This talk focuses on the control and understanding of a
gravitationally interacting elementary quantum system
using the techniques of resonance spectroscopy. It offers a
new way of looking at gravitation at short distances based
on quantum interference. The ultra-cold neutron reflects
from a mirror in well-defined quantum states in the gravity
potential of the earth allowing the application of gravity
resonance spectroscopy (GRS). GRS relies on frequency
measurements, which provide a spectacular sensitivity.