Spurious Strange Correlators in Symmetry-Protected Topological Phases
Pith reviewed 2026-05-17 01:28 UTC · model grok-4.3
The pith
An ill-chosen reference state creates spurious long-range strange correlators in trivial phases.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
An ill-chosen reference state can induce spurious long-range strange correlators in trivial SPT phases. Focusing on 1D gapped bosonic/spin systems described by matrix product states, the origin of these spurious signals is traced to the magnitude-degeneracy of the transfer matrix. Three distinct mechanisms are classified: the presence of high-dimensional irreducible representations in the entanglement spectrum, a phase mismatch in symmetry representations between the target and reference states, and long-range order arising from symmetry breaking.
What carries the argument
Magnitude-degeneracy of the transfer matrix in the matrix product state description, which permits non-decaying strange correlators to appear in trivial phases.
Load-bearing premise
The systems are one-dimensional gapped bosonic or spin chains represented by matrix product states.
What would settle it
A numerical or analytic calculation in a known trivial 1D MPS phase that still shows long-range strange correlators after the reference state is chosen to eliminate all three listed sources of transfer-matrix degeneracy.
Figures
read the original abstract
Strange correlator is a powerful tool widely used in detecting symmetry-protected topological (SPT) phases. However, the result of strange correlator crucially relies on the adoption of the reference state. In this work, we report that an ill-chosen reference state can induce spurious long-range strange correlators in trivial SPT phases, leading to false positives in SPT diagnosis. Focusing on 1D gapped bosonic/spin systems described by matrix product states (MPS), we trace the origin of these spurious signals in trivial SPT phases to the magnitude-degeneracy of the transfer matrix. We systematically classify three distinct mechanisms responsible for such degeneracy, each substantiated by concrete examples: (1) the presence of high-dimensional irreducible representations (abbreviated as \emph{irrep}) in the eigenspace corresponding to the entanglment spectrum (entanglement space); (2) a phase mismatch in symmetry representations between the target and reference states; and (3) long-range order arising from symmetry breaking. Our findings clarify the importance of the choice of proper reference states, providing a guideline to avoid pitfalls and correctly identify SPT order using strange correlators.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript claims that strange correlators, a common diagnostic for symmetry-protected topological (SPT) phases, can produce spurious long-range signals in trivial phases when an inappropriate reference state is chosen. Restricting to 1D gapped bosonic/spin systems represented by matrix product states (MPS), the authors trace these artifacts to magnitude degeneracy of the transfer matrix and partition the degeneracy into three mechanisms—high-dimensional irreducible representations in the entanglement spectrum, phase mismatch between symmetry representations of target and reference states, and symmetry-breaking long-range order—each illustrated by explicit examples.
Significance. If the central claim holds, the work is significant for the field because strange correlators are widely used to detect SPT order in both numerical and analytical studies; identifying concrete mechanisms that generate false positives and supplying explicit MPS examples provides a practical guideline for reference-state selection that can reduce diagnostic errors in 1D systems.
minor comments (2)
- [Abstract] Abstract: 'entanglment spectrum' is a typographical error and should read 'entanglement spectrum'.
- The manuscript would benefit from a short table or flowchart summarizing the three mechanisms, their signatures in the transfer-matrix spectrum, and the corresponding reference-state choices that avoid them.
Simulated Author's Rebuttal
We thank the referee for the positive assessment of our work and the recommendation for minor revision. The referee's summary accurately reflects our central claim that ill-chosen reference states can induce spurious long-range strange correlators in trivial SPT phases via magnitude degeneracy of the MPS transfer matrix, along with the three classified mechanisms supported by explicit examples.
Circularity Check
No significant circularity detected
full rationale
The paper's analysis of spurious strange correlators in trivial SPT phases is self-contained within its stated scope of 1D gapped bosonic/spin systems represented by MPS. It directly traces the issue to magnitude degeneracy of the transfer matrix and partitions this degeneracy into three concrete, independently illustrated mechanisms (high-dimensional irreps in the entanglement spectrum, phase mismatch between symmetry representations, and symmetry-breaking long-range order), each backed by explicit examples. No load-bearing step reduces a claimed result to a fitted input, self-definition, or unverified self-citation chain; the central claim follows from the enumerated mechanisms without circular reduction.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption 1D gapped bosonic and spin systems admit faithful matrix product state representations
- domain assumption Strange correlators require a reference state whose symmetry representation interacts with the target state
Reference graph
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Spurious Strange Correlators in Symmetry-Protected Topological Phases
Introduction——Symmetry-protected topological (SPT) phases in strongly interacting systems are short-range entan- gled (SRE) states that cannot be adiabatically connected to a trivial product state without breaking a specific protecting symmetry [1]. The Affleck–Kennedy–Lieb–Tasaki (AKLT) model serves as a paradigmatic example for understanding SPT phases ...
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Origin of long-range strange correlators——In this sec- tion, we aim to establish the analytical nature of the long- range strange correlator. We first write all correlators in the form of matrix product states (MPS), then prove that the long- range behavior of the connected strange correlator can be at- tributed to what we name as magnitude-degeneracy (ex...
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Spurious Strange Correlators: Examples and Mecha- nisms——However, the magnitude-degeneracy of eigenval- ues can also appear in a trivial SPT phase, and so does the long-range strange correlator. This mechanism shares the same mathematical origin (Schur’s lemma) as nontrivial SPT phases (SM Sec. B), but arises in a topologically trivial phase. Example 1(SO...
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Theorem 1 In the main text, we focus on theconnected strange correlator. Analytic analysis (Theorem5,Mechanism4) has shown that it is the connected strange correlator that reflects the contribution from the nontrivial SRE state. Since we are discussing spurious signature of the original strange correlator, we show in this section that the connected strang...
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Theorem 2 Theorem 5(Magnitude-degeneracy implies long-range strange correlator).Suppose the eigenvalue ofM ω with the largest magnitude isD-fold magnitude-degenerate. i.e. we assume that|λ 0|=|λ 1|=· · ·=|λ D−1|>|λ D|>· · ·. IfD= 1, then∀O a, Ob, the connected strange correlator is zero: OaOb S,connected := OaOb S − ⟨Oa⟩S Ob S = 0.(D5) IfD >1, then there ...
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Theorem 3 Theorem 6.IfD >1, we define the direct sum of spaces of operator matricesM a, Mb asM:={(M a, Mb)|Ma := P σ ⟨ω|O a |σ⟩M σ, Mb := P σ ⟨ω|O b |σ⟩M σ}, then Oa i Ob j S,connected does not converge to zero (i.e. exhibits long-range behavior) almost everywhere inM, where the exceptions have zero Lebesgue measure inM. Theorem 3claims that if the larges...
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Symmetry Breaking Case In the main text, we claim that if the MPS target state is the symmetric ground state of a symmetry breaking Hamiltonian (this state can be selected by linear combining bases of all ground state subspaces), then we can always choose the reference state |Ω⟩= N i |ω⟩i, such that the largest-magitude eigenvalue of the transfer matrixM ...
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