Quantum Chromodynamics (QCD) is the well-established theory of the strong nuclear force, with
quarks and gluons as the elementary degrees of freedom. However, the emergence of its most
prominent phenomena, i.e., the quark confinement into hadrons and the generation of hadronic mass, remains under intense investigation. Over the last two decades, another remarkable phenomenon has been discovered, namely the quark-gluon plasma (QGP), with transport properties that are often
referred to as a ``nearly perfect liquid", as they are close to conjectured limits set by quantum
After a brief introduction to QCD and its phase diagram, the focus will be on the use of the heavy
charm and bottom quarks as "Brownian markers" of the QGP's transport properties. We will introduce a quantum many-body approach for the interactions of heavy quarks in the QGP that provides a basis for transport simulations that aim at describing heavy-flavor observables in heavy-ion collisions.
A critical role in constraining these interactions, which operate in the deeply non-perturbative
regime of QCD, is played by first-principles computations of the QCD partition function (or
"lattice QCD"). We will highlight progress that has been made over the last decade in scrutinizing
the various components of phenomenological applications to heavy-flavor observables in heavy-ion
collisions, and how this has advanced our understanding of the heavy-quark diffusion coefficient
and hadronization mechanisms. It appears that remnants of the confining force in the QGP are at
the origin of its extraordinary transport properties.