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Localized Fock space cages in kinetically constrained models

Cheryne Jonay1 and Frank Pollmann2,3

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Phys. Rev. B 113, 134313 – Published 27 April, 2026

DOI: https://doi.org/10.1103/wz33-vczt

Abstract

We investigate a mechanism for nonergodic behavior arising from destructive interference in Fock space, leading to Fock space cages (FSCs)—exact zero-energy eigenstates localized on O(1) to O(Lp) configurations within an exponentially large connected Krylov sector. Unlike many-body localization or Hilbert space fragmentation, FSCs emerge from graph-theoretic interference cancellations in disorder-free kinetically constrained systems with chiral symmetry. We develop systematic algorithms that enable explicit construction of these cages in four representative models, revealing this as a universal mechanism for kinetically constrained systems with chiral symmetry. Dynamically, FSCs produce persistent plateaus in the Loschmidt echo and magnetization: return probabilities scale as L−2 for O(L) cages and remain O(1) for ultralocal ones, while the magnetization saturates to O(1) instead of decaying to zero. This may provide new routes for engineering long-lived nonthermal states in quantum simulation platforms.

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Corrections

17 September, 2026

Correction: The source information in Ref. [48] was incorrect and has been fixed.

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Outline

Information

Phys. Rev. B 113, 134313– Published 27 April, 2026
  • Received 12 May 2025
  • Revised 18 March 2026
  • Accepted 9 April 2026
  • Corrected 17 September 2026
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