We present analytic results for the ground state properties and entanglement in the Lieb-Liniger model and q-deformed Affleck-Kennedy-Lieb-Tasaki (AKLT) models. The translational invariance of the Lieb-Liniger model is broken by an external harmonic potential in one case and by coarse-grained measurements of particle number in another. Meanwhile, anisotropy is introduced into the AKLT model by generalizing its SU(2) invariant hamiltonian into one that is SUq(2) invariant. The Lieb-Liniger model describes a one-dimensional gas of bosons that mutually interact by a zero-range interaction term. Under longitudinal harmonic confinement, the exact spectrum of the hamiltonian is known only for free and impenetrable bosons. We use a pseudopotential approach and perturbation theory to calculate the ground state energy of this gas near the limit of infinite repulsion. We further study entanglement in the periodic homogeneous Lieb-Liniger model. When the particle number in a spatial partition of the ground state is measured, entanglement in the resulting state arises only from interparticle interactions. We demonstrate that entanglement in these projected states increases monotonically with the strength of interactions. Finally, we discuss how anisotropy reduces entanglement in the valence-bond-solid ground state of the q-deformed AKLT model. We propose effective thermal models that describe block entanglement in terms of boundary degrees of freedom. The anisotropy parameter q enters these models as an effective temperature. | 116 pages
In the ground state of gapped systems, the entanglement entropy of a subsystem A scales with the length of the boundary of A. This observation suggests that the entanglement properties of the sub- system can be described in terms of degrees of freedom living in the boundary of A. We will discuss the the connection between en- tanglement properties and effective boundary descriptions in spin systems in one and two dimensions. In one dimension we present analytic results for the spin S = 1, Affleck-Kennedy-Lieb-Tasaki (AKLT) ground state entanglement, characterized by negativity and entanglement spectrum. We also discuss a generalization of the AKLT model, based on the quantum group Uq (sl(2)) for gen- eral integer spin S. In two dimensions, we study two spin systems whose ground state can be written in terms of tensor product states of bond dimension two, the AKLT model in the hexagonal lattice and the Ising projected entangled pair state (Ising PEPS) in the square lattice. We show how the reduced density matrix of a parti- tion is associated with a thermal state of a one dimensional model along the boundary of that partition. We also present arguments supporting this correspondence for arbitrary gapped systems. Fi- nally we discuss the behavior of this boundary theory when the original two dimensional model is tuned through a quantum phase transition,126 pages
We investigate the entanglement properties of the valence-bond-solid (VBS) state defined on two-dimensional lattices, which is the exact ground state of the Affleck-Kennedy-Lieb-Tasaki model. It is shown that the entanglement entropy obeys an area law and the nonuniversal prefactor of the leading term is strictly less than
ln2. The analysis of entanglement spectra for various lattices reveals that the reduced density matrix associated with the VBS state is closely related to a thermal density matrix of a holographic spin chain, the spectrum of which is reminiscent of that of the spin-1/2 Heisenberg chain. This correspondence is further supported by comparing the entanglement entropy in the holographic spin chain with conformal field theory predictions.
In this paper we compute the entanglement, as quantified by negativity, between two blocks of length
LA and
LB, separated by
L sites in the one-dimensional spin-1 Affleck-Kennedy-Lieb-Tasaki (AKLT) model. We took the model with two different boundary conditions. We consider the case of
N spin-1 particles in the bulk and one spin-1/2 particle at each boundary, which constitute a unique ground state, and the case of just spin 1, even at the end of the chain, where the degeneracy of the ground state is 4. In both scenarios we made a partition consisting of two blocks
A and
B, containing
LA and
LB sites, respectively. The separation of these two blocks is
L. In both cases we explicitly obtain the reduced density matrix of the blocks
A and
B. We prove that the negativity in the first case vanishes identically for $L\ensuremath{\geqslant}1$, while in the second scenario it may approach a constant value
N=1/2 for each degenerate eigenstate depending on the way one constructs these eigenstates. However, as there is some freedom in constructing these eigenstates, vanishing entanglement is also possible in the latter case. Additionally, we also compute the entanglement between noncomplementary blocks in the case of periodic boundary conditions for the spin-1 AKLT model for which there is a unique ground state. Even in this case, we find that the negativity of separated blocks of spins is zero.
We study quantum entanglement in the ground state of the Affleck–Kennedy–Lieb–Tasaki model defined on two-dimensional graphs with reflection and/or inversion symmetry. The ground state of this spin model is known as the valence-bond-solid state. We investigate the properties of a reduced density matrix of a subsystem which is a mirror image of the other one. Thanks to reflection symmetry, the eigenvalues of the reduced density matrix can be obtained by numerically diagonalizing a real symmetric matrix whose elements are calculated by Monte Carlo integration. We calculate the von Neumann entropy of the reduced density matrix. The obtained results indicate that there is some deviation from the naive expectation that the von Neumann entropy per valence bond on the boundary between the subsystems is ln 2. This deviation is interpreted in terms of the hidden spin chain along the boundary between the subsystems. In some cases where graphs are on ladders, the numerical results are analytically or algebraically confirmed.
We propose a simple variational wave function that captures the correct ground-state energy of the spin-1 Heisenberg chain model to within 0.04%. The wave function is written in the matrix product state (MPS) form with the bond dimension
D=8 and is characterized by three fugacity parameters. The proposed MPS generalizes the Affleck-Kennedy-Lieb-Tasaki state by dressing it with dimers, trimers, and general
q-mers. The fugacity parameters control the number and the average size of the
q-mers. Furthermore, the
D=8 variational MPS state captures the ground states of the entire family of the bilinear-biquadratic Hamiltonian belonging to the Haldane phase to high accuracy. The 2-4-2 degeneracy structure in the entanglement spectrum of our MPS state is found to match well the results of the density matrix renormalization group (DMRG) calculation, which is computationally much heavier. Spin-spin correlation functions also find an excellent fit with those obtained by DMRG.