  The sensitivity of an interferometer depends on its readout scheme. However,
little attention has been paid to the readout schemes of atom interferometers
from the viewpoint of their sensitivity. The difference in sensitivity between
readout schemes or their optimization has not been considered in the
literature. Herein, we analytically calculate the sensitivities of an atom
interferometer with typical readout schemes by applying the two-photon
formalism, which was developed for optical interferometers to deal with quantum
noise. Our calculations reveal that by using sinusoidal phase modulation, the
sensitivity can surpass that obtained by the conventional phase sweeping
scheme. The superiority of this phase modulation scheme for both cold and
thermal atomic beams is demonstrated. In addition, we show that the phase
modulation scheme is advantageous for atom-flux fluctuation and resists
atom-flux drift. This study performs a general analysis of the sensitivity of
atom interferometers and identifies an advantageous readout scheme.
