Preprint and reproducibility archive; not peer reviewed. WHAT IS THE QUESTION? Quantum phases are often characterized through properties of their many-body states: order parameters, correlations, reduced density matrices, entanglement spectra, and local-unitary equivalence classes. Transport, however, is a response to a specified physical probe. This paper asks a sharp version of that distinction: Can two exact condensates be indistinguishable by every spatial reduced-density spectrum, yet exhibit different charge stiffness when the same physical gauge field is applied to both? The answer is yes. The paper constructs an exactly solvable finite-range flat-band pairing model in which two number-projected condensates are related by onsite product unitaries and therefore have identical spatial reduced-density spectra for every bipartition, while their response to the same fixed physical U(1) Peierls twist is qualitatively different. MAIN EXACT RESULT The model contains an exactly degenerate manifold of number-projected condensates. Two branches in this manifold are related by onsite product unitaries. For every spatial region R, their reduced density matrices are therefore unitarily conjugate and have exactly identical spectra. Consequently, all spatial entanglement spectra and all Rényi entropies coincide between the two branches. Nevertheless, under the same fixed physical U(1) twist, one branch is exactly soft while the other has positive static charge curvature. The difference is exact rather than a small perturbative correction. The soft tower has zero curvature, while the rigid branch satisfies an exact all-filling response law K_N^r(epsilon) = [N(M-N)/(M-1)] K_1(epsilon), with M = 2L, and retains a nonzero response density in the fixed-filling density limit evaluated in the paper. An arbitrarily weak symmetry-allowed selector epsilon > 0 uniquely chooses the rigid texture while the odd-charge gap remains open. The odd-charge gap is obtained exactly: Delta_odd(epsilon) = 3/4 + epsilon/2. Thus the switch between zero and finite static curvature is not produced by closing the protected odd-charge gap. WHAT INFORMATION IS MISSING? The two condensates are equivalent as static states under onsite local unitary transformations. However, the external gauge coupling is held physically fixed rather than being transformed together with the state. The paper formulates this distinction as an equivariance problem for the twist-defect map. Static onsite-local-unitary orbit invariants determine the charge curvature only if the physical twist-defect map transforms equivariantly along the orbit. In the constructed model, it does not. The conceptual conclusion is therefore more precise than the familiar observation that condensate fraction and superfluid density are different quantities. Here the two states agree on an exceptionally strong collection of static state information: every spatial reduced-density spectrum. They nevertheless respond differently to the same physical probe. The missing datum is the relative orientation between the state orbit and the physical gauge embedding. In other words: state equivalence does not automatically imply response equivalence when the probe is specified independently of the equivalence transformation. The result does not deny that a full microscopic wavefunction written in a fixed physical basis, together with the complete physical coupling to the external field, determines the response. Rather, it identifies exactly which information is lost when one reduces the system to probe-independent state-orbit data. TWIST-INDUCED UNSELECTION AND NONCOMMUTING LIMITS The work also studies competition between the infinitesimal selector and a finite gauge twist. In the joint scaling limit epsilon -> 0+ with A_x^2/epsilon fixed, the leading texture energies cross at lim_(epsilon -> 0+) A_(x,c)^2(epsilon) / epsilon = 24/7. The twist can therefore drive the selected rigid texture back toward the soft texture while the states remain within the same static onsite-local-unitary orbit. This provides an explicit example of noncommuting limits: selecting the state with epsilon > 0 and then taking epsilon -> 0+ does not reproduce every feature of setting epsilon = 0 from the outset. WHY THIS MATTERS For readers outside flat-band superconductivity, the broader issue is: What information about a quantum state is sufficient to determine its response to an external probe? Entanglement spectra, reduced-density spectra, and local-unitary orbit data are powerful state descriptors. This construction shows that even the complete collection of spatial reduced-density spectra does not, by itself, determine static charge response when the physical gauge coupling supplies additional orientation information. The distinction is conceptually relevant to: - quantum many-body response; - quantum geometry; - transport inferred from wavefunction information; - exact paired states; - entanglement-based classification; - local-unitary invariants; - gauge coupling and probe