From pure Schrödingerian to statistical dynamics

Many processes in nature seem to be entirely controlled by transition rates and the corresponding statistical dynamics. Some of them are in essence quantum, like the decay of excited states, the tunneling through barriers or the decay of unstable nuclei. Thus, starting from first principles, those systems should be analyzed on the basis of the Schrödinger equation. In the present paper, we consider a two-level system coupled to an environment which is basically described by a two-band energy scheme. For appropriately tuned environment parameters, the excitation probability of the two-level system exhibits statistical dynamics, while the full system follows the coherent, unitary pure-state, evolution generated by the Schrödinger equation.