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arXiv:1901.04117 · Nuclear Physics B

Honeycomb lattice Kitaev model with Wen–Toric-code interactions, and anyon excitations

Kazuhiko Minami

Published Jan 9, 2019• 9 citations
arXivHTMLPDF

Abstract

The honeycomb lattice Kitaev model H K with two kinds of Wen–Toric-code four-body interactions H W T is investigated exactly using a new fermionization method, and the ground state phase diagram is obtained. Six kinds of three-body interactions are also considered. A Hamiltonian equivalent to the honeycomb lattice Kitaev model is also introduced. The fermionization method is generalized to two-dimensional systems, and the two-dimensional Jordan–Wigner transformation is obtained as a special case of this formula. The model H K + H W T is symmetric in four-dimensional space of coupling constants, and the anyon type excitations appear in each phase.

DOI
10.1016/j.nuclphysb.2018.12.029

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The honeycomb lattice Kitaev model H K with two kinds of Wen–Toric-code four-body interactions H W T is investigated exactly using a new fermionization method, and the ground state phase diagram is obtained. Six kinds of three-body interactions are also considered. A Hamiltonian equivalent to the honeycomb lattice Kitaev model is also introduced. The fermionization method is generalized to two-dimensional systems, and the two-dimensional Jordan–Wigner transformation is obtained as a special case of this formula. The model H K + H W T is symmetric in four-dimensional space of coupling constants, and the anyon type excitations appear in each phase.
We study an anyon model in a toric honeycomb lattice. The ground states and the low-lying excitations coincide with those of Kitaev toric code model and then the excitations obey mutual semionic statistics. This model is helpful to understand the toric code of anyons in a more symmetric way. On the other hand, there is a direct relation between this toric honeycomb model and a boundary coupled Ising chain array in a square lattice via Jordan—Wigner transformation. We discuss the equivalence between these two models in the low-lying sector and realize these anyon excitations in a conventional fermion system. The analysis for the ground state degeneracy in the last section can also be thought of as a complementarity of our previous work [ Phys. A: Math. Theor. 43 (2010) 105306 ].
We present a solution of Kitaev's spin model on the honeycomb lattice and of related topologically ordered spin models. We employ a Jordan-Wigner-type fermionization and find that the Hamiltonian takes a BCS-type form, allowing the system to be solved by Bogoliubov transformation. Our fermionization does not employ nonphysical auxiliary degrees of freedom and the eigenstates we obtain are completely explicit in terms of the spin variables. The ground state is obtained as a BCS condensate of fermion pairs over a vacuum state which corresponds to the toric-code state with the same vorticity. We show in detail how to calculate all eigenstates and eigenvalues of the model on the torus. In particular, we find that the topological degeneracy on the torus descends directly from that of the toric-code, which now supplies four vacua for the fermions, one for each choice of periodic vs antiperiodic boundary conditions. The reduction of the degeneracy in the non-Abelian phase of the model is seen to be due to the vanishing of one of the corresponding candidate BCS ground states in that phase. This occurs in particular in the fully periodic vortex-free sector. The true ground state in this sector is exhibited and shown to be gapped away from the three partially antiperiodic ground states whenever the non-Abelian phase is gapped.
We study the gapped phase of the Kitaev model on the honeycomb lattice using perturbative continuous unitary transformations. The effective low-energy Hamiltonian is found to be an extended toric code with interacting anyons. High-energy excitations are emerging free fermions which are composed of hard-core bosons with an attached string of spin operators. The excitation spectrum is mapped onto that of a single particle hopping on a square lattice in a magnetic field. We also illustrate how to compute correlation functions in this framework. The present approach yields analytical perturbative results in the thermodynamical limit without using the Majorana or the Jordan-Wigner fermionization initially proposed to solve this problem.
The non-Abelian Berry phase is an essential feature of non-Abelian anyons for the realization of topological quantum computation. This thesis is primarily a study about the numerical calculation of the Berry phase of non-Abelian anyons in the Kitaev honeycomb lattice model. It is also a guide for experimental the realizations of the actual brading process. We give an introduction to the theory of non-Abelian anyons, briefly discussing in what kind of systems they are realized, and their possible use in topological quantum computation. Non-Abelian anyons are studied within the Kitaev honeycomb model where they are realized on the plaquettes of the honeycomb lattice. The Kitaev honeycomb model can be solved exactly by using various fermionization methods. In this thesis, we review a solution based on Jordan-Wigner types of fermions which transform Hamiltonian to a fermionic quadratic form. This kind of fermionization procedure is quite general and can be applied to any trivalent spin lattice models. Moreover, we introduce Hartree-Fock-Bogoliubov method to solve general quadratic fermionic Hamiltonian and employ Bloch-Messiah theorem in order to write ground state wave function explicitly. Later, we apply these methods to honeycomb model and study the eigenstates of the model so that we can do the Berry phase calculation. The final chapter explains the details of the numerical calculation of the non-Abelian Berry phase. First, we show how to create and adiabatically move vortices in the honeycomb model. A brief review of the Berry phase is given including some discussion about a numerical approach. Later on Thouless’ representation of the ground state is introduced to calculate the Berry phase. All these theoretical tools are applied to a 4 vortex configuration of the model to calculate the non-Abelian Berry phase of the system on a particular path in the parameter space.
We introduce a spin-12\frac{1}{2}21​ model in three dimensions which is a generalization of the well-known Kitaev model on a honeycomb lattice. Following Kitaev, we solve the model exactly by mapping it to a theory of noninteracting fermions in the background of a static Z2{\mathbb{Z}}_{2}Z2​ gauge field. The phase diagram consists of a gapped phase and a gapless one, similar to the two-dimensional case. Interestingly, unlike in the two-dimensional model, in the gapless phase the gap vanishes on a contour in the k\mathbf{k}k space. Furthermore, we show that the flux excitations of the gauge field, due to some local constraints, form looplike structures; such loops exist on a lattice formed by the plaquettes in the original lattice and is topologically equivalent to the pyrochlore lattice. Finally, we derive a low-energy effective Hamiltonian that can be used to study the properties of the excitations in the gapped phase.
Abstract The Kitaev honeycomb model is an approximate topological quantum error correcting code in the same phase as the toric code, but requiring only a 2-body Hamiltonian. As a frustrated spin model, it is well outside the commuting models of topological quantum codes that are typically studied, but its exact solubility makes it more amenable to analysis of effects arising in this noncommutative setting than a generic topologically ordered Hamiltonian. Here we study quantum error correction in the honeycomb model using both analytic and numerical techniques. We first prove explicit exponential bounds on the approximate degeneracy, local indistinguishability, and correctability of the code space. These bounds are tighter than can be achieved using known general properties of topological phases. Our proofs are specialized to the honeycomb model, but some of the methods may nonetheless be of broader interest. Following this, we numerically study noise caused by thermalization processes in the perturbative regime close to the toric code renormalization group fixed point. The appearance of non-topological excitations in this setting has no significant effect on the error correction properties of the honeycomb model in the regimes we study. Although the behavior of this model is found to be qualitatively similar to that of the standard toric code in most regimes, we find numerical evidence of an interesting effect in the low-temperature, finite-size regime where a preferred lattice direction emerges and anyon diffusion is geometrically constrained. We expect this effect to yield an improvement in the scaling of the lifetime with system size as compared to the standard toric code.