  This work focuses on the control and understanding of a gravitationally
interacting elementary quantum system. It offers a new way of looking at
gravitation based on quantum interference: an ultracold neutron, a quantum
particle, as an object and as a tool. The ultracold neutron as a tool reflects
from a mirror in well-defined quantum states in the gravity potential of the
earth allowing to apply the concept of gravity resonance spectroscopy (GRS).
GRS relies on frequency measurements, which provide a spectacular sensitivity.
