Operator space fragmentation in perturbed Floquet-Clifford circuits
Pith reviewed 2026-05-23 21:56 UTC · model grok-4.3
The pith
Perturbed Floquet-Clifford circuits localize operators through fragmentation of operator space into disjoint sectors created by wall configurations.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In the interacting model the appearance of wall configurations fragments operator space into disjoint sectors for 0 ≤ p < 1. The walls give rise to emergent local integrals of motion that are constructed exactly and produce strong localization of operators. The average length of operator spreading is tunable by p. Although the circuit is not separable across any bipartition, the fragmentation creates an entanglement bottleneck. Analytic arguments establish stability against generic single-qubit unitary perturbations, and the spectral form factor shows a fragmentation timescale before circular-unitary-ensemble behavior at p = 1.
What carries the argument
Wall configurations that fragment operator space into disjoint sectors and generate exact local integrals of motion.
Load-bearing premise
The walls and resulting local integrals of motion remain stable and exactly constructible under generic single-qubit unitary perturbations sampled uniformly.
What would settle it
A numerical simulation of operator evolution for p = 0.5 in which operators spread across the full system size without bound would falsify the localization claim.
Figures
read the original abstract
Floquet quantum circuits are able to realise a wide range of non-equilibrium quantum states, exhibiting quantum chaos, topological order and localisation. In this work, we investigate the stability of operator localisation and emergence of chaos in random Floquet-Clifford circuits subjected to unitary perturbations which drive them away from the Clifford limit. We construct a nearest-neighbour Clifford circuit with a brickwork pattern and study the effect of including disordered non-Clifford gates. The perturbations are uniformly sampled from single-qubit unitaries with probability $p$ on each qubit. We show that the interacting model exhibits strong localisation of operators for $0 \le p < 1$ that is characterised by the fragmentation of operator space into disjoint sectors due to the appearance of wall configurations. Such walls give rise to emergent local integrals of motion for the circuit that we construct exactly. We analytically establish the stability of localisation against generic perturbations and calculate the average length of operator spreading tunable by $p$. Although our circuit is not separable across any bi-partition, we further show that the operator localisation leads to an entanglement bottleneck, where initially unentangled states remain weakly entangled across typical fragment boundaries. Finally, we study the spectral form factor (SFF) to characterise the chaotic properties of the operator fragments and spectral fluctuations as a probe of non-ergodicity. In the $p = 1$ model, the emergence of a fragmentation time scale is found before random matrix theory sets in after which the SFF can be approximated by that of the circular unitary ensemble. Our work provides an explicit description of quantum phases in operator dynamics and circuit ergodicity which can be realised on current NISQ devices.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies operator dynamics in a nearest-neighbor brickwork Floquet-Clifford circuit with random single-qubit non-Clifford perturbations applied at each site with probability p. For 0 ≤ p < 1 it claims that random placement of Clifford gates produces 'wall configurations' that fragment the operator space into disjoint sectors; these walls are asserted to yield exactly constructible emergent local integrals of motion (LIOMs) that remain conserved under the full circuit evolution, including the generic perturbations. The work further claims an analytically tunable average operator-spreading length, an entanglement bottleneck across fragment boundaries, and distinct SFF regimes (including a fragmentation timescale at p=1 before RMT behavior).
Significance. If the exact LIOM construction and its invariance under generic perturbations hold, the manuscript supplies an explicit, analytically tractable mechanism for operator-space fragmentation and localization in a family of circuits that can be realized on NISQ hardware. The parameter p controlling the density of walls and the resulting tunable spreading length constitute a concrete handle on the crossover between Clifford and chaotic regimes.
major comments (1)
- [Abstract / stability and LIOM construction sections] Abstract (stability paragraph) and the section on exact LIOM construction: the central claim that wall configurations produce LIOMs that are 'constructed exactly' and remain conserved under conjugation by arbitrary single-qubit unitaries (uniformly sampled from the Haar measure) is load-bearing. Generic unitaries do not in general preserve the support or commutation relations of an operator localized on one side of a putative wall; without an explicit invariance condition (e.g., the LIOM being supported exclusively on Clifford sites or commuting independently of the unitary parameters), the fragmentation into disjoint sectors is not guaranteed to survive for p < 1. The manuscript must supply the explicit operator form of the LIOMs and demonstrate their commutation or invariance under the perturbation.
minor comments (1)
- [Abstract] The abstract states that the average length of operator spreading is 'calculated analytically' and 'tunable by p'; the corresponding derivation (presumably in the main text) should be cross-referenced to the wall-density formula so that the p-dependence is transparent.
Simulated Author's Rebuttal
We thank the referee for their careful reading and for identifying the central claim regarding LIOM stability as load-bearing. We respond to the major comment below.
read point-by-point responses
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Referee: [Abstract / stability and LIOM construction sections] Abstract (stability paragraph) and the section on exact LIOM construction: the central claim that wall configurations produce LIOMs that are 'constructed exactly' and remain conserved under conjugation by arbitrary single-qubit unitaries (uniformly sampled from the Haar measure) is load-bearing. Generic unitaries do not in general preserve the support or commutation relations of an operator localized on one side of a putative wall; without an explicit invariance condition (e.g., the LIOM being supported exclusively on Clifford sites or commuting independently of the unitary parameters), the fragmentation into disjoint sectors is not guaranteed to survive for p < 1. The manuscript must supply the explicit operator form of the LIOMs and demonstrate their commutation or invariance under the perturbation.
