Nanoscale ultrafast energy and charge transfer: From approximate theories to numerically exact simulations

Detailed understanding of mechanisms of ultrafast charge and energy transfer processes is important for practical realization of novel nanodevices with intriguing physical and chemical properties. Various perturbative schemes have been developed to date to perform theoretical analysis of such processes. However, the nature of the processes often renders the applicability of these schemes questionable in realistic situtations and calls for a development of non-perturbative methodologies. Once developed, such methodologies can provide theoretical benchmarks to validate approximate schemes.;In this thesis we discuss two examples of nanoscale ultrafast energy/charge transfer: condensed phase electron transport in the presence of mode-mixing and phononic heat transport through nanojunction. For both cases we develop a numerically exact methodology capable of describing dynamics of energy/charge transfer in a broad variety of physical regimes. Along with this, approximate methods based on the generalized quantum master equation formalism are developed. Finally, exact and approximate methodologies are used to study quantum dynamics in the two mentioned examples. The ranges of applicability of approximate theories are clarified.