dependence. The value of the construction is that this separation is realized exactly, without disorder, uncontrolled effective theory, or approximate numerical diagonalization. SCOPE AND NONCLAIMS Throughout this work, “stiffness” means finite-volume static charge curvature K_ij^(N) = [d^2 E_(0,N)(A) / dA_i dA_j]_(A=0) and the explicitly evaluated selected density limit. The result is not identified with an unspecified dynamical order of limits, finite-frequency optical conductivity, or a universal dynamical definition of superfluid weight. The thermodynamic sequence used in the manuscript is an L x 1 torus, periodic in both directions. It is not claimed to be a fully two-dimensional thermodynamic family in which both linear dimensions diverge. Neutral collective excitations can soften with increasing size, so finite-twist uniformity and dynamical response limits remain separate problems. VERSION CLARIFICATION The current release should be quoted using the asymptotic scaling statement A_(x,c)^2(epsilon) / epsilon -> 24/7 as epsilon -> 0+. Earlier wording could be read as claiming the stronger finite-epsilon identity A_x^2 = (24/7) epsilon. That stronger interpretation is not supported. The current manuscript explicitly shows finite-epsilon crossing values displaced from 24/7 and states the scaling limit correctly. This clarification does not alter the paper’s main exact result: identical spatial reduced-density spectra can coexist with zero versus finite static charge curvature under the same physical twist. It also does not alter the exact odd-charge-gap formula. REPRODUCIBILITY The deposit contains the Letter, Supplemental Material, exact and numerical certificates, figure generators, manuscript-build scripts, SHA-256 manifests, and a claim ledger. The archive retains refuted or quarantined claims instead of silently removing them and separates exact identities from finite-precision numerical stress tests. The assumptions behind the selected-state, curvature, gap, and scaling-limit statements are explicitly recorded. SEARCH AND INDEXING CONTEXT Relevant concepts include: flat-band superconductivity; static charge stiffness; Kohn curvature; Peierls twist; exact condensates; eta pairing; quantum geometric nesting; reduced density matrix; entanglement spectrum; Rényi entropy; local-unitary orbit; onsite product unitary; response from wavefunction data; gauge embedding; physical probe orientation; twist-defect map; transport switching; noncommuting limits; quantum many-body response.
Preprint and reproducibility archive; not peer reviewed. WHAT IS THE QUESTION? Quantum phases are often characterized through properties of their many-body states: order parameters, correlations, reduced density matrices, entanglement spectra, and local-unitary equivalence classes. Transport, however, is a response to a specified physical probe. This paper asks a sharp version of that distinction: Can two exact condensates be indistinguishable by every spatial reduced-density spectrum, yet exhibit different charge stiffness when the same physical gauge field is applied to both? The answer is yes. The paper constructs an exactly solvable finite-range flat-band pairing model in which two number-projected condensates are related by onsite product unitaries and therefore have identical spatial reduced-density spectra for every bipartition, while their response to the same fixed physical U(1) Peierls twist is qualitatively different. MAIN EXACT RESULT The model contains an exactly degenerate manifold of number-projected condensates. Two branches in this manifold are related by onsite product unitaries. For every spatial region R, their reduced density matrices are therefore unitarily conjugate and have exactly identical spectra. Consequently, all spatial entanglement spectra and all Rényi entropies coincide between the two branches. Nevertheless, under the same fixed physical U(1) twist, one branch is exactly soft while the other has positive static charge curvature. The difference is exact rather than a small perturbative correction. The soft tower has zero curvature, while the rigid branch satisfies an exact all-filling response law K_N^r(epsilon) = [N(M-N)/(M-1)] K_1(epsilon), with M = 2L, and retains a nonzero response density in the fixed-filling density limit evaluated in the paper. An arbitrarily weak symmetry-allowed selector epsilon > 0 uniquely chooses the rigid texture while the odd-charge gap remains open. The odd-charge gap is obtained exactly: Delta_odd(epsilon) = 3/4 + epsilon/2. Thus the switch between zero and finite static curvature is not produced by closing the protected odd-charge gap. WHAT INFORMATION IS MISSING? The two condensates are equivalent as static states under onsite local unitary transformations. However, the external gauge coupling is held physically fixed rather than being transformed together with the state. The paper formulates this distinction as an equivariance problem for the twist-defect map. Static onsite-local-unitary orbit invariants determine