Authors: The manuscript already supplies the explicit operator form of the LIOMs in the exact LIOM construction section: they are defined as Pauli-string operators whose support is strictly confined to one side of each wall (i.e., products of Pauli operators on the qubits belonging to a given fragment). Because the walls are realized by Clifford gates, these operators are eigenoperators of the Clifford evolution by construction and therefore commute with every gate forming the wall. For the Haar-random single-qubit perturbations applied only at non-Clifford sites (with probability p < 1), the fragmentation guarantees that each LIOM has no support on the perturbed sites; consequently its commutation relation with any local unitary on those sites holds identically and independently of the unitary parameters. This invariance is shown analytically in the stability paragraph by verifying that conjugation by the perturbation leaves both the support and the eigenvalue of the LIOM unchanged. We therefore maintain that the fragmentation survives for p < 1 and that the required explicit form and invariance condition are already present. revision: no
Circularity Check
No circularity: exact analytical construction of LIOMs from walls is independent of inputs and numerical checks
full rationale
The paper's central derivation claims an exact construction of emergent local integrals of motion arising from wall configurations in the perturbed Floquet-Clifford circuit, with analytical stability established against generic single-qubit unitary perturbations for p < 1. This construction is presented as self-contained and does not reduce to fitted parameters, self-citations, or renaming of known results. The SFF analysis is a separate numerical characterization of chaos within fragments and does not define or force the fragmentation sectors or localisation length. No load-bearing self-citation chains or ansatzes smuggled via prior work are indicated. The derivation chain therefore remains independent of its inputs.
Axiom & Free-Parameter Ledger
free parameters (1)
- p
axioms (1)
- standard math Unitarity of the circuit evolution and the brickwork nearest-neighbour structure
invented entities (1)
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wall configurations
no independent evidence
Reference graph
Works this paper leans on
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In this case we could clearly find some sig- nal to generate any element ofGleft combined with any element of the left symplectic subspace. This would allow us to join the central site to the L subsytem forming a 0-wall and again contradicting that the wall be irreducible. The internal subspace must therefore be a proper and non-trivial subspace of symple...
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[2]
Let us define the subspace of left-conserved and right-conserved (2l + 1)-qubit operators of the form: L = { ¯Pl ⊗ {1, σc} ⊗ 1}\1, (17) R = {1 ⊗ {1, σc} ⊗ ¯Pl}\1, (18) 10 FIG. 7. A symmetric region of the circuit, with exactly one 1-wall on the central qubit (shown in red). Due to frag- mentation of the Pauli space, the perturbations in green act within i...
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[3]
(22) This shows that random rotations create, on average, a uniform mixture between local Paulis and thus in typical circuit realisations, rotations will de-stabilise the walls exponentially quickly in time. A similar analysis establishes that any perturbation within the k qubits between wall edges will break lo- calisation for k > 1. Although for larger ...
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[4]
The p = 0 limit The Clifford nature of the p = 0 model naturally suggests to consider the entanglement of stabiliser states as operator spreading is equivalent to entan- glement spreading in that case. An n-qubit stabiliser state |ψ⟩ is one which fulfils gi |ψ⟩ = |ψ⟩ for the gener- ators of an Abelian subgroup {gi} of the n-qubit Pauli group. A pure state...
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[5]
Average entanglement for p > 0 We argue that the above bound persists across walls which have no perturbation on the central qubit even when p > 0. Starting from a stabiliser state, the single-particle perturbations generate a superposition of stabilisers similarly to a branching process. If the peturbations act within the conserved subspace L, all stabil...
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[6]
Circuit regions In this section, we present exact diagonalisation re- sults for finite-size circuits to study the spreading of entanglement across wall with and without perturba- tions. The lack of symmetries in random unitaries limits the numerically attainable system sizes in ex- act diagonalisation to small qubit numbers. We resort to exact methods to ...
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Entropy growth across the wall We show the time-evolution of average entangle- ment entropy and entropy fluctuations in Figure 8 for p = 0 .5. For an unperturbed wall, the bound based on stabiliser equipartition is corroborated re- flecting the fact that circuit disorder generate walls with X, Y, Z conserved subspaces in equal numbers on average and there...
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[8]
Fluctuations in the entropy distribution Localisation is also observable from the large sam- ple fluctuations of the entropy on Figure 8. For an un- perturbed wall, entropy may only be generated across fragments through the Clifford wall gates, therefore the entropy of the state will be between 0 and 1 there- fore producing large fluctuations in the disor...
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We observe the ap- proach to oretical bound in steps of two in the system size
Convergence to the localised entropy bound We also performed a scaling analysis of reaching oretical bound on average entanglement for p = 0 .5 and p = 1 shown on Figure 9. We observe the ap- proach to oretical bound in steps of two in the system size. This finite size effect is due to our selection of bi- partite subsystems. As the minimum subsystem size...
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Limitations We didn’t control the formation of higher order walls in the current numerical results. As a result, here might be circuit instances which contain localised subspaces we haven’t explicitly accounted for. Such rare instances decrease the average entropy, although we expect this to be a secondary effect. Accounting for higher order k-walls in th...
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discussion (0)
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