Quantum Spin Chains Research Articles

Subsystem distance after a local operator quench

We investigate the time evolution of the subsystem trace distance and Schatten distances after local operator quenches in two-dimensional conformal field theory (CFT) and in one-dimensional quantum spin chains. We focus on the case of a subsystem being an interval embedded in the infinite line. The initial state is prepared by inserting a local operator in the ground state of the theory. We only consider the cases in which the inserted local operator is a primary field or a sum of several primaries. While a nonchiral primary operator can excite both left-moving and right-moving quasiparticles, a holomorphic primary operator only excites a right-moving quasiparticle and an anti-holomorphic primary operator only excites a left-moving one. The reduced density matrix (RDM) of an interval hosting a quasiparticle is orthogonal to the RDM of the interval without any quasiparticles. Moreover, the RDMs of two intervals hosting quasiparticles at different positions are also orthogonal to each other. We calculate numerically the entanglement entropy, Rényi entropy, trace distance, and Schatten distances in time-dependent states excited by different local operators in the critical Ising and XX spin chains. These results match the CFT predictions in the proper limit.

Emergence of conformal symmetry in quantum spin chains: Antiperiodic boundary conditions and supersymmetry

Universal properties of a critical quantum spin chain are encoded in the underlying conformal field theory (CFT). This underlying CFT is fully characterized by its conformal data. We propose a method to extract the conformal data from a critical quantum spin chain with both periodic and anti-periodic boundary conditions (PBC and APBC) based on low-energy eigenstates, generalizing previous work on spin chains with only PBC. First, scaling dimensions and conformal spins are extracted from the energies and momenta of the eigenstates. Second, the Koo-Saleur formula of lattice Virasoro generators is generalized to APBC and used to identify conformal towers. Third, local operators and string operators on the lattice are identified with CFT operators with PBC and APBC, respectively. Finally, operator product expansion coefficients are extracted by computing matrix elements of lattice primary operators in the low-energy subspaces with PBC and APBC. To go beyond exact diagonalization, tensor network methods based on periodic uniform matrix product states are used. We illustrate our approach with critical and tricritical Ising quantum spin chains. In the latter case, we propose lattice operators that correspond to supervirasoro generators and verify their action on low-energy eigenstates. In this way we explore the emergence of superconformal symmetry in the quantum spin chain.

Conformal Fields and Operator Product Expansion in Critical Quantum Spin Chains.

At a quantum critical point, the low-energy physics of a quantum spin chain is described by conformal field theory (CFT). Given the Hamiltonian of a critical spin chain, we propose a variational method to build an approximate lattice representation ϕ_{α} of the corresponding primary CFT operators ϕ_{α}^{CFT}. We then show how to numerically compute the operator product expansion coefficients C_{αβγ}^{CFT} governing the fusion of two primary fields. In this way, we complete the implementation of Cardy's program, outlined in the 1980s, which aims to extract the universality class of a phase transition, as encoded in the so-called conformal data of the underlying CFT, starting from a microscopic description. Our approach, demonstrated here for the critical quantum Ising model, only requires a generic (i.e., in general, nonintegrable) critical lattice Hamiltonian as its input.

Calculated magnetic exchange interactions in the quantum spin chain materials K2CuSO4Cl2 and K2CuSO4Br2

Using density functional calculations, we present a comprehensive study of the electronic and magnetic properties of the quantum spin chain materials ${\mathrm{K}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Cl}}_{2}$ and ${\mathrm{K}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Br}}_{2}$. With the first-principles linear response method, we calculate the magnetic exchange parameters, and the theoretical values are in a quantitatively good agreement with experiments. Depending on the signs and magnitudes of the interchain magnetic exchange interactions, we find that the magnetic order of ${\mathrm{K}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Br}}_{2}$ along the $b$ axis is antiferromagnetic, while ${\mathrm{K}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Cl}}_{2}$ has strong frustration along the $b$ axis. Based on the obtained magnetic exchange parameters, we successfully reproduce the experimental spin-wave dispersion. We also calculate Dzyaloshinskii-Moriya interactions, which are found to dominate in these compounds compared to the interchain magnetic exchange interactions. Finally, we obtain the phase diagram for these two materials using Monte Carlo simulations. Our results are consistent with previous experiments, giving a complete explanation of the magnetic structure.

