  We analyze the time evolution of an initial spatial coherence for a two level
atom whose internal degrees of freedom interact with a single mode of a cavity
field. When the qubit-field subsystem is taken as an environment, the
translational dynamics experiences a decoherence process which may be encoded
in a decoherence factor D. We find that the field statistics affects D through
the alternative paths the system-environment may follow along their
entanglement, while eventual field phase properties give rise to an imaginary
part of D which is related to the atomic translation. From the decoherence
perspective, we analyze the relation between the atomic momentum and the
imaginary part of the atomic spatial density matrix, and some considerations on
its asymptotic behavior are brought into question at the conclusion of the
paper.
