Tensor Product States and 2D Gapped Phases

Leo Molinaro

Tensor Networks are a tool from quantum information theory to describe many-body quantum systems. It has been shown that states arising from Tensor Networks in 1D (Matrix Product States - MPS) accurately describe ground states of local, gapped Hamiltonians. The same project for higher dimensional systems is currently the subject of great efforts. However, quantum many-body systems in dimension 2 or greater can exhibit much richer behaviour than their one-dimensional counterparts. One of the hallmarks of this is the scarcely understood notion of topological order. It has been shown recently, that Projected Entangled-Pair States (PEPS - Tensor Networks in 2D) can describe chiral topological states, that lie in the ground state manifold of a certain class of gapless Hamiltonians. In this work, we try to perturb the Hamiltonian into a gapped chiral phase. We will then study, whether this gapped, chiral phase can be described in terms of PEPS or how the PEPS-construction has to be modified to accommodate for this kind of behaviour.

Bei Zeng, Xie Chen, D. L. Zhou +1 more

Chen Liu, Anders W. Sandvik, Yu-Cheng Su +1 more

Paul Herringer

Certain symmetry-protected topological (SPT) phases also possess the property that every state in the phase is a universal resource for measurement-based quantum computation (MBQC). These phases are called computational phases of quantum matter. Computational phases have been classified in 1D, and several examples are known in 2D. 2D computational phases are of interest because we can simulate any quantum computation using a 2D phase. Therefore, the classification of 2D phases may lead to a better understanding of the relationship between symmetry and quantum computation. Many known 2D phases have a projected entangled pair state (PEPS) representation where the local tensor symmetries define a quantum cellular automaton (QCA) acting on correlation space. However, not every tensor symmetry defines a QCA, which motivates the definition and study of a larger class of PEPS tensors called stabilizer tensors. Stabilizer tensors have symmetries that act as a tensor product of Pauli operators on correlation space but do not necessarily define a QCA. In this thesis, I present a classification of stabilizer tensors by their channel capacity for quantum wire. In particular, I consider the quantum wire capacity of translationally invariant, cylindrical PEPS constructed from stabilizer tensors and find that it falls into one of 13 classes. Furthermore, I prove that no other types of channel capacity exist for stabilizer tensors, regardless of the size of the cylinder. I also discuss the significance of this result for the classification of 2D computational phases along with possible next steps.

Zheng‐Cheng Gu, Michael Levin, Xiao-Gang Wen

Traditional mean-field theory is a generic variational approach for analyzing symmetry breaking phases. However, this simple approach only applies to symmetry breaking states with short-range entanglement. In this paper, we describe a generic approach for studying two-dimensional (2D) quantum phases with long-range entanglement (such as topological phases). The method is based on (a) a general class of trial wave functions known as tensor-product states and (b) a 2D real-space renormalization group algorithm for efficiently calculating expectation values for these states. We demonstrate our method by studying several simple 2D quantum spin models exhibiting both symmetry breaking phase transitions and topological phase transitions. Our approach can be viewed as a unified mean-field theory for both symmetry breaking phases and topological phases.

Timothy R Field, Jeeva S. Anandan