Thermalization and irreversibility of an isolated quantum system II

Abstract: The irreversible entropy increase described by the second law of thermodynamics is fundamentally tied to thermalization and the emergence of equilibrium. In the first part of our work (Ref: arXiv.2503.04152), we constructed an isolated gas system model and numerically demonstrated irreversible growth of entanglement entropy caused by erasure of spread non-equilibrium state information. Here, we mathematically prove that for any macroscopic system in a non-equilibrium state $|\phi_0\rangle$, the quantum state $|\phi'_0\rangle = \hat{O}(t)|\phi_0\rangle$ will inevitably evolve toward equilibrium. Our work demonstrates that the second law of thermodynamics, and consequently the ergodic hypothesis in statistical physics, can be understood and proven from a quantum information perspective. From this perspective, the second law can be stated as: In generic macroscopic physical systems, the spreading and erasure of non-equilibrium information is unavoidable.