  In this work the non-equilibrium density operator approach introduced by
Zubarev more than 50 years ago to describe quantum systems at local
thermodynamic equilibrium is revisited. This method - which was used to obtain
the first ''Kubo" formula of shear viscosity, is especially suitable to
describe quantum effects in fluids. This feature makes it a viable tool to
describe the physics of the Quark Gluon Plasma in relativistic nuclear
collisions.
