In the case of an external Hamiltonian at most quadratic in position and
momentum operators, we use the ABCD formulation of atom optics to establish an
exact analytical phase shift expression for atom interferometers with arbitrary
spatial or temporal beam splitter configurations. This result is expressed in
terms of coordinates and momenta of the wave packet centers at the interaction
vertices only.Comment: 11 pages, 3 figures, submitted to Phys. Lett.