Symmetries of L\'evy processes on compact quantum groups, their Markov semigroups and potential theory

TL;DR

This paper explores the symmetries of quantum Markov semigroups on compact quantum groups, linking them to Lévy processes and potential theory, and characterizing their invariance properties and spectral triples.

Contribution

It establishes a correspondence between translation-invariant quantum Markov semigroups and Lévy processes, and characterizes their symmetries via invariance of generating functionals.

Findings

Quantum Markov semigroups correspond to Lévy processes on $*$-Hopf algebras.

GNS- and KMS-symmetries are characterized by invariance under antipode and unitary antipode.

Complete description of invariant generating functionals on free orthogonal quantum groups $O_n^+$.

Abstract

Strongly continuous semigroups of unital completely positive maps (i.e. quantum Markov semigroups or quantum dynamical semigroups) on compact quantum groups are studied. We show that quantum Markov semigroups on the universal or reduced C${}^*$-algebra of a compact quantum group which are translation invariant (w.r.t. to the coproduct) are in one-to-one correspondence with L\'evy processes on its $*$-Hopf algebra. We use the theory of L\'evy processes on involutive bialgebras to characterize symmetry properties of the associated quantum Markov semigroup. It turns out that the quantum Markov semigroup is GNS-symmetric (resp. KMS-symmetric) if and only if the generating functional of the L\'evy process is invariant under the antipode (resp. the unitary antipode). Furthermore, we study L\'evy processes whose marginal states are invariant under the adjoint action. In particular, we give a complete description of generating functionals on the free orthogonal quantum group $O_n^+$ that are invariant under the adjoint action. Finally, some aspects of the potential theory are investigated. We describe how the Dirichlet form and a derivation can be recovered from a quantum Markov semigroup and its L\'evy process and we show how, under the assumption of GNS-symmetry and using the associated Sch\"urmann triple, this gives rise to spectral triples. We discuss in details how the above results apply to compact groups, group C$^*$-algebras of countable discrete groups, free orthogonal quantum groups $O_n^+$ and the twisted $SU_q (2)$ quantum group.