Open-system eigenstate thermalization in a noninteracting integrable model

We study thermalization in isolated quantum systems from an open quantum systems perspective. We argue that for a small system connected to a macroscopic bath, the system observables are thermal if the combined system-bath configuration is in an eigenstate of its Hamiltonian, even for fully integrable models (unless thermalization is suppressed by localization due to strong coupling). We illustrate our claim for a single fermionic level coupled to a noninteracting fermionic bath. We further show that upon quenching the system Hamiltonian, the system occupancy relaxes to the thermal value corresponding to the new Hamiltonian. Finally, we demonstrate that system thermalization also arises for a system coupled to a bath initialized in a typical eigenstate of its Hamiltonian. Our findings show that chaos and nonintegrability are not the sole drivers of thermalization and complementary approaches are needed to offer a more comprehensive understanding of how statistical mechanics emerges.