  Atomic nuclei are complex, quantum many-body systems whose structure
manifests itself through intrinsic quantum states associated with different
excitation modes or degrees of freedom. Collective modes (vibration and/or
rotation) dominate at low energy (near the ground-state). The associated states
are usually employed, within a truncated model space, as a basis in (coherent)
coupled channels approaches to low-energy reaction dynamics. However, excluded
states can be essential, and their effects on the open (nuclear) system
dynamics are usually treated through complex potentials. Is this a complete
description of open system dynamics? Does it include effects of quantum
decoherence? Can decoherence be manifested in reaction observables? In this
contribution, I discuss these issues and the main ideas of a coupled-channels
density-matrix approach that makes it possible to quantify the role and
importance of quantum decoherence in low-energy nuclear reaction dynamics.
Topical applications, which refer to understanding the astrophysically
important collision $^{12}$C + $^{12}$C and achieving a unified quantum
dynamical description of relevant reaction processes of weakly-bound nuclei,
are highlighted.
