In a conventional optical interferometer, a light beam is split into two or more paths that are recombined later on a detector. If the beams from the different paths are in phase, their fields add up and the detector measures bright light. If they are out of phase, on the other hand, the fields cancel each other and the signal at the detector indicates darkness. Similarly, in an atom interferometer, as in Figure 3–4–1, one can observe the coherent addition or cancellation of matter waves. By carefully monitoring the variations of “bright” and “dark” periods, one can determine with exquisite precision the physical processes that influenced the dynamics of the atoms in the two paths of the interferometer. For instance, atom interferometers have been used to carry out tests of Einstein’s equivalence principle at the atomic level and are being developed into rotation and inertial sensors with unprecedented sensitivity and accuracy.