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arXiv:2603.29287

Entanglement in the θθθ-vacuum

Sebastian Grieninger, Dmitri E. Kharzeev, Eliana Marroquin

hep-phcond-mat.str-elhep-lathep-thquant-phSubmitted Mar 31, 2026• Announced Apr 1, 2026• 0 citations
v1 · Mar 31, 2026v2 · Oct 13, 2026
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Abstract

We compute the entanglement entropy and the entanglement spectrum of the vacuum state in the massive Schwinger model at a finite $θ$ angle. The $θ$ term is implemented through a chirally rotated lattice Hamiltonian that preserves the periodicity in $θ$ already at the operator level and maintains the correct massless limit without $θ$-dependent lattice artifacts. We clarify the physical origin of entanglement entropy enhancement at $θ=π$ by relating it to the competition between distinct electric-flux vacuum branches. We show that the peak near $θ=π$ persists across the range of masses studied and corresponds to the point of maximal competition between distinct vacuum branches with opposite electric-field orientation, where quantum fluctuations due to fermion pair creation are maximized. While this entropy enhancement is generic, a pronounced narrowing of the entanglement gap occurs only near the critical mass ratio $m/g\simeq0.33$. Using the Bisognano--Wichmann (BW) theorem, we construct a lattice BW entanglement Hamiltonian and compare it with the exact modular Hamiltonian obtained from the reduced density matrix. We observe agreement between these Hamiltonians in the infrared sector, indicating that the entanglement Hamiltonian is well approximated by a spatially weighted microscopic Hamiltonian. These results establish entanglement observables as sensitive probes of the $θ$-dependent vacuum structure and highlight the chirally rotated formulation as a natural framework for open boundary conditions. Additionally, we discuss possible applications to entanglement in topological insulators and quantum wires.

Comments
16+3 pages, 13+2 figures

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We compute the entanglement entropy and the entanglement spectrum of the vacuum state in the massive Schwinger model at a finite θθθ angle. The θθθ term is implemented through a chirally rotated lattice Hamiltonian that preserves the periodicity in θθθ already at the operator level and maintains the correct massless limit without θθθ-dependent lattice artifacts. We clarify the physical origin of entanglement entropy enhancement at θ=πθ=πθ=π by relating it to the competition between distinct electric-flux vacuum branches. We show that the peak near θ=πθ=πθ=π persists across the range of masses studied and corresponds to the point of maximal competition between distinct vacuum branches with opposite electric-field orientation, where quantum fluctuations due to fermion pair creation are maximized. While this entropy enhancement is generic, a pronounced narrowing of the entanglement gap occurs only near the critical mass ratio m/g≃0.33m/g\simeq0.33m/g≃0.33. Using the Bisognano--Wichmann (BW) theorem, we construct a lattice BW entanglement Hamiltonian and compare it with the exact modular Hamiltonian obtained from the reduced density matrix. We observe agreement between these Hamiltonians in the infrared sector, indicating that the entanglement Hamiltonian is well approximated by a spatially weighted microscopic Hamiltonian. These results establish entanglement observables as sensitive probes of the θθθ-dependent vacuum structure and highlight the chirally rotated formulation as a natural framework for open boundary conditions. Additionally, we discuss possible applications to entanglement in topological insulators and quantum wires.
We define string breaking by the midpoint electric field falling to half a flux unit. In the massive Schwinger model, an energy balance between intact string and screened configuration predicts this separation using only vacuum observables and meson masses as input. We observe numerically that this half flux point corresponds to equal weights of the two configurations thus explaining the observed peak of the entanglement entropy as maximal uncertainty of the binary flux. The same binary uncertainty explains the entanglement peak at the vacuum transition. Near breaking, screening organizes part of the spatial entanglement between the string interior and exterior into a nearly maximally entangled bit. Within the selected flux sectors, this approximate encoded Bell pair records whether the flux is screened and nearly separates from the remaining correlations. The vacuum transition exhibits the same structure. The string and the θθθ-vacuum reveal a matching structure of the capacity of entanglement minimum at their transition point, linking their entanglement spectra. The transition rearranges degrees of freedom coherently as revealed by the entanglement asymmetry and Robustness of Magic of disjoint subregions. Finally, we construct the θθθ-vacuum and string wavefunctions with high fidelity using a restricted Boltzmann machine ansatz.
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We consider a lattice version of the Bisognano-Wichmann (BW) modular Hamiltonian as an ansatz for the bipartite entanglement Hamiltonian of the quantum critical chains. Using numerically unbiased methods, we check the accuracy of the BW ansatz by both comparing the BW R\'enyi entropy to the exact results and investigating the size scaling of the norm distance between the exact reduced density matrix and the BW one. Our study encompasses a variety of models, scanning different universality classes, including integrable models such as the transverse field Ising, three-state Potts and XXZ chains, and the nonintegrable bilinear-biquadratic model. We show that the R\'enyi entropies obtained via the BW ansatz properly describe the scaling properties predicted by conformal field theory. Remarkably, the BW R\'enyi entropies also faithfully capture the corrections to the conformal field theory scaling associated with the energy density operator. In addition, we show that the norm distance between the discretized BW density matrix and the exact one asymptotically goes to zero with the system size: this indicates that the BW ansatz also can be employed to predict properties of the eigenvectors of the reduced density matrices and is thus potentially applicable to other entanglement-related quantities such as negativity.
We study the charge resolved structure of the pairwise mode entanglement produced by the Schwinger effect. Using the U(1)U(1)U(1) electric charge of quantum electrodynamics, we decompose the entanglement of the reduced density matrices of momentum modes into individual charge sectors, applying the symmetry resolved entanglement entropy to the pure bipartition reductions and the charge imbalance resolved negativity to the mixed particle-antiparticle pair. For a constant electric field we obtain closed form results for every mode pairing, with all field dependence controlled by the single pair production probability ∣β∣2=e−πμ|β|^2=e^{-πμ}∣β∣2=e−πμ. For the particle, particle and antiparticle antiparticle pairs the entanglement entropy within each charge sector is field independent, so that the field acts only by redistributing statistical weight among the sectors; for the mixed particle antiparticle pair the populated sector additionally carries a field dependent entropy. The vacuum created pair is the richest case: its resolved negativity is distributed over three imbalance sectors and is transported from one sector to the next as the field increases, the sectors contributing equally only at the critical field. The entanglement across a fixed momentum bipartition, by contrast, is frozen at its initial value because pair creation conserves the sector charge. Symmetry resolution thus refines the total entanglement measures into a sector resolved picture, exposing where in charge space the entanglement of the Schwinger mechanism resides and how the electric field moves it, information beyond that contained in the total measures or in particle number distributions.