Emergence and Entanglement - 2010

Matrix Product States (MPS) and their higher dimensional extensions, the Projected Entangled-Pair States (PEPS) can efficiently describe the ground and thermal states of interacting systems with short-range interactions. We will describe some mathematical properties of this families of states, as well as possible extensions. Work in collaboration with N. Schuch, D. Perez-Garcia, M. Sanz, M. Wolf, F. Verstraete and G. Sierra.

This talk will be concerned with three new results (or a subset thereof) on the idea of grasping quantum many-body systems in terms of suitable tensor networks, such as finitely correlated states (FCS), tree tensor networks (TTN), projected entangled pair states (PEPS) or entanglement renormalization (MERA). We will first briefly introduce some basic ideas and relate the feasibility of such approaches to entanglement properties and area laws. We will then see that (a) surprisingly, any MERA can be efficiently encoded in a PEPS, hence in a sense unifying these approaches. (b) We will also find that the ground state-manifold of any frustration-free spin-1/2 nearest neighbor Hamiltonian can be completely characterized in terms of tensor networks, how all such ground states satisfy an area law, and in which way such states serve as ansatz states for simulating almost frustration-free systems. (c) The last part will be concerned with using flow techniques to simulate interacting quantum fields with finitely correlated state approaches, and with simulating interacting fermions using efficiently contractible tensor networks.

Closed quantum systems evolve unitarily and therefore cannot converge in a strong sense to an equilibrium state starting out from a generic pure state. Nevertheless for large system size one observes temporal typicality. Namely, for the overwhelming majority of the time instants, the statistics of observables is practically indistinguishable from an effective equilibrium one. In this talk we will discuss he Loschmidt echo (LE) to study this sort of unitary equilibration after a quench. In particular we ll address the issue of typicality with respect the initial state preparation and the influence of quantum criticality of the long-time probability distribution of LE.

A lattice gauge theory is described by a redundantly large vector space that is subject to local constraints, and it can be regarded as the low energy limit of a lattice model with a local symmetry. I will describe a coarse-graining scheme capable of exactly preserving local symmetries. The approach results in a variational ansatz for the ground state(s) and low energy excitations of a lattice gauge theory. This ansatz has built-in local symmetries, which are exploited to significantly reduce simulation costs. I will describe benchmark results in the context of Kitaev’s toric code with a magnetic field or, equivalently, Z2 lattice gauge theory, for lattices with up to 16 x 16 sites (16^2 x 2 = 512 spins) on a torus.

Using the mapping of the Fokker-Planck description of classical stochastic dynamics onto a quantum Hamiltonian, we argue that a dynamical glass transition in the former must have a precise definition in terms of a quantum phase transition in the latter. At the dynamical level, the transition corresponds to a collapse of the excitation spectrum at a critical point. At the static level, the transition affects the ground state wavefunction: while in some cases it could be picked up by the expectation value of a local operator, in others the order may be non-local, and impossible to be determined with any local probe. Here we propose instead to use concepts from quantum information theory that are not centered around local order parameters, such as fidelity and entanglement measures. We show that for systems derived from the mapping of classical stochastic dynamics, singularities in the fidelity susceptibility translate directly into singularities in the heat capacity of the classical system. In classical glassy systems with an extensive number of metastable states, we find that the prefactor of the area law term in the entanglement entropy jumps across the transition. We also discuss how entanglement measures can be used to detect a growing correlation length that diverges at the transition. Finally, we illustrate how static order can be hidden in systems with a macroscopically large number of degenerate equilibrium states by constructing a three dimensional lattice gauge model with only short-range interactions but with a finite temperature continuous phase transition into a massively degenerate phase.

In this talk, I will present two schemes which would result in substantial entanglement between distant individual spins of a spin chain. One relies on a global quench of the couplings of a spin chain, while the other relies on a bond quenching at one end. Both of the schemes result in substantial entanglement between the ends of a chain so that such chains could be used as a quantum wire to connect quantum registers. I will also examine the resource of entanglement already existing between separated parts of a many-body system at criticality as the size of the parts and their separation is varied. This form of entanglement displays an interesting scale invariance.

