  We provide a general macrostatistical formulation of nonequilibrium steady
states of reservoir driven quantum systems. This formulation is centred on the
large scale properties of the locally conserved hydrodynamical observables, and
our basic assumptions comprise (a) a chaoticity hypothesis for the nonconserved
currents carried by these observables, (b) an extension of Onsager's regression
hypothesis to the fluctuations about nonequilibrium states, and (c) a certain
mesoscopic local equilibrium hypothesis. On this basis we obtain a picture
wherein the fluctuations of the hydrodynamical observables about a
nonequilibrium steady state execute a Gaussian Markov process of a generalized
Onsager-Machlup type, which is completely determined by the position dependent
transport coefficients and the equilibrium entropy function of the system. This
picture reveals that the transport coefficients satisfy a generalized form of
the Onsager reciprocity relations in the nonequilibrium situation and that the
spatial correlations of the hydrodynamical observables are generically of long
range. This last result constitutes a model-independent generalization of that
obtained for special classical stochastic systems and marks a striking
difference between the steady nonequilibrium and equilibrium states, since it
is only at critical points that the latter carry long range correlations.
