  The decoherence of nonclassical motional states of a trapped $^9 {\rm Be^+}$
ion in a recent experiment is investigated theoretically. Sources of
decoherence considered here destroy the characteristic coherent quantum
dynamics of the system but do not cause energy dissipation. Here they are first
introduced phenomenologically and then described using a microscopic
Hamiltonian formulation. Theoretical predictions are compared to experimental
results.
