  Quantum states in the Earth's gravitational field can be observed, when
ultra-cold neutrons fall under gravity. In an