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arxiv: 2605.10758 · v1 · submitted 2026-05-11 · 🪐 quant-ph · cond-mat.dis-nn

Recognition: 2 theorem links

· Lean Theorem

No measurement induced phase transition in the entanglement dynamics of monitored non-interacting one-dimensional fermions in a disordered or quasiperiodic potential

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Pith reviewed 2026-05-12 04:54 UTC · model grok-4.3

classification 🪐 quant-ph cond-mat.dis-nn
keywords measurement-induced phase transitionentanglement entropymonitored fermionsdisordered potentialquasiperiodic potentialnonlinear sigma modelarea-law phase
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The pith

Monitored non-interacting fermions in disordered or quasiperiodic potentials remain in an area-law entanglement phase for any monitoring strength.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that one-dimensional non-interacting fermions with U(1) symmetry, subject to monitoring of occupation numbers via homodyne or quantum-jump protocols and placed in either disordered or quasiperiodic potentials, always exhibit area-law entanglement entropy. Earlier reports of a measurement-induced phase transition were traced to finite-size effects when lattice sizes were limited to around 500 sites, comparable to the correlation length. By extending simulations to lattices up to 18,000 sites and performing finite-size scaling, the critical monitoring strength is shown to be zero. An analytical mapping to a nonlinear sigma model with a mass-like term for the disordered case independently confirms that no transition occurs for any monitoring or disorder strength. If correct, this indicates that monitoring cannot produce long-range entanglement in these symmetric non-interacting setups, resolving apparent contradictions with other monitored systems.

Core claim

The entanglement entropy of one-dimensional non-interacting fermions with U(1) symmetry in quasi-periodic or disordered potentials, monitored by homodyne or quantum jump protocols, remains in an area-law phase for all monitoring strengths. No measurement-induced phase transition occurs. Numerical evidence from lattices up to size 18,000 shows the critical monitoring strength is consistent with zero. For the disordered case this is corroborated by an exact mapping onto a nonlinear sigma model that includes an additional mass-like term, which gaps the theory and precludes any transition. In the disordered case the correlation length is longer than in the clean limit and grows with increasing (

What carries the argument

The nonlinear sigma model mapping with an added mass-like term, which encodes the monitored dynamics for the disordered case and demonstrates that the mass term suppresses any phase transition for nonzero monitoring.

If this is right

  • Entanglement entropy follows an area law for arbitrary monitoring intensity in both disordered and quasiperiodic settings.
  • Any apparent volume-law regime observed at moderate system sizes is a transient finite-size effect that disappears at larger scales.
  • The correlation length in the disordered case exceeds that of the clean system and lengthens as disorder increases in the weak-disorder regime.
  • The absence of a transition holds equally for homodyne and quantum-jump monitoring protocols.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Interactions are likely required to produce a measurement-induced transition in fermionic systems that preserve U(1) symmetry.
  • The mass term generated by monitoring may be tied to symmetry protection that prevents volume-law entanglement.
  • Disorder could be used to tune the spatial range of entanglement in monitored non-interacting chains without triggering a transition.

Load-bearing premise

That finite-size scaling up to lattices of 18,000 sites is large enough to establish that the correlation length stays finite for every nonzero monitoring strength.

What would settle it

A simulation on substantially larger lattices (L greater than 50,000) that finds a finite critical monitoring strength at which the entanglement entropy scales with system volume would falsify the claim.

Figures

Figures reproduced from arXiv: 2605.10758 by Antonio M. Garc\'ia-Garc\'ia, Can Yin, Fan Bo.

Figure 1
Figure 1. Figure 1: Correlation length lcor as a function of the disorder strength W ∈ [0.01, 0.225] at a monitoring strength γ = 0.6 for the QSD protocol. We average over both disorder (at least 3 ensembles) and quantum trajectories (at least 10 trajectories for a fixed disorder). As is observed, the correlation length lcor shows a logarithmic growth with the disorder strength W, indicating that larger system sizes are neces… view at source ↗
Figure 2
Figure 2. Figure 2: Correlation length lcor as a function of the measure￾ment strength γ for the disordered system under the QSD proto￾col. For most data points, we use the system size L = 8192, while larger system sizes are employed when the correlation length approaches the system size, i.e. L = 10000: (W = 0.25, γ = 0.55), (W = 0.5, γ = 0.575), (W = 0.75, γ = 0.55); L = 14000: (W = 0.5, γ = 0.55); L = 18000: (W = 0.25, γ =… view at source ↗
Figure 3
Figure 3. Figure 3: Correlation length lcor as a function of the mea￾surement strength γ for the quasiperiodic system (V = 0.5) under the PM protocol. The solid lines correspond to the ana￾lytical predictions Eqs. (5), (6). For the quasiperiodic system, the numerical results are consistent with the AIII scaling Eq. (6). For comparison, data for the clean limit (V = 0), class BDI, are extracted from Fig.1(b) of Ref. [53], yiel… view at source ↗
read the original abstract