the charge curvature only if the physical twist-defect map transforms equivariantly along the orbit. In the constructed model, it does not. The conceptual conclusion is therefore more precise than the familiar observation that condensate fraction and superfluid density are different quantities. Here the two states agree on an exceptionally strong collection of static state information: every spatial reduced-density spectrum. They nevertheless respond differently to the same physical probe. The missing datum is the relative orientation between the state orbit and the physical gauge embedding. In other words: state equivalence does not automatically imply response equivalence when the probe is specified independently of the equivalence transformation. The result does not deny that a full microscopic wavefunction written in a fixed physical basis, together with the complete physical coupling to the external field, determines the response. Rather, it identifies exactly which information is lost when one reduces the system to probe-independent state-orbit data. TWIST-INDUCED UNSELECTION AND NONCOMMUTING LIMITS The work also studies competition between the infinitesimal selector and a finite gauge twist. In the joint scaling limit epsilon -> 0+ with A_x^2/epsilon fixed, the leading texture energies cross at lim_(epsilon -> 0+) A_(x,c)^2(epsilon) / epsilon = 24/7. The twist can therefore drive the selected rigid texture back toward the soft texture while the states remain within the same static onsite-local-unitary orbit. This provides an explicit example of noncommuting limits: selecting the state with epsilon > 0 and then taking epsilon -> 0+ does not reproduce every feature of setting epsilon = 0 from the outset. WHY THIS MATTERS For readers outside flat-band superconductivity, the broader issue is: What information about a quantum state is sufficient to determine its response to an external probe? Entanglement spectra, reduced-density spectra, and local-unitary orbit data are powerful state descriptors. This construction shows that even the complete collection of spatial reduced-density spectra does not, by itself, determine static charge response when the physical gauge coupling supplies additional orientation information. The distinction is conceptually relevant to: - quantum many-body response; - quantum geometry; - transport inferred from wavefunction information; - exact paired states; - entanglement-based classification; - local-unitary invariants; - gauge coupling and probe dependence. The value of the construction is that this separation is realized exactly, without disorder, uncontrolled effective theory, or approximate numerical diagonalization. SCOPE AND NONCLAIMS Throughout this work, “stiffness” means finite-volume static charge curvature K_ij^(N) = [d^2 E_(0,N)(A) / dA_i dA_j](A=0) and the explicitly evaluated selected density limit. The result is not identified with an unspecified dynamical order of limits, finite-frequency optical conductivity, or a universal dynamical definition of superfluid weight. The thermodynamic sequence used in the manuscript is an L x 1 torus, periodic in both directions. It is not claimed to be a fully two-dimensional thermodynamic family in which both linear dimensions diverge. Neutral collective excitations can soften with increasing size, so finite-twist uniformity and dynamical response limits remain separate problems. VERSION CLARIFICATION The current release should be quoted using the asymptotic scaling statement A(x,c)^2(epsilon) / epsilon -> 24/7 as epsilon -> 0+. Earlier wording could be read as claiming the stronger finite-epsilon identity A_x^2 = (24/7) epsilon. That stronger interpretation is not supported. The current manuscript explicitly shows finite-epsilon crossing values displaced from 24/7 and states the scaling limit correctly. This clarification does not alter the paper’s main exact result: identical spatial reduced-density spectra can coexist with zero versus finite static charge curvature under the same physical twist. It also does not alter the exact odd-charge-gap formula. REPRODUCIBILITY The deposit contains the Letter, Supplemental Material, exact and numerical certificates, figure generators, manuscript-build scripts, SHA-256 manifests, and a claim ledger. The archive retains refuted or quarantined claims instead of silently removing them and separates exact identities from finite-precision numerical stress tests. The assumptions behind the selected-state, curvature, gap, and scaling-limit statements are explicitly recorded. SEARCH AND INDEXING CONTEXT Relevant concepts include: flat-band superconductivity; static charge stiffness; Kohn curvature; Peierls twist; exact condensates; eta pairing; quantum geometric nesting; reduced density matrix; entanglement spectrum; Rényi entropy; local-unitary orbit; onsite product unitary; response from wavefunction data; gauge embedding; physical probe orientation; twist-defect map; transport switching; noncommuting limits; quantum many-body response.
