Matter wave interferometry
Just like materials with an inhomogeneous index of refraction force straight electromagnetic waves on e.g. parabolas, gravity acts on matter waves. Hence, the center of mass of matter-wave packets follows the trajectory of classical bodies (fig. 1, left). Under the influence of gravity, packets of matter-waves also gain phase shifts depending on g.
We use a Mach-Zehnder like interferometer to detect the phase shifts and therefore g by atom-interferometric methods. A schematic of such a measurement is displayed in fig. 1, right. The leading order phase shift caused by accelerations during an interferometer cycle is
with the effective wave vector keff, acceleration a and drift time T between two light pulses, derived from the Schrödinger equation.
To test EEP, the experiment is operated simultaneosly with both potassium and rubidium. By detecting g for Rb and K, one can test the EEP at an expected sensitivity of ~10-9 g after a few thousand seconds of integration in a differential measurement.
Fig. 1: left: Free falling wave packets without and with gravity. right: Schematic of the propagation of matter-wave packets of rubidium (black lines) and potassium (red lines) during an interferometric cycle.