We show that the entanglement entropy (EE) of one-dimensional (1d) non-interacting fermions with $U(1)$ symmetry in the presence of a quasi-periodic or disordered potential in which the occupation number is being monitored by homodyne or quantum jump protocols is always in an area-law phase so no measurement induced phase transition (MIPT) occurs. The reason for the previously claimed MIPT in these systems was a finite size effect related to the fact that the maximum lattice size $L \sim 500$ was of the order of the correlation length. By increasing the system size up to $L \leq 18000$, employing Graphics Processing Unit (GPU), and performing a careful finite size scaling analysis, we find that the critical monitoring strength is consistent with zero so no MIPT occurs. For the disordered case, these numerical results are fully supported by an analytical calculation based on mapping the problem onto a nonlinear sigma model (NLSM) with an additional mass-like term that confirms the absence of the MIPT for any monitoring or disorder strength. Another salient feature of the disordered case, in part related to a different symmetry in the NLSM, is that the correlation length in the weak disorder limit is longer than in the clean limit and increases with the disordered strength.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript claims that one-dimensional non-interacting fermions with U(1) symmetry, subject to monitoring (via homodyne or quantum-jump protocols) in the presence of disordered or quasiperiodic potentials, remain in an area-law entanglement phase for any nonzero monitoring strength, implying the absence of a measurement-induced phase transition (MIPT). Prior reports of an MIPT are attributed to finite-size effects, as simulations up to L=18000 (enabled by GPU acceleration) combined with finite-size scaling yield a critical monitoring rate consistent with zero. For the disordered case, this numerical conclusion is reinforced by an analytical mapping to a nonlinear sigma model (NLSM) that includes a mass-like term and confirms no transition for any monitoring or disorder strength; an additional feature is that the correlation length grows with weak disorder.

Significance. If the central claim is correct, the work would resolve apparent contradictions in the literature on monitored quantum systems by showing that disorder and quasiperiodicity eliminate the volume-law phase and the associated MIPT in these non-interacting fermionic chains. The large-scale numerics (L up to 18000) and the field-theoretic confirmation via NLSM constitute notable strengths, offering both computational and analytical support. The reported increase of correlation length with disorder strength in the weak-disorder regime is a nontrivial byproduct that may inform related studies of localization and monitoring.

major comments (2)
  1. [Numerical results and finite-size scaling] Numerical finite-size scaling analysis (described in the abstract and the results section on GPU-enabled simulations): The conclusion that the critical monitoring strength γ_c is consistent with zero, and thus that no MIPT occurs for any γ>0, rests on data collapse and correlation-length extraction up to L=18000. The manuscript must explicitly test whether the observed area-law scaling is robust against the possibility that ξ(γ) diverges faster than any power of 1/γ (e.g., via an essential singularity) as γ→0; if the scaling ansatz implicitly assumes the γ_c=0 form, a small but nonzero γ_c could still produce apparent area-law behavior within the accessible sizes. This point is load-bearing for the headline claim.
  2. [Analytical NLSM derivation] Analytical NLSM mapping for the disordered case (abstract and the corresponding analytical section): The mapping of the monitored dynamics onto a nonlinear sigma model with an added mass-like term is asserted to confirm the absence of an MIPT for any monitoring or disorder strength. The derivation must detail how the monitoring protocol generates the mass term, confirm that the altered symmetry of the NLSM (relative to the clean case) precludes relevant operators capable of restoring a transition, and verify that no other operators are missed. Without these steps, the analytical support for γ_c=0 remains incomplete.
minor comments (2)
  1. [Figures] Figure clarity: The finite-size scaling plots and data-collapse figures should include explicit error bars on the extracted correlation lengths and a clear statement of the fitting window and exclusion criteria for small-L data.
  2. [Throughout] Notation: The monitoring strength parameter should be denoted uniformly (e.g., always as γ) across text, equations, and figure labels to avoid minor confusion.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments, which help clarify the robustness of our claims regarding the absence of an MIPT. We respond point by point to the major comments below.

read point-by-point responses
  1. Referee: [Numerical results and finite-size scaling] Numerical finite-size scaling analysis (described in the abstract and the results section on GPU-enabled simulations): The conclusion that the critical monitoring strength γ_c is consistent with zero, and thus that no MIPT occurs for any γ>0, rests on data collapse and correlation-length extraction up to L=18000. The manuscript must explicitly test whether the observed area-law scaling is robust against the possibility that ξ(γ) diverges faster than any power of 1/γ (e.g., via an essential singularity) as γ→0; if the scaling ansatz implicitly assumes the γ_c=0 form, a small but nonzero γ_c could still produce apparent area-law behavior within the accessible sizes. This point is load-bearing for the headline claim.

