  Transient and steady state currents through dc-biased quantum impurity models
beyond the linear response regime are of considerable interest, both from an
experimental and a theoretical point of view. Here we present a new analytical
approach for the calculation of such currents based on the flow equation method
(method of infinitesimal unitary transformations). Specifically, we analyze the
Anderson impurity model in its mixed valence regime where the coupling to the
leads is switched on suddenly at time $t=0$. We observe the real time buildup
of the current until it reaches its steady state limit.