Preprint and reproducibility archive; not peer reviewed. WHAT IS THE QUESTION? In an ordinary dispersive band, charge stiffness is closely related to kinetic energy. In a perfectly flat band, ordinary band velocity vanishes, so superconducting or superfluid stiffness can instead depend on quantum geometry and interactions. In the quantum-geometric-nesting (QGN) class of frustration-free flat-band models, previous work established a rigorous bound relating the many-body superfluid stiffness to the inverse mass of a Cooper pair and reported numerical saturation at approximately the 0.1% level. This raises a natural question: Is perfect quantum geometric nesting by itself sufficient to make the pair-mass bound an exact equality? This work does not assume that the answer is yes and does not attempt to prove a universal saturation theorem. Instead, it calculates the finite-size static charge-twist response exactly and identifies the structural obstruction that decides whether saturation occurs. MAIN RESULT: TWO EXACT RESPONSE CHANNELS For the isolated QGN condensates studied here, the finite-size second derivative of the ground-state energy under a charge twist has the exact structure E_0^(N)''(0) = alpha C_3(N) + gamma C_2(N), where C_3(N) and C_2(N) are fixed hypergeometric filling factors. The amplitudes alpha and gamma are independent of pair number N and can be determined entirely within the one-pair sector. The second amplitude satisfies gamma >= 0, so its contribution has a fixed sign. This converts the question of saturation from a numerical observation into a structural condition: The familiar one-channel pair-mass form is saturated exactly when the second channel vanishes. The paper then determines how this second channel can arise. The spin trace first decides whether a generator contributes at all. Traceless spin structures make the relevant contribution vanish identically. For the contributing charge-type sector of the on-site construction, the remaining finite-size term is an aliasing contribution associated with nonzero lattice Fourier or Poisson modes rather than the continuum mode. EXPLICIT FINITE-SIZE COUNTEREXAMPLE The paper constructs an explicit admissible QGN model with a numerically certified unique gapped ground state for which gamma / alpha = 0.846 at N_k = 4. Relative to the one-channel saturating form, the finite-size departure is approximately 42% at half filling and rises to approximately 169% at N = 3, where the reference one-channel contribution is itself small. These percentages are finite-size departures from the one-channel form. They are not claimed to be thermodynamic violations of the rigorous pair-mass bound. THERMODYNAMIC RECOVERY The same analysis also explains why near-perfect saturation may appear in larger regular systems. Under the stated joint analyticity and regularity assumptions, uniform normalization assumptions, and a uniform charge-gap condition, the finite-size aliasing contribution decays and the conjectured one-channel relation is recovered in the thermodynamic limit. The work therefore separates two logically different statements: 1. perfect QGN alone does not force exact finite-size saturation; 2. with additional regularity and gap assumptions, the finite-size obstruction disappears in the thermodynamic limit. This distinction is important because an exact identity at finite system size and a thermodynamic definition of stiffness are not interchangeable statements. ADDITIONAL STRUCTURAL RESULTS The paper derives further constraints on the pairing form factor. When the generators that actually enter the Hamiltonian act irreducibly on each flat-band fibre and the momentum graph generated by them is connected, the pairing form factor has a degenerate, momentum-independent singular-value spectrum and the condensate is unique up to an overall scale. Counterexamples to converses are given, so these properties are not claimed to follow from QGN alone. The admissible band-space bilinears are also organized by a Z2-graded real structure. In an appropriate real basis, the even sector is real symmetric and the odd sector is i times a real antisymmetric matrix, with dimensions m(m+1)/2 and m(m-1)/2, respectively. This grading clarifies which algebraic sectors can carry the finite-size obstruction. The work additionally checks that, in the translation-invariant setting considered, optimization over the relevant all-spin matrix-valued antisymmetrized-geminal-power variational family reproduces the exact finite-size curvature to second order. The additional response channel is therefore not merely an artifact of evaluating a single fixed trial vector. WHY THIS MATTERS The scientific point is not simply that one numerical conjecture has a finite-size exception. The stronger result is that the apparent equality between a collective many-body charge response and a two-particle pair-mass quantity has an identifiable structural condition. The exact two-channel decomposition explains both: - why many QGN examples may appear almost perfectly saturated, and - how a valid QGN system can depart strongly from the one-channel form at finite size. For a broader reader, the work illustrates an important problem in many-body physics: When can information obtained from a low-particle sector determine a collective many-body response? In this QGN class, the one-pair sector carries unexpectedly strong information: it determines both response amplitudes. However, an additional algebraic and finite-size aliasing condition decides whether the response collapses to the commonly assumed pair-mass form. The lesson is therefore not that few-particle information is generally insufficient, but that its sufficiency must be established structurally rather than inferred from numerical agreement. SCOPE AND NONCLAIMS The observable treated here is the static charge-twist curvature in the conventions stated in the manuscript. The work does not claim that the 42% or 169% finite-size departures survive as thermodynamic violations. The thermodynamic recovery theorem requires the regularity, normalization, and gap hypotheses stated in the paper. Finite-range or infinite-volume situations outside those hypotheses are separate questions. The surjectivity theorem in release v1.1 explicitly assumes a momentum-independent, invertible singlet nesting matrix. That hypothesis is essential and should not be omitted when quoting the theorem. The relation between the graded algebraic sectors and the complete QGN interaction class also requires the stated orbital-coverage conditions; the manuscript gives cases in which those conditions fail. VERSION CLARIFICATION Release v1.1 tightens the formal scope of the v1.0 statements. Most importantly, it explicitly adds the momentum-independence hypothesis required by the surjectivity argument and separates analytically proved statements from numerically certified properties of the explicit counterexample. The finite-size two-channel response result and the thermodynamic scope should be quoted in the v1.1 form given above. REPRODUCIBILITY The deposit includes the manuscript and a fail-closed reproducibility package containing exact or symbolic checks, numerical certificates, figure generators, deterministic reference outputs, and a frozen manifest. The explicit finite-size counterexample, response formulas, convention factors, gap diagnostics, and auxiliary algebraic claims are mapped to executable verification routines. SEARCH AND INDEXING CONTEXT Relevant concepts include: flat-band superconductivity; quantum geometric nesting; QGN; superfluid stiffness; static charge curvature; Kohn twist; Cooper-pair mass; pair-mass bound; quantum metric; quantum geometry; exact pairing; antisymmetrized geminal power; AGP; finite-size aliasing; Poisson summation; thermodynamic limit; frustration-free Hamiltonian; Z2 grading.
