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arXiv:2512.16979 · Physical Review A

Bundling of bipartite entanglement

Maike Drieb-Schön, Florian Dreier, Wolfgang Lechner

quant-phmath-phSubmitted Dec 18, 2025• Announced Dec 22, 2025• 0 citations• Revised Jul 2, 2026 (v3)
v1 · Dec 18, 2025v2 · Jun 8, 2026v3 · Jul 2, 2026
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Abstract

We investigate bipartite entanglement and prove that in constrained energy subspaces, the entanglement spectra of multiple bipartitions are the same across the whole subspace. We show that in quantum many-body systems the bipartite entanglement entropy is affected in such a way that it forms "bundles" under unitary time evolution. Leveraging the structure of the subspace, we present methods to verify whether the entanglement spectrum of two bipartitions is identical throughout the entire subspace. For the subspace defined by the parity embedding, we further provide an algorithm that can determine this in polynomial time.

Journal reference
Phys. Rev. A 114, 012401 (2026)
DOI
10.1103/ss6r-ktrr
Comments
24 pages, 10 figures

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We investigate bipartite entanglement and prove that in constrained energy subspaces, the entanglement spectra of multiple bipartitions are the same across the whole subspace. We show that in quantum many-body systems the bipartite entanglement entropy is affected in such a way that it forms "bundles" under unitary time evolution. Leveraging the structure of the subspace, we present methods to verify whether the entanglement spectrum of two bipartitions is identical throughout the entire subspace. For the subspace defined by the parity embedding, we further provide an algorithm that can determine this in polynomial time.
Quantum entanglement in multipartite systems cannot be shared freely. In order to illuminate basic rules of entanglement sharing between qubits, we introduce a concept of an entangled structure (graph) such that each qubit of a multipartite system is associated with a point (vertex), while a bipartite entanglement between two specific qubits is represented by a connection (edge) between these points. We prove that any such entangled structure can be associated with a pure state of a multiqubit system. Moreover, we show that a pure state corresponding to a given entangled structure is a superposition of vectors from a subspace of the 2N{2}^{N}2N-dimensional Hilbert space, whose dimension grows linearly with the number of entangled pairs.
A bstract In this paper, we investigate the entanglement dynamics induced by a composite operator, defined as the local operator evolved with the time evolution operator constructed out of the Euclidean and Lorentzian ones. The systems under consideration are described by two-dimensional holographic conformal field theories (2d holographic CFTs), the theories described by gravity. Then, we find that the time ordering of Euclidean and Lorentzian time evolutions determines whether the time evolution, inducing the operator growth, is unitary. We also establish the relation between entanglement entropy and energy densities. This relation provides the entanglement entropy as a function of those densities. By exploiting this relation, we find that the time ordering, i.e., whether the time evolution is unitary, determines the late-time behavior of bipartite entanglement and non-local correlation. We also investigate how the decay of the quasiparticle influences the bipartite entanglement and non-local correlation. Furthermore, we investigate the gravity dual of the systems considered in this paper.
We consider physical Hamiltonians that can be represented by the multiparametric Gaussian ensembles, theoretically derive the state ensembles for its eigenstates and analyze the effect of varying system conditions on its bipartite entanglement entropy. Our approach leads to a single parametric based common mathematical formulation for the evolution of the entanglement statistics of different states of a given Hamiltonian or different Hamiltonians subjected to same symmetry constraints. The parameter turns out to be a single functional of the system parameters and thereby reveals a deep web of connection hidden underneath different quantum states.
The entanglement structure of a bipartite pure state is characterized by its entanglement spectrum, or equivalently, by the family of Rényi-kkk entanglement entropies with integer kkk. Taking the difference of two such entropies defines a quantity known as an entanglement spread, that is monotonous under local operations, and that vanishes if and only if the entanglement spectrum of the state is flat. We introduce a multipartite generalization of this notion which, for a fixed number D≥3D\geq 3D≥3 of parties, results in a collection of maps indexed by pairs of DDD-edge-colored graphs (obeying some condition). Each member of this collection takes the form of a difference of two multipartite Rényi entanglement entropies, each associated to a local unitary polynomial invariant (known in this context as a trace-invariant), and is shown to be monotonous under local operations. We then argue that a previously introduced family of so-called hypergraph-tensor (HT) states can be understood as a multipartite counterpart to the family of flat bipartite states. Indeed, we first prove that, when D=3D=3D=3, a tripartite pure state is HT if and only if it minimizes a particular (infinite) family of entanglement spreads. Second, we embed the set of HT states into the much larger family of spectral hypergraph-tensor (SHT) states, defined by a new Ansatz we introduce. We then prove that HT states can be uniquely characterized as the minimizers of some fixed entanglement spread among SHT states, for arbitrary D≥3D\geq 3D≥3. We also investigate SHT states in their own right: in particular, we characterize their orbits under local unitary transformations. Finally, along the way, the asymptotic large-NNN expectation values of a number of entanglement spreads are computed in the Haar-random state of local dimension NNN.