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arXiv:1211.5446 · Brazilian Journal of Probability and Statistics

A Mermin–Wagner theorem on Lorentzian triangulations with quantum spins

Mark Kelbert, Yu. M. Suhov, Anatoly Yambartsev

Published Jul 30, 2014• 8 citations
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Abstract

We consider infinite random causal Lorentzian triangulations emerging in quantum gravity for critical values of parameters. With each vertex of the triangulation we associate a Hilbert space representing a bosonic particle moving in accordance with the standard laws of Quantum Mechanics. The particles interact via two-body potentials decaying with the graph distance. A Mermin–Wagner type theorem is proven for infinite-volume reduced density matrices related to solutions to DLR equations in the Feynman–Kac (FK) representation.

DOI
10.1214/13-bjps222

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We consider infinite random causal Lorentzian triangulations emerging in quantum gravity for critical values of parameters. With each vertex of the triangulation we associate a Hilbert space representing a bosonic particle moving in accordance with the standard laws of Quantum Mechanics. The particles interact via two-body potentials decaying with the graph distance. A Mermin–Wagner type theorem is proven for infinite-volume reduced density matrices related to solutions to DLR equations in the Feynman–Kac (FK) representation.
This paper is the second in a series of papers considering symmetry properties of bosonic quantum systems over 2D graphs, with continuous spins, in the spirit of the Mermin-Wagner theorem. In the model considered here the phase space of a single spin is ℋ1=L2(M), where M is a d -dimensional unit torus M=ℝd/ℤd with a flat metric. The phase space of k spins is ℋk=L2sym(Mk) , the subspace of L2(Mk) formed by functions symmetric under the permutations of the arguments. The Fock space H =⊕k=0,1,… ℋk yields the phase space of a system of a varying (but finite) number of particles. We associate a space H≃H(i) with each vertex i∈Γ of a graph (Γ,ℰ) satisfying a special bidimensionality property. (Physically, vertex i represents a heavy “atom” or “ion” that does not move but attracts a number of “light” particles.) The kinetic energy part of the Hamiltonian includes (i) -Δ/2 , the minus a half of the Laplace operator on M , responsible for the motion of a particle while “trapped” by a given atom, and (ii) an integral term describing possible “jumps” where a particle may join another atom. The potential part is an operator of multiplication by a function (the potential energy of a classical configuration) which is a sum of (a) one-body potentials U(1)(x) , x∈M , describing a field generated by a heavy atom, (b) two-body potentials U(2)(x,y) , x,y∈M , showing the interaction between pairs of particles belonging to the same atom, and (c) two-body potentials V(x,y) , x,y∈M , scaled along the graph distance d(i,j) between vertices i,j∈Γ , which gives the interaction between particles belonging to different atoms. The system under consideration can be considered as a generalized (bosonic) Hubbard model. We assume that a connected Lie group G acts on M , represented by a Euclidean space or torus of dimension d'≤d , preserving the metric and the volume in M . Furthermore, we suppose that the potentials U(1) , U(2) , and V are G -invariant. The result of the paper is that any (appropriately defined) Gibbs states generated by the above Hamiltonian is G -invariant, provided that the thermodynamic variables (the fugacity z and the inverse temperature β ) satisfy a certain restriction. The definition of a Gibbs state (and its analysis) is based on the Feynman-Kac representation for the density matrices.
Les systèmes de spin antiferromagnétiques en une dimension présentent des caractéristiques remarquables. Le théorème de Mermin-Wagner montre que même dans l'état fondamental de leur Hamiltonien, ces systèmes ne possèdent pas d'ordre à longue distance dans leurs fonctions de corrélation spin-spin s'ils possèdent une symétrie continue. Néanmoins, un quasi-ordre à longue distance peut exister, avec des fonctions de corrélation spin-spin décroissant en loi de puissance avec la distance, comme par exemple dans la chaîne de spin-1/2. Ce cas est décrit par la théorie des liquides de Luttinger. Même dans le cas d'un ordre à courte distance où les fonctions de corrélations spin-spin décroissent exponentiellement, un ordre topologique peut être présent. Il peut être mis en évidence par un paramètre d'ordre dépendant des opérateurs de spin de façon non-locale ou par la présence d'excitations de bord, comme par exemple dans le cas de la chaînes de spin-1. D'autre part, il existe une équivalence formelle entre les opérateurs de spin-1/2 et les bosons de coeur dur, qui permet de traduire les propriétés des systèmes magnétiques dans le langage des bosons en interaction. Après une rapide revue des méthodes théoriques utilisées pour la description des systèmes de basse dimensionnalité, je décrirais mes travaux sur les systèmes échelles de spin antiferromagnétiques et les applications aux systèmes de bosons en interaction.
This paper focuses on infinite-volume bosonic states for a quantum particle system (a quantum gas) in Rd. The kinetic energy part of the Hamiltonian is the standard Laplacian (with a boundary condition at the border of a ‘box’). The particles interact with each other through a two-body finite-range potential depending on the distance between them and featuring a hard core of diameter a>0. We introduce a class of so-called FK-DLR functionals containing all limiting Gibbs states of the system. As a justification of this concept, we prove that for d=2, any FK-DLR functional is shift-invariant, regardless of whether it is unique or not. This yields a quantum analog of results previously achieved by Richthammer.
The paper focuses on infinite-volume bosonic states for a quantum particle system (a quantum gas) in a Euclidean space. The kinetic energy part of the Hamiltonian is the standard Laplacian (with a Dirichlet's boundary condition at the border of a `box'). The particles interact with each other through a two-body finite-range potential depending on the distance between them and featuring a hard core of a positive diameter. We introduce a class of so-called FK-DLR functionals containing all limiting Gibbs states of the system. In the next paper we will prove that any FK-DLR functional is shift-invariant, regardless of whether it is unique or not.
A new theorem on random walks suggest some possible revisions of the foundations of Quantum Mechanics. This is presented below in the simplified framework of the description of the evolution of a material point in space. Grossly speaking, it is shown that the probabilities generated by normalizing the square modulus of a sum of probability amplitudes, in the setup of Quantum Mechanics, becomes asymptotically close (under the appropriate limiting conditions) to the probabilities generated by the usual causal processes of Classical Mechanics. This limiting coincidence has a series of interesting potential applications. In particular it allows us to reintroduce the concept of causality within the core of Quantum Mechanics. Moreover, it suggests, among other consequences, that gravitational interaction may not even exist. Even though the interpretations of Quantum Mechanics which follow from this mathematical result may seem to bring some unexpected innovations in the context of theoretical physics, there is an obvious necessity to study its theoretical impact on Quantum Mechanics. The first steps toward this aim are taken in the present article.
We solve the problem of the spin quantum Hall transition on random networks using a mapping to classical percolation that focuses on the boundary of percolating clusters. Using tools of two-dimensional quantum gravity, we compute critical exponents that characterize this transition and confirm that these are related to the exponents for the regular (square) network through the KPZ relation. Our results demonstrate the relevance of the geometric randomness of the networks and support conclusions of numerical simulations of random networks for the integer quantum Hall transition.