  We explore the correlated quantum dynamics of a single atom, regarded as an
open system, with a spatio-temporally localized coupling to a finite bosonic
environment. The single atom, initially prepared in a coherent state of low
energy, oscillates in a one-dimensional harmonic trap and thereby periodically
penetrates an interacting ensemble of $N_A$ bosons, held in a displaced trap.
We show that the inter-species energy transfer accelerates with increasing
$N_A$ and becomes less complete at the same time. System-environment
correlations prove to be significant except for times when the excess energy
distribution among the subsystems is highly imbalanced. These correlations
result in incoherent energy transfer processes, which accelerate the early
energy donation of the single atom and stochastically favour certain energy
transfer channels depending on the instantaneous direction of transfer.
Concerning the subsystem states, the energy transfer is mediated by
non-coherent states of the single atom and manifests itself in singlet and
doublet excitations in the finite bosonic environment. These comprehensive
insights into the non-equilibrium quantum dynamics of an open system are gained
by ab-initio simulations of the total system with the recently developed
Multi-Layer Multi-Configuration Time-Dependent Hartree Method for Bosons.
