Quantum thermalization of translation-invariant systems at high temperature

Abstract: Quantum thermalization describes how closed quantum systems can effectively reach thermal equilibrium, resolving the apparent incongruity between the reversibility of Schr\"odinger's equation and the irreversible entropy growth dictated by the second law of thermodynamics. Despite its ubiquity and conceptual significance, a complete proof of quantum thermalization has remained elusive for several decades. Here, we prove that quantum thermalization must occur in any qubit system with local interactions satisfying three conditions: (i) high effective temperature, (ii) translation invariance, and (iii) no perfect resonances in the energy spectrum. Specifically, we show that a typical, initially unentangled pure state drawn from any ensemble with maximum entropy becomes locally indistinguishable from a Gibbs state upon unitary evolution. Our proof relies on a recent breakthrough in quantum information theory proving the separability of high-temperature thermal states, as well as on new technical results identifying sufficient conditions for quantum thermalization, whose applicability extends beyond our main result. Our work illustrates that statistical physics can be understood as an emergent phenomenon, explicitly derived from the first principles of quantum mechanics.