  A solid state electronic nanodevice is an intrinsically open quantum system,
exchanging both energy with the host material and carriers with connected
reservoirs. Its out-of-equilibrium behavior is determined by a non-trivial
interplay between electronic dissipation and decoherence induced by inelastic
processes within the device, and the coupling of the latter to metallic
electrodes. We propose a unified description, based on the density matrix
formalism, that accounts for both these aspects, enabling to predict various
steady-state as well as ultrafast nonequilibrium phenomena, nowadays
experimentally accessible. More specifically, we derive a generalized
density-matrix equation, particularly suitable for the design and optimization
of a wide class of electronic and optoelectronic quantum devices. The power and
flexibility of this approach is demonstrated with the application to a
photoexcited triple-barrier nanodevice.
