Hilbert Subspace Ergodicity

Ergodicity has been one of the fundamental concepts underpinning our understanding of thermalisation in isolated systems since the first developments in classical statistical mechanics. Recently, a similar notion has been introduced for quantum systems, termed Complete Hilbert Space Ergodicity (CHSE), in which the evolving quantum state explores all of the available Hilbert space. This contrasts with the Eigenstate Thermalisation Hypothesis (ETH), in which thermalisation is formulated via the properties of matrix elements of local operators in the energy eigenbasis. In this work we explore how ETH-violation mechanisms, including quantum many-body scars and Hilbert space fragmentation can affect Complete Hilbert Space Ergodicity. We find that the presence of these mechanisms leads to CHSE in decoupled subspaces, a phenomenon we call Hilbert Subspace Ergodicity, and which represents a protocol for constructing t-designs in subspaces.