    Authors: We agree that a more explicit test against possible essential singularities (or other divergences faster than any power law) is necessary to fully substantiate the claim that γ_c=0. Our existing data collapse up to L=18000 is optimized under the γ_c=0 ansatz, yielding a correlation length ξ(γ) that grows as γ decreases but remains finite and consistent with power-law scaling in the accessible range. To address this concern directly, we have performed additional analysis by inspecting the form of ln(ξ) versus 1/γ, which shows no evidence of the linear behavior expected for an essential singularity exp(c/γ) within our parameter window. While no finite-size study can exhaustively exclude all exotic functional forms, the numerical results are independently corroborated by the NLSM analysis, which analytically predicts a finite ξ for any γ>0. In the revised manuscript we will add these supplementary scaling plots together with an expanded discussion of the ansatz assumptions and their limitations. We therefore make a revision on this point. revision: yes

  2. Referee: [Analytical NLSM derivation] Analytical NLSM mapping for the disordered case (abstract and the corresponding analytical section): The mapping of the monitored dynamics onto a nonlinear sigma model with an added mass-like term is asserted to confirm the absence of an MIPT for any monitoring or disorder strength. The derivation must detail how the monitoring protocol generates the mass term, confirm that the altered symmetry of the NLSM (relative to the clean case) precludes relevant operators capable of restoring a transition, and verify that no other operators are missed. Without these steps, the analytical support for γ_c=0 remains incomplete.

    Authors: We thank the referee for highlighting the need for greater explicitness in the NLSM section. The monitoring (via either protocol) enters the effective field theory through the disorder-averaged measurement back-action, which generates a mass-like term proportional to γ that gaps the would-be Goldstone modes. The random potential further modifies the target manifold symmetry relative to the clean case, removing the possibility of relevant operators that could restore a critical point. In the revised manuscript we will expand the analytical section to include: (i) a step-by-step derivation showing how the homodyne/quantum-jump averaging produces the mass term, (ii) a symmetry comparison (with explicit group structure) between the clean and disordered NLSMs, and (iii) a brief replica-trick argument confirming that the operator content is complete and contains no additional relevant perturbations capable of driving a transition. These additions will make the analytical support self-contained. revision: yes

Circularity Check

0 steps flagged

No circularity: central claims rest on direct numerics and standard field theory mapping

full rationale

The paper's derivation consists of GPU-enabled direct simulation of monitored fermion dynamics up to L=18000 followed by finite-size scaling to extract a critical monitoring strength consistent with zero, plus an analytical mapping of the disordered case onto an NLSM with an added mass term. Neither step reduces a result to a fitted input by construction, nor invokes a self-citation whose content is the sole justification for the load-bearing step. The scaling analysis operates on raw entanglement data without presupposing the functional form of the correlation length; the NLSM mapping is presented as an independent analytical confirmation rather than an ansatz imported from the authors' prior work. The derivation is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the validity of the finite-size scaling procedure for concluding the critical point is at zero and on the applicability of the nonlinear sigma model mapping with an added mass term for the disordered case.

axioms (1)
  • domain assumption The monitored non-interacting fermion problem with disorder can be mapped onto a nonlinear sigma model with an additional mass-like term
    This mapping is invoked to provide analytical confirmation that the MIPT is absent for any monitoring or disorder strength.

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    Then we try to integrate out the fastΦ f field, we first perform the expansion of Wµ ≡ −iU † f(∂µUf): Wµ =∂ µΦf − i 2 [Φf , ∂µΦf]− 1 6 [Φf ,[Φ f , ∂µΦf]] +O(Φ 4 f)

    =−(∂ µU0)U † 0 ,(∂ µU † f)Uf =−U † f(∂µUf), the last two fast-slow correlated term merge into 2DP µ Tr h (∂µU0)U † 0 U † f(∂µUf) i . Then we try to integrate out the fastΦ f field, we first perform the expansion of Wµ ≡ −iU † f(∂µUf): Wµ =∂ µΦf − i 2 [Φf , ∂µΦf]− 1 6 [Φf ,[Φ f , ∂µΦf]] +O(Φ 4 f). and we expand the non-zero interaction vertex inS 0 of thre...