Taking the temperature of quantum many-body scars

A quantum many-body scar is an eigenstate of a chaotic many-body Hamiltonian that displays two apparently contradictory features: its energy corresponds to a high temperature, yet its entanglement characteristics resemble those of low-temperature eigenstates, such as ground states. In standard treatments, temperature is associated with an energy eigenvalue through the canonical temperature–energy relation. In contrast, this work employs the eigenstate subsystem temperature—a recently introduced measure that assigns a temperature directly to an energy eigenstate based on the structure of its reduced density matrix. For thermal eigenstates, this quantity closely matches the canonical temperature. Since quantum many-body scars exhibit ground-state-like entanglement, it is not obvious that their eigenstate subsystem temperature should align with their canonical temperature. Surprisingly, we find that it does: quantum many-body scars appear to possess an approximate “awareness” of their spectral location encoded in the structure of their reduced density matrices.