In recent years the characterization of many-body ground states via the entanglement of their wave-function has attracted a lot of attention. One useful measure of entanglement is provided by the entanglement entropy S. The interest in this quantity has been sparked, in part, by the result that at one dimensional quantum critical points (QCPs) S scales logarithmically with the subsystem size with a universal coefficient related to the central charge of the conformal field theory describing the QCP. On the other hand, in spatial dimension d > 1 the leading contribution to the entanglement entropy scales as the area of the boundary of the subsystem. The coefficient of this ''area law'' is non-universal. However, in the neighbourhood of a QCP, S is believed to possess subleading universal corrections. In this talk, I will present the first field-theoretic study of entanglement entropy in dimension d > 1 at a stable interacting QCP - the quantum O(N) model. Our results confirm the presence of universal corrections to the entanglement entropy and exhibit a number of surprises such as different epsilon -> 0 limits of the Wilson-Fisher and Gaussian fixed points, violation of large N counting and subtle dependence on replica index.

Condensed matter theorists have recently begun exploiting the properties of entanglement as a resource for studying quantum materials. At the forefront of current efforts is the question of how the entanglement of two subregions in a quantum many-body groundstate scales with the subregion size. The general belief is that typical groundstates obey the so-called "area law", with entanglement entropy scaling as the boundary between regions. This has lead theorists to propose that sub-leading corrections to the area law provide new universal quantities at quantum critical points and in exotic quantum phases (i.e. topological Mott insulators). However, away from one dimension, entanglement entropy is difficult or impossible to calculate exactly, leaving the community in the dark about scaling in all but the simplest non-interacting systems. In this talk, I will discuss recent breakthroughs in calculating entanglement entropy in two dimensions and higher using advanced quantum Monte Carlo simulation techniques. We show, for the first time, evidence of leading-order area law scaling in a prototypical model of strongly-interacting quantum spins. This paves the way for future work in calculating new universal quantities derived from entanglement, in the plethora of real condensed matter systems amenable to numerical simulation.

Many one dimensional random quantum systems exhibit infinite randomness phases, such as the random singlet phase of the spin-1/2 Heisenberg model. These phases are typically the result of destabilizing systems described by a conformal field theory with disorder. Interestingly, entanglement entropy in 1d infinite randomness phases also exhibits a universal log scaling with length. In my talk I will touch upon calculating the entanglement entropy for inifinite-randomness phases, as well as describe the exotic infinite randomness phases realized in chains of non-abelian anyon chains. It was speculated that the entanglement entropy of an infinite-randomness phase is associated with the direction of RG flow, just as the c-theorem dictates the direction of RG flows for CFT's. I will also show that the entanglement entropy in disordered non-abelian chains provide the only known counter example.

The quantum states postulated to occur in situations of the "Schroedinger's Cat" type are essentially N-particle GHZ states with N very large compared to 1,and their observation would thus be particularly compelling evidence for the ubiquity of the phenomenon of entanglement. However, in the traditional quantum measurement literature considerable scepticism has been expressed about the observability of this kind of "macroscopically entangled" state, primarily because of the putatively disastrous effect on it of decoherence. In this talk I first examine why much of the literature has grossly overestimated the effects of decoherence,and then review the current experimental situation with respect to such states, as they (may) occur in fullerene diffraction, magnetic biomolecules, quantum-optical systems and Josephson devices; I also consider the prospects for their observation in nanomechanical systems. I conclude by reviewing and the theoretical implications of the experiments of the last decade in this area.

Many one dimensional random quantum systems exhibit infinite randomness phases, such as the random singlet phase of the spin-1/2 Heisenberg model. These phases are typically the result of destabilizing systems described by a conformal field theory with disorder. Interestingly, entanglement entropy in 1d infinite randomness phases also exhibits a universal log scaling with length. In my talk I will touch upon calculating the entanglement entropy for inifinite-randomness phases, as well as describe the exotic infinite randomness phases realized in chains of non-abelian anyon chains. It was speculated that the entanglement entropy of an infinite-randomness phase is associated with the direction of RG flow, just as the c-theorem dictates the direction of RG flows for CFT's. I will also show that the entanglement entropy in disordered non-abelian chains provide the only known counter example.