  Current quantum noise can be pictured as a sum over transitions through which
the electronic system exchanges energy with its environment. We formulate this
picture and use it to show which type of current correlators are measurable,
and in what measurement the zero point fluctuations will play a role (the
answer to the latter is as expected: only if the detector excites the system.)
Using the above picture, we calculate and give physical interpretation of the
finite-frequency finite-temperature current noise in a noninteracting
Landauer-type system, where the chemical potentials of terminals 1 and 2 are
$\mu+eV/2$ and $\mu-eV/2$ respectively, and derive a detailed-balance condition
for this nonequilibrium system. Finally, we derive a generalized form of the
Kubo formula for a wide class of interacting nonequilibrium systems, relating
the differential conductivity to the current noise.
