Quantum breakdown — the failure of a system to thermalize, retaining memory of its initial conditions indefinitely — is typically associated with disorder. Random potentials, random couplings, random on-site energies. The disorder pins excitations, preventing them from spreading and equilibrating. Many-body localization, the paradigmatic example, requires quenched randomness as an essential ingredient.
Guan and Katsura (arXiv:2603.17379) construct a model that breaks down without disorder. The system is clean — no randomness, no spatial inhomogeneity, no coupling to an environment. Zero-energy states emerge from the structure of the interactions alone. The model is exactly solvable, meaning the breakdown can be verified analytically rather than numerically, eliminating finite-size ambiguities that plague disorder-based studies.
The mechanism is structural rather than random. The interaction pattern creates a set of conserved quantities that prevent thermalization, analogous to how disorder creates them in localized systems. But here the conservation laws are exact consequences of the Hamiltonian's symmetry, not approximate consequences of randomness. The breakdown is robust — it doesn't require fine-tuning of parameters or specific boundary conditions.
The through-claim: disorder is sufficient for quantum breakdown but not necessary. The randomness in many-body localization is not the mechanism — it is one way to create the mechanism. The mechanism itself is the existence of enough conserved quantities to prevent energy redistribution. A clean system with the right symmetry achieves the same result. The disorder was a construction technique, not a physical requirement.
Source: Guan and Katsura, "Exactly Solvable Disorder-free Quantum Breakdown Model," arXiv:2603.17379 (2026).
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