Preprint and reproducibility archive; not peer reviewed. What this record contains. The full text of the manuscript "Identical Few-Particle Gauge Hamiltonians, Programmable Collective Charge Response" (M. J. Baek), its Supplementary Information with complete proofs, and the archive that certifies every closed-form value the paper states. The result. Few-particle dynamics are routinely used to infer collective response in paired quantum matter. This work constructs finite-range interacting response twins that defeat that inference exactly. For any prescribed finite p, two positive finite-range parents have complete static gauge Hamiltonians that coincide at every Peierls link phase on every sector N ≤ p and first separate at N = p+1; their zero-twist finite-density ground rays are the same Dicke condensate, yet their collective charge curvatures differ extensively. The obstruction has no universal depth, and the paper locates it exactly: the blind depth equals the spectator body order, the algebraic bound is independent of geometric range, and bounded range still bounds depth only through the number of sites a ball can hold — so no arbitrary-depth family of uniformly fixed range is claimed. Conversely, a structurally closed exact-pair tower closes at depth two. What is hidden is designable: with d(d+1)/2 displacement families the whole symmetric response-tensor difference is locally realizable at interior filling, and a triangular-lattice twin pair differs only in shear while both diagonal responses stay equal. A 4×4 hard-core-qubit discriminator gives an exact factor-of-two contrast with an internal null control and a state-preparation certificate in 17 equal-time settings. What is certified, and what is not. Results proved in closed form are stated as theorems and proved in full in the Supplementary Information; results established by exact rational or symbolic computation are labelled computer-assisted and carry the script that produces them; the 16-qubit discriminator is a blueprint with an end-to-end noise simulation, not a hardware demonstration, which is not claimed anywhere. Scope limits, open proof lines and one refuted conjecture are recorded explicitly rather than omitted. The archive. response_twins_unified_v1.0.zip contains the manuscript and Supplementary sources, the proof notes, the complete fail-closed verification harness (31 executed stages in release mode — 27 claim-bearing certificate stages, 1 non-claim diagnostic stage and three fail-closed gates — and 24 in quick mode) with one frozen log per stage, independent reimplementations of the load-bearing calculations, negative controls that must fail, the exact-arithmetic certificates behind every closed-form value in the paper, the figure-generation scripts, the scientific claim and provenance record (79 rows with status and scope notes, 50 of them naming the in-archive script or certificate that checks them, the rest saying in words why not), and a per-file SHA-256 manifest of all 209 payload files. Verifying it. Extract the archive and run python3 REPRODUCIBILITY/run_all.py --mode quick for the fast subset or --mode release for all 31 stages; bash BUILD.sh rebuilds both PDFs from source. The three gates check journal-format limits, the release payload against its manifest, and every number the documentation states about the artefacts. Running the harness does not modify the archive: all runtime output is written outside the immutable tree, so verification cannot invalidate the manifest. README.md separates reproducing the content (TeX Live 2023 or newer) from reproducing the archived PDF bytes, which needs the exact producing build recorded inside the PDFs. Licences. Manuscript, Supplementary Information and documentation CC BY 4.0; source code MIT; certificate data CC0 1.0. LICENSE.md is the authoritative per-file mapping and is included both in the archive and as a separate file on this record.
