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arXiv:2607.25625 · Physical review. E

Entanglement asymmetry in the gapped XYZ spin-$\frac12$ chain

Felipe Taha Sant'Ana

cond-mat.stat-mechhep-thquant-phSubmitted Jul 28, 2026• Announced Jul 29, 2026• 0 citations
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Abstract

The entanglement asymmetry measures how strongly a symmetry is broken inside a subsystem. Analytic results at equilibrium have so far covered free theories and, perturbatively, the critical XXZ chain. We compute the Rényi entanglement asymmetries of a large interval in the gapped, $U(1)$-breaking phase of the interacting XYZ chain. The calculation combines three ingredients. A charged-moment identity, which we prove for fermionic Gaussian and for injective matrix-product ground states, ties the asymmetry to the static susceptibility of the broken charge. A non-conservation sum rule then evaluates the susceptibility from sine-Gordon form factors, its two-kink and one-breather channels providing a lower bound on the universal amplitude. The Baxter--Johnson--Krinsky--McCoy solution supplies the kink mass for different couplings. Infinite-system density-matrix renormalization group simulations built on these masses reproduce the master formula.

Comments
5 pages, 3 figures; Supplemental Material: 13 pages, 2 figures, 2 tables

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Entanglement asymmetry in the gapped XYZ spin-12\frac1221​ chain
The entanglement asymmetry measures how strongly a symmetry is broken inside a subsystem. Analytic results at equilibrium have so far covered free theories and, perturbatively, the critical XXZ chain. We compute the Rényi entanglement asymmetries of a large interval in the gapped, U(1)U(1)U(1)-breaking phase of the interacting XYZ chain. The calculation combines three ingredients. A charged-moment identity, which we prove for fermionic Gaussian and for injective matrix-product ground states, ties the asymmetry to the static susceptibility of the broken charge. A non-conservation sum rule then evaluates the susceptibility from sine-Gordon form factors, its two-kink and one-breather channels providing a lower bound on the universal amplitude. The Baxter--Johnson--Krinsky--McCoy solution supplies the kink mass for different couplings. Infinite-system density-matrix renormalization group simulations built on these masses reproduce the master formula.
Abstract We study the explicit breaking of a SU(2) symmetry to a U(1) subgroup employing the entanglement asymmetry, a recently introduced observable that measures how much symmetries are broken in a part of extended quantum systems. We consider as specific model the critical XXZ spin chain, which breaks the SU(2) symmetry of spin rotations except at the isotropic point, and is described by the massless compact boson in the continuum limit. We examine the U(1) subgroup of SU(2) that is broken outside the isotropic point by applying conformal perturbation theory, which we complement with numerical simulations on the lattice. We also analyse the entanglement asymmetry of the full SU(2) group. By relying on very generic scaling arguments, we derive an asymptotic expression for it.
Symmetry and symmetry breaking are two pillars of modern quantum physics. Still, quantifying how much a symmetry is broken is an issue that has received little attention. In extended quantum systems, this problem is intrinsically bound to the subsystem of interest. Hence, in this work, we borrow methods from the theory of entanglement in many-body quantum systems to introduce a subsystem measure of symmetry breaking that we dub entanglement asymmetry. As a prototypical illustration, we study the entanglement asymmetry in a quantum quench of a spin chain in which an initially broken global U(1) symmetry is restored dynamically. We adapt the quasiparticle picture for entanglement evolution to the analytic determination of the entanglement asymmetry. We find, expectedly, that larger is the subsystem, slower is the restoration, but also the counterintuitive result that more the symmetry is initially broken, faster it is restored, a sort of quantum Mpemba effect, a phenomenon that we show to occur in a large variety of systems.
Abstract Entanglement asymmetry is a novel quantity that, using entanglement methods, measures how much a symmetry is broken in a part of an extended quantum system. So far, it has only been used to characterise the breaking of continuous Abelian symmetries. In this paper, we extend the concept to cyclic Z N groups. As an application, we consider the XY spin chain, in which the ground state spontaneously breaks the Z 2 spin parity symmetry in the ferromagnetic phase. We thoroughly investigate the non-equilibrium dynamics of this symmetry after a global quantum quench, generalising known results for the standard order parameter.
Abstract Entanglement asymmetry is a quantity recently introduced to measure how much a symmetry is broken in a part of an extended quantum system. It has been employed to analyze the non-equilibrium dynamics of a broken symmetry after a global quantum quench with a Hamiltonian that preserves it. In this work, we carry out a comprehensive analysis of the entanglement asymmetry at equilibrium taking the ground state of the XY spin chain, which breaks theU(1) particle number symmetry, and provide a physical interpretation of it in terms of superconducting Cooper pairs. We also consider quenches from this ground state to the XX spin chain, which preserves theU(1) symmetry. In this case, the entanglement asymmetry reveals that the more the symmetry is initially broken, the faster it may be restored in a subsystem, a surprising and counter-intuitive phenomenon that is a type of a quantum Mpemba effect. We obtain a quasi-particle picture for the entanglement asymmetry in terms of Cooper pairs, from which we derive the microscopic conditions to observe the quantum Mpemba effect in this system, giving further support to the criteria recently proposed for arbitrary integrable quantum systems. In addition, we find that the power law governing symmetry restoration depends discontinuously on whether the initial state is critical or not, leading to new forms of strong and weak Mpemba effects.
A bstract The entanglement asymmetry is an information based observable that quantifies the degree of symmetry breaking in a region of an extended quantum system. We investigate this measure in the ground state of one dimensional critical systems described by a CFT. Employing the correspondence between global symmetries and defects, the analysis of the entanglement asymmetry can be formulated in terms of partition functions on Riemann surfaces with multiple non-topological defect lines inserted at their branch cuts. For large subsystems, these partition functions are determined by the scaling dimension of the defects. This leads to our first main observation: at criticality, the entanglement asymmetry acquires a subleading contribution scaling as log ℓ/ℓ for large subsystem length ℓ . Then, as an illustrative example, we consider the XY spin chain, which has a critical line described by the massless Majorana fermion theory and explicitly breaks the U(1) symmetry associated with rotations about the z -axis. In this situation the corresponding defect is marginal. Leveraging conformal invariance, we relate the scaling dimension of these defects to the ground state energy of the massless Majorana fermion on a circle with equally-spaced point defects. We exploit this mapping to derive our second main result: the exact expression for the scaling dimension associated with n defects of arbitrary strengths. Our result generalizes a known formula for the n = 1 case derived in several previous works. We then use this exact scaling dimension to derive our third main result: the exact prefactor of the log ℓ/ℓ term in the asymmetry of the critical XY chain.
We obtain a formula for the determinant of a block Toeplitz matrix associated with a quadratic fermionic chain with complex coupling. Such couplings break reflection symmetry and/or charge conjugation symmetry. We then apply this formula to compute the R\'enyi entropy of a partial observation to a subsystem consisting of contiguous sites in the limit of large size. The present work generalizes similar results due to Its, Jin, and Korepin [Fields Institute Communications, Universality and Renormalization, Vol. 50, p. 151, 2007] and Its, Mezzadri, and Mo [Commun. Math. Phys. 284, 117 (2008)]. A striking feature of our formula for the entanglement entropy is the appearance of a term scaling with the logarithm of the size. This logarithmic behavior originates from certain discontinuities in the symbol of the block Toeplitz matrix. Equipped with this formula we analyze the entanglement entropy of a Dzyaloshinski-Moriya spin chain and a Kitaev fermionic chain with long-range pairing.