Compact quantum metric spaces and ergodic actions of compact quantum groups

Abstract

We show that for any co-amenable compact quantum group A = C ( G ) there exists a unique compact Hausdorff topology on the set EA ( G ) of isomorphism classes of ergodic actions of G such that the following holds: for any continuous field of ergodic actions of G over a locally compact Hausdorff space T the map T → EA ( G ) sending each t in T to the isomorphism class of the fibre at t is continuous if and only if the function counting the multiplicity of γ in each fibre is continuous over T for every equivalence class γ of irreducible unitary representations of G . Generalizations for arbitrary compact quantum groups are also obtained. In the case G is a compact group, the restriction of this topology on the subset of isomorphism classes of ergodic actions of full multiplicity coincides with the topology coming from the work of Landstad and Wassermann. Podleś spheres are shown to be continuous in the natural parameter as ergodic actions of the quantum SU ( 2 ) group. We also introduce a notion of regularity for quantum metrics on G , and show how to construct a quantum metric from any ergodic action of G , starting from a regular quantum metric on G . Furthermore, we introduce a quantum Gromov–Hausdorff distance between ergodic actions of G when G is separable and show that it induces the above topology.