A non-invertible topological line fixes global defect data but does not select a reduced density matrix (RDM) in a spatially twisted sector. For the Kramers-Wannier defect of the critical Ising chain, we choose the equal-weight incoherent mixture of the two charge-sector ground states and reduce it to the ordinary full spin algebra of a complete non-wrapping prefix anchored at the defect endpoint. The finite-size RDM is the convex average of two charge-sector RDMs rather than a sign-zero Gaussian proxy, and an exact descended antiunitary enforces many-body Kramers pairing without determining the entropy. At fixed prefix, the RDMs converge in trace norm to a common Gaussian state as the circumference grows. Convergence is uniform over all ground-doublet preparations, including coherent pure states. The subsequent large-prefix entropy limit follows from the odd-even Toeplitz/Fisher-Hartwig endpoint, yielding half the natural logarithm of two above the homogeneous chain. The matched spin-flip defect has zero excess. The exact joint energy-modified-translation character of the same twisted Hamiltonian records the finite-size roots and their multiplicities, retains chirality in the marked scaling limit, and, using the standard Ising character identities, resolves the four Virasoro towers of the Ising duality-twisted sector.
Exactly solvable quantum many-body models are rare, and even when their spectra are algebraically organized, dynamical responses generally remain difficult to obtain because they probe an extensive number of excited states. Here we show that quantum geometric nesting (QGN) models admit an unusually strong form of solvability rooted in \emph{Fock-space fragmentation}: excitations on top of the exact frustration-free ground states decouple into Krylov subspaces with a fixed number of particle and hole operators, and hence remain dynamically invariant. Exploiting this structure, we prove that the stiffness of the spontaneously broken continuous symmetry in QGN models is exactly equal to its variational value in the Gaussian manifold, confirming a conjecture from quantum many-body bootstrap~\cite{GaoHanKhalaf2026}. The proof shows that an infinitesimal phase twist couples the ground state only to the one-particle, one-hole fragment, which coincides with the tangent space of the ground state within the variational manifold, thereby making the variational curvature exact. More generally, perturbations whose action remains within a fixed Fock-space fragment have response functions determined exactly by the corresponding few-body sector, enabling exact access to quantities including static susceptibility, optical conductivity, dynamical structure factors, and single-particle Green's functions.
A charge-sector structure and the gauge dynamics built on it need not be controlled by the same microscopic information. We show this for a charge on a graph G = (V, E) with compact link variables, held near one site by a penalty Delta, with epsilon = t/Delta. The Gauss-sector combinatorics is fixed by the vertex count and the first Betti number b1 = |E| - |V| + 1, while the magnetic stiffness kappa_B generated by transport around closed cycles appears only at the order set by the girth g, as kappa_B = c_g Delta epsilon^g. The two are independent: a five-cycle and a four-cycle with a pendant edge agree in |V|, |E|, b1, record-site degree z, sector data and leading record leakage, yet their stiffnesses differ by a whole power of epsilon. With 1 - F = z epsilon^2 to leading order for the retained record weight F, this gives the compatibility relation kappa_B = c_g Delta [(1-F)/z]^{g/2} in the same regime. We also prove, from an exact symmetry of the transfer graph rather than order by order, that a shift-type link forces the within-sector electric stiffness to vanish at every coupling; a second electric channel then suffices to complete the dynamics within the static class studied. A finite record lifetime sets a further validity condition with no counterpart when a Gauss constraint is imposed exactly. The low-energy theory is standard compact gauge dynamics; the contribution is the constraint that localizing the charge places on which completions are accessible.
Finite coherent quantum systems exhibit a nontrivial response to local sources of phase curvature, which cannot be reduced to conventional forces, disorder-induced localization, or simple gap opening. Here we show that, in finite fermionic rings, a localized symmetry-breaking perturbation acts as a source of phase curvature in the many-body Hilbert space, inducing an anomalous breakdown of global phase rigidity. Starting from a Hubbard-Peierls description, we derive an effective field-theoretic functional in which the inverse local susceptibility defines a phase-rigidity scale controlled by system size and electronic correlations. This rigidity quantifies the resistance of a coherent many-body state to geometric deformation of its phase structure, rather than to energetic localization. We demonstrate that interactions enhance phase rigidity in finite systems, counter to naive expectations based on single-particle localization, and that rigidity loss may occur without a direct correspondence to gap formation. Molecular pi-electron rings and mesoscopic quantum circuits provide experimentally accessible realizations of this regime, establishing a direct connection between local phase curvature, geometric rigidity, and coherence-driven phenomena across finite quantum matter.