  We present a theory of finite-frequency noise in non-equilibrium conductors.
It is shown that Non-Markovian correlations are essential to describe the
physics of quantum noise. In particular, we show the importance of a correct
treatment of the initial system-bath correlations, and how these can be
calculated using the formalism of quantum master equations. Our method is
particularly important in interacting systems, and when the measured
frequencies are larger that the temperature and applied voltage. In this
regime, quantum-noise steps are expected in the power spectrum due to vacuum
fluctuations. This is illustrated in the current noise spectrum of single
resonant level model and of a double quantum dot --charge qubit-- attached to
electronic reservoirs. Furthermore, the method allows for the calculation of
the single-time counting statistics in quantum dots, measured in recent
experiments.

Mesoscale and Nanoscale Physics