dynamics of thermalization in closed quantum systems.The paper explores the emergence of statistical mechanics in a quantum state, focusing on the role of quantum entanglement in facilitating thermalization. The authors experimentally study a Bose-Einstein condensate of ${ }^{87} \mathrm{Rb}$ atoms in a two-dimensional optical lattice, observing thermalization on a local scale while maintaining a globally pure quantum state. They measure entanglement entropy and observe how it assumes the role of thermal entropy in thermalization. Despite the full state remaining pure, entanglement creates local entropy, validating the use of statistical physics for local observables. The measurements agree with the Eigenstate Thermalization Hypothesis (ETH) and the presence of a near-volume law in entanglement entropy. The study highlights the equivalence between entanglement entropy and thermal entropy, providing insights into the dynamics of thermalization in closed quantum systems.