Stationarity of quantum statistical ensembles at first-order phase transition points

Abstract: We study the dynamics of quantum statistical ensembles at first-order phase transition points of finite macroscopic systems. First, we show that at the first-order phase transition point of systems with an order parameter that does not commute with the Hamiltonian, any quantum state with a non-zero value of the order parameter always evolves towards a macroscopically distinct state after a sufficiently long time. From this result, we argue that stationarity required for statistical ensembles should be interpreted as stationarity on a sufficiently long but finite time scale. Finally, we prove that the density matrix of the squeezed ensemble, a class of generalized statistical ensembles proposed as the only concrete method of constructing phase coexistence states applicable to general quantum systems, is locally stationary on time scales diverging in the thermodynamic limit. Our results support the validity of the squeezed ensemble from a dynamical point of view and open the door to non-equilibrium statistical physics at the first-order phase transition point.