New higher anomalies, SU(N) Yang–Mills gauge theory and [formula omitted] sigma model

We hypothesize a new and more complete set of anomalies of certain quantum field theories (QFTs) and then give an eclectic verification. First, we propose a set of ’t Hooft higher anomalies of 4d time-reversal symmetric pure SU(N)-Yang–Mills (YM) gauge theory with a second-Chern-class topological term at θ=π, via 5d cobordism invariants (higher symmetry-protected topological states), with N =2,3,4 and others. Second, we propose a set of ’t Hooft anomalies of 2d CPN−1-sigma models with a first-Chern-class topological term at θ=π, by enlisting all possible 3d cobordism invariants and selecting the matched terms. Based on algebraic/geometric topology, QFT analysis, manifold generator correspondence, condensed matter inputs such as stacking PSU(N)-generalized Haldane quantum spin chains, and additional physics criteria, we derive a correspondence between 5d and 3d new invariants. Thus we broadly prove a potentially complete anomaly-matching between 4d SU(N) YM and 2d CPN−1 models at N = 2, and suggest new (but maybe incomplete) anomalies at N = 4. We formulate a higher-symmetry analog of “Lieb–Schultz–Mattis theorem” to constrain the low-energy dynamics.

Continuum Limits of Homogeneous Binary Trees and the Thompson Group.

Tree tensor network descriptions of critical quantum spin chains are empirically known to reproduce correlation functions matching conformal field theory (CFT) predictions in the continuum limit. It is natural to seek a more complete correspondence, additionally incorporating dynamics. On the CFT side, this is determined by a representation of the diffeomorphism group of the circle. In a remarkable series of papers, Jones outlined a research program where the Thompson group T takes the role of the latter in the discrete setting, and representations of T are constructed from certain elements of a subfactor planar algebra. He also showed that, for a particular example of such a construction, this approach only yields-in the continuum limit-a representation which is highly discontinuous and hence unphysical. Here we show that the same issue arises generically when considering tree tensor networks: the set of coarse-graining maps yielding discontinuous representations has full measure in the set of all isometries. This extends Jones's no-go example to typical elements of the so-called tensor planar algebra. We also identify an easily verified necessary condition for a continuous limit to exist. This singles out a particular class of tree tensor networks. Our considerations apply to recent approaches for introducing dynamics in holographic codes.

One-way street for the energy current: A ubiquitous phenomenon in boundary-driven quantum spin chains

Focusing on the description of nontrivial properties of the energy transport at quantum scale, we investigate asymmetrical quantum spin chains described by boundary-driven and Heisenberg models. We search for symmetries properties of the Lindblad master equation related to the dynamics of the system in order to establish properties of the steady state. Under rather general assumptions for the target polarization at the boundaries, we show the occurrence of an effect related to (but stronger than) energy rectification, namely, the one-way street phenomenon, which is the existence of an unique way for the energy flow. Precisely, the energy current does not change in magnitude and direction as we invert the baths at the boundaries: its direction is completely determined by the asymmetry in the bulk of the chain. The results follow independently of the system size and of the transport regime. Our findings show the ubiquitous occurrence of the one-way street phenomenon for the energy flow in boundary-driven spin systems and, we believe, they shall be an useful contribution to the area devoted to the investigation and building of efficient quantum devices used to control and manipulate the energy current.

Classification of symmetry protected topological phases in quantum spin chains

We consider the classification problem of symmetry protected topological (SPT) phases on quantum spin systems. SPT phases are gapped short-range-entangled quantum phases with a symmetry $G$. We explain that in one and two-dimensional quantum spin systems, there are $H^{2}(G, U(1))$/ $H^{3}(G, U(1))$-valued invariant, confirming a physicists conjecture.

Measuring von Neumann entanglement entropies without wave functions

We present a method to measure the von Neumann entanglement entropy of ground states of quantum many-body systems which does not require access to the system wave function. The technique is based on a direct thermodynamic study of lattice entanglement Hamiltonians—recently proposed in the paper [Dalmonte et al 2018 Nat. Phys. 14 827] via field theoretical insights—and can be performed by quantum Monte Carlo methods. We benchmark our technique on critical quantum spin chains, and apply it to several two-dimensional quantum magnets, where we are able to unambiguously determine the onset of area law in the entanglement entropy, the number of Goldstone bosons, and to check a recent conjecture on geometric entanglement contribution at critical points described by strongly coupled field theories. The protocol can also be adapted to measure entanglement in experiments via quantum quenches.

High-temperature analysis of the transverse dynamical two-point correlation function of the XX quantum-spin chain

We analyze the transverse dynamical two-point correlation function of the XX chain by means of a thermal form factor series. The series is rewritten in terms of the resolvent and the Fredholm determinant of an integrable integral operator. This connects it with a matrix Riemann-Hilbert problem. We express the correlation function in terms of the solution of the matrix Riemann-Hilbert problem. The matrix Riemann-Hilbert problem is then solved asymptotically in the high-temperature limit. This allows us to obtain the leading high-temperature contribution to the two-point correlation function at any fixed space-time separation.

Spinorial R operator and Algebraic Bethe Ansatz

We propose a new approach to the spinor-spinor R-matrix with orthogonal and symplectic symmetry. Based on this approach and the fusion method we relate the spinor-vector and vector-vector monodromy matrices for quantum spin chains. We consider the explicit spinor R matrices of low rank orthogonal algebras and the corresponding RTT algebras. Coincidences with fundamental R matrices allow to relate the Algebraic Bethe Ansatz for spinor and vector monodromy matrices.

Universal Signature from Integrability to Chaos in Dissipative Open Quantum Systems.

We study the transition between integrable and chaotic behavior in dissipative open quantum systems, exemplified by a boundary driven quantum spin chain. The repulsion between the complex eigenvalues of the corresponding Liouville operator in radial distance s is used as a universal measure. The corresponding level spacing distribution is well fitted by that of a static two-dimensional Coulomb gas with harmonic potential at inverse temperature β∈[0,2]. Here, β=0 yields the two-dimensional Poisson distribution, matching the integrable limit of the system, and β=2 equals the distribution obtained from the complex Ginibre ensemble, describing the fully chaotic limit. Our findings generalize the results of Grobe, Haake, and Sommers, who derived a universal cubic level repulsion for small spacings s. We collect mathematical evidence for the universality of the full level spacing distribution in the fully chaotic limit at β=2. It holds for all three Ginibre ensembles of random matrices with independent real, complex, or quaternion matrix elements.

Completing the solution for the OSp(1|2) spin chain

The periodic OSp(1|2) quantum spin chain has both a graded and a non-graded version. Naively, the Bethe ansatz solution for the non-graded version does not account for the complete spectrum of the transfer matrix, and we propose a simple mechanism for achieving completeness. In contrast, for the graded version, this issue does not arise. We also clarify the symmetries of both versions of the model, and we show how these symmetries are manifested in the degeneracies and multiplicities of the transfer-matrix spectrum. While the graded version has OSp(1|2) symmetry, the non-graded version has only SU(2) symmetry. Moreover, we obtain conditions for selecting the physical singular solutions of the Bethe equations. This analysis solves a lasting controversy over signs in the Bethe equations.

Fractionalized Edge Spins and Boundary Conditions in Frustrated Ferromagnetic Quantum Spin Chains

We examine frustrated ferromagnetic S = 1 spin chains at zero temperature by using the density-matrix renormalization group method. In open boundary conditions, the S = 1 quantum spin chains are known to produce fractionalized free spins on each edge, which causes difficulty in the numerical calculation of the thermodynamic ground states. To overcome this, we propose modified open boundary conditions to suppress the formation of fractionalized edge spins. The comparison of the physical properties of the ground states, which are obtained from the modified and the standard open boundary conditions, reveals that the modified open boundary conditions are more effective in improving the convergence behavior and reducing the computational costs.

Off-diagonal Bethe Ansatz for the {D}_3^{(1)} model

The exact solutions of the {D}_3^{(1)} model (or the so(6) quantum spin chain) with either periodic or general integrable open boundary conditions are obtained by using the off-diagonal Bethe Ansatz. From the fusion, the complete operator product identities are obtained, which are sufficient to enable us to determine spectrum of the system. Eigenvalues of the fused transfer matrices are constructed by the T - Q relations for the periodic case and by the inhomogeneous T- Q one for the non-diagonal boundary reflection case. The present method can be generalized to deal with the {D}_n^{(1)} model directly.

Dynamics and correlations in Motzkin and Fredkin spin chains

The Motzkin and Fredkin quantum spin chains are described by frustration-free Hamiltonians recently introduced and studied because of their anomalous behaviors in the correlation functions and in the entanglement properties. In this paper we analyze their quantum dynamical properties, focusing in particular on the time evolution of the excitations driven by a quantum quench, looking at the correlations functions of spin operators defined along different directions, and discussing the results in relation with the cluster decomposition property. We show that, for such models, one can have absence of cone-like structure in the propagation of the excitations after a quantum quench, together with a violation of the cluster decomposition property for the ground state characterized by only a logarithmic violation of the area law for the entanglement entropy.

Quantum counterpart of measure synchronization: A study on a pair of Harper systems

Measure synchronization is a well-known phenomenon in coupled classical Hamiltonian systems over last two decades. Here, synchronization in a pair of coupled Harper systems is investigated both in classical and quantum contexts. It seems that the concept of measure synchronization is restricted in the classical limit as it involves with the phase space. We show the quantum counterpart of the synchronization in a pair of coupled quantum kicked Harper chains. In the quantum context, the coupling occurs between two spins chains via a time and site dependent potential. We use the average interaction energy between the participating systems as an order parameter in both the contexts to establish a connection between the classical and the quantum scenarios. Besides, we also study the entanglement between the chains and difference between the average bare energies in the quantum context. Interestingly, all such indicators suggest a connection between the MS transition in classical maps and a phase transition in quantum spin chains.

Persistence of power-law correlations in nonequilibrium steady states of gapped quantum spin chains

The existence of quasi-long range order is demonstrated in nonequilibrium steady states in isotropic $XY$ spin chains including of two types of additional terms that each generate a gap in the energy spectrum. The system is driven out of equilibrium by initializing a domain-wall magnetization profile through application of an external magnetic field and switching off the magnetic field at the same time the energy gap is activated. An energy gap is produced by either applying a staggered magnetic field in the $z$ direction or introducing a modulation to the $XY$ coupling. The magnetization, spin current, and spin-spin correlation functions are computed analytically in the thermodynamic limit at long times after the quench. For both types of systems, we find the persistence of power-law correlations despite the ground-state correlation functions exhibiting exponential decay.

Characterization of localized effective spins in gapped quantum spin chains

We study properties of localized effective spins induced in gapped quantum spin chains by local inhomogeneities of the lattice. As a prototype, we study effective spins induced in impunity sites doped AKLT model by constructing the exact ground state in a matrix product state (MPS) form. We characterize their responses to external fields by studying an extended Zeeman interaction. We also study the antiferromagnetic bond-alternating Heisenberg chain with defect structures. For this model, an MPS representation similar to that for the AKLT model, "a uniform MPS with windows", is constructed, and it gives a good approximation of the ground state. We discuss the trade-off relation between the window length and the precision of the MPS ansatz. The effective exchange interaction between the induced spins is also investigated by using this representation.

Power-law entanglement growth from typical product states

Generic quantum many-body systems typically show a linear growth of the entanglement entropy after a quench from a product state. While entanglement is a property of the wave function, it is generated by the unitary time evolution operator and is therefore reflected in its increasing complexity as quantified by the operator entanglement entropy. Using numerical simulations of a static and a periodically driven quantum spin chain, we show that there is a robust correspondence between the entanglement entropy growth of typical product states with the operator entanglement entropy of the unitary evolution operator, while special product states, e.g. $\sigma_z$ basis states, can exhibit faster entanglement production. In the presence of a disordered magnetic field in our spin chains, we show that both the wave function and operator entanglement entropies exhibit a power-law growth with the same disorder-dependent exponent, and clarify the apparent discrepancy in previous results. These systems, in the absence of conserved densities, provide further evidence for slow information spreading on the ergodic side of the many-body localization transition.