  This document presents the contents of three lectures delivered by the author
at the Erd\H{o}s Center School ``Optimal Transport on Quantum Structures'',
Septemer 19-23, 2022 in Budapest, Hungary. It presents a fairly self contained
account of an active topic of current research, and this account should be
accessible to most graduate students, as befits lectures for a school. The main
results are known, but there a number of new proofs and some new results.
