  The robustness of an atomic fountain interferometer with respect to
variations in the initial velocity of the atoms and deviations from the optimal
pulse amplitude is examined. We numerically simulate the dynamics of an
interferometer in momentum space with a maximum separation of $20 \hbar k$ and
map out the expected signal contrast depending on the variance of the initial
velocity distribution and the value of the laser field amplitude. We show that
an excitation scheme based on rapid adiabatic passage significantly enhances
the expected signal contrast compared to the commonly used scheme consisting of
a series of Rabi pulses. We demonstrate further substantial increase of the
robustness by using optimal control theory to identify splitting and swapping
pulses that perform well on an ensemble average of pulse amplitudes and
velocities. Our results demonstrate the ability of optimal control to
significantly enhance future implementations of atomic fountain interferometry.
