Long and short time quantum dynamics: III. Transients

The quantum transport equations for fast transients have the structure of a Generalized Master Equations for the single-particle distribution, with causal memory terms. Nonequilibrium Green's functions are reduced to GME if the Generalized Kadanoff-Baym Ansatz is applied. This Ansatz has been used with success both to non-linear transport and to optical transients in semi-conductors; further progress is linked with its extension to a family of the Causal Ansatzes, differing primarily in renormalization of the propagators. For the switch-on non-equilibrium states, generated by a perturbation from equilibrium, the renormalization to the dark dressed Green's function followed by calculation of the induced self-energies is a productive direction. It also circumvents the problem of correlated initial conditions, far from a general solution otherwise. Such initial conditions appear as incompatible with a Causal Ansatz in general. The presently available formalism permits to study a transient process in the whole time range using the complete NGF, but making a flexible Ansatz-based reduction appropriate to the stage of dynamic evolution.