  Light-pulse atom interferometers are powerful quantum sensors, however, their
accuracy for example in tests of the weak equivalence principle is limited by
various spurious influences like magnetic stray fields or blackbody radiation.
Pushing the accuracy therefore requires a detailed assessment of the size of
such deleterious effects. Here, we present a systematic operator expansion to
obtain phase shifts and contrast analytically in powers of the perturbation.
The result can either be employed for robust straightforward order-of-magnitude
estimates or for rigorous calculations. Together with general conditions for
the validity of the approach, we provide a particularly useful formula for the
phase including wave-packet effects.
