  The reduced dynamics of an atomic qubit coupled both to its own quantized
center of mass motion through the spatial mode functions of the electromagnetic
field, as well as the vacuum modes, is calculated in the influence functional
formalism. The formalism chosen can describe the entangled non-Markovian
evolution of the system with a full account of the coherent back-action of the
environment on the qubit. We find a slight increase in the decoherence due to
the quantized center of mass motion and give a condition on the mass and qubit
resonant frequency for which the effect is important. In optically resonant
alkali-metal atom systems, we find the effect to be negligibly small. The
framework presented here can nevertheless be used for general considerations of
the coherent evolution of qubits in moving atoms in an electromagnetic field.
