  Complementarity lies at the heart of conceptual foundation of orthodox
quantum mechanics. The wave-particle duality makes it impossible to tell which
slit each particle passes through and still observe an interference pattern in
a Young's double-slit experiment. In this paper, this duality is appraised
under the standard formulation of quantum mechanics for atom interferometers.
It is found that the internal freedoms like electronic states of an atom can be
used to detect the which-path information of each atom while uphold the
interference pattern of atoms when the center-of-mass motion of atoms is
detected.
