It is the 50th anniversary of ultra-cold neutrons and in this talk I will acknowledge the ultra-cold neutron as a tool for experiments with unprecedented precision. A focuses will be given 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.