New characteristic of quantum many-body chaotic systems.

An isolated quantum system in a pure state may be perceived as thermal if only a substantially small fraction of all degrees of freedom is probed. We propose that in a quantum chaotic many-body system all states with sufficiently small energy fluctuations are approximately thermal. We refer to this hypothesis as canonical universality (CU). The CU hypothesis complements the eigenstate thermalization hypothesis which proposes that for chaotic systems individual energy eigenstates are thermal. Integrable and many-body localization systems do not satisfy CU. We provide theoretical and numerical evidence supporting the CU hypothesis.