Spectral truncations in noncommutative geometry and operator systems

Abstract

In this paper we extend the traditional framework of noncommutative geometry in order to deal with spectral truncations of geometric spaces (i.e. imposing an ultraviolet cutoff in momentum space) and with tolerance relations which provide a coarse grain approximation of geometric spaces at a finite resolution. In our new approach the traditional role played by $C^{\ast }$-algebras is taken over by operator systems. As part of the techniques we treat $C^{\ast }$-envelopes, dual operator systems and stable equivalence. We define a propagation number for operator systems, which we show to be an invariant under stable equivalence and use to compare approximations of the same space. We illustrate our methods for concrete examples obtained by spectral truncations of the circle. These are operator systems of finite-dimensional Toeplitz matrices and their dual operator systems which are given by functions in the group algebra on the integers with support in a fixed interval. It turns out that the cones of positive elements and the pure state spaces for these operator systems possess a very rich structure which we analyze including for the algebraic geometry of the boundary of the positive cone and the metric aspect i.e. the distance on the state space associated to the Dirac operator. The main property of the spectral truncation is that it keeps the isometry group intact. In contrast, if one considers the other finite approximation provided by circulant matrices the isometry group becomes discrete, even though in this case the operator system is a $C^{\ast }$-algebra. We analyze this in the context of the finite Fourier transform. The extension of noncommutative geometry to operator systems allows one to deal with metric spaces up to finite resolution by considering the relation $d(x,y)<\epsilon$ between two points, or more generally a tolerance relation which naturally gives rise to an operator system.

46 pages, 5 figures. Version to appear in Commun. Math. Phys

References in corpus (9)

• Metrics on State Spaces
• The noncommutative Choquet boundary
• Quanta of Geometry: Noncommutative Aspects
• The Choquet boundary of an operator system
• Geometry and the Quantum: Basics
• Spectral geometry with a cut-off: topological and metric aspects
• A View on Optimal Transport from Noncommutative Geometry
• Understanding truncated non-commutative geometries through computer simulations
• Reconstructing manifolds from truncated spectral triples

Cited by in corpus (23)

• Tutorial: Classifying Photonic Topology Using the Spectral Localizer and Numerical $K$-Theory
• Gromov-Hausdorff convergence of state spaces for spectral truncations
• Phase Transition in Random Noncommutative Geometries
• On multimatrix models motivated by random Noncommutative Geometry I: the Functional Renormalization Group as a flow in the free algebra
• The Podles spheres converge to the sphere
• On multimatrix models motivated by random noncommutative geometry II: A Yang-Mills-Higgs matrix model
• Gromov--Hausdorff Convergence of Spectral Truncations for Tori
• Truncated Geometry on the Circle
• Characterizations of Ordered Self-adjoint Operator Spaces
• Convergence of Fourier truncations for compact quantum groups and finitely generated groups
• A categorical approach to injective envelopes
• Tolerance Relations and Quantization
• Asymptotic commutativity of quantized spaces: the case of ${\mathbb{CP}}^{p,q}$
• Reconstructing manifolds from truncated spectral triples
• An extension property for noncommutative convex sets and duality for operator systems
• Modular spectral triples and deformed Fredholm modules
• A comparison of two quantum distances
• Truncated quantum observables and their semiclassical limit
• The Gauge Group and Perturbation Semigroup of an Operator System
• A noncommutative integral on spectrally truncated spectral triples, and a link with quantum ergodicity
• Product systems and their representations: an approach using Fock spaces and Fell bundles
• On the embeddings of selfadjoint operator spaces
• The loop equations for noncommutative geometries on quivers