Quantum mechanics

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Theoretical physics

Biology

Hilbert space

Classical mechanics

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Physics

Superconductivity

Quantum

Quasiparticle

Non-equilibrium thermodynamics

Annual Review of Condensed Matter Physics

Weakly interacting quasiparticles play a central role in the low-energy description of many phases of quantum matter. At higher energies, however, quasiparticles cease to be well-defined in generic many-body systems due to a proliferation of decay channels. In this review, we discuss the phenomenon of quantum many-body scars, which can give rise to certain species of stable quasiparticles throughout the energy spectrum. This goes along with a set of unusual non-equilibrium phenomena including many-body revivals and non-thermal stationary states. We provide a pedagogical exposition of this physics via a simple yet comprehensive example, that of a spin-1 XY model. We place our discussion in the broader context of symmetry-based constructions of many-body scar states, projector embeddings, and Hilbert space fragmentation. We conclude with a summary of experimental progress and theoretical puzzles.

Quantum Many-Body Scars: A Quasiparticle Perspective