Quantum Many-Body Scars Constructed from Integrable Boundary States*

Recently, there has been a great deal of interest in studying non-integrable systems exhibiting nonthermal eigenstates called quantum many-body scars (QMBS). They are expected to shed light on the mechanisms of thermalization in isolated quantum many-body systems. There have been a number of attempts to construct models with exact QMBS. Here we present a method to construct quantum spin models with multi-body interactions that exhibit exact QMBS. Our method employs integrable boundary states that are annihilated by all parity-odd conserved charges in integrable spin chains. By adding a perturbation that breaks the integrability but leaves the integrable boundary states as eigenstates, one can construct scarred models. To illustrate the power of these methods, we construct several models with QMBS and compute physical quantities, such as the entanglement entropy of each eigenstate. Furthermore, in a model with a tower of QMBS, we find that a superposition of these states exhibits non-thermalizing dynamics that cannot occur for typical thermal states.