 The ergodic hierarchy has also been used to understand quantum chaos. Castagnino and Lombardi (2007) analyze the problem of quantum chaos as a particular case of the classical limit of quantum mechanics and identify mixing in the classical limit as the condition that a quantum system must satisfy to be nonintegrable. Gomez and Castagnino (2014, 2015) generalize the entire ergodic hierarchy to the quantum context and argue that EH thus generalized is a helpful tool to understand quantum chaos; Fortin and Lombardi (2018) use EH to understand decoherence; and Gomez (2018) discusses the KS entropy in quantum mixing systems.