Single-photon transitions for atom-interferometric detectors*

Atom interferometers have evolved into a competitive tool for inertial sensing and tests of fundamental physics. Most proposals for the detection of dark matter or gravitational waves with atom interferometers rely on differential measurements of two spatially separated interferometers, operated with the same diffraction beams. To suppress differential laser phase noise on the timescale of light propagation that inevitably arises in Raman or Bragg diffraction, single-photon transitions are the method of choice for such sensors. In this contribution, we present a theoretical discussion of such single-photon transitions and distinguish between different processes:  direct transitions and transitions induced by static magnetic fields. We explicitly derive the latter by consistently eliminating an auxiliary state and show the effect of hypothetical dark-matter fields on the process itself as well as gravity. Moreover, we study the influence of the atomic motion on the resonance condition during diffraction and find an effective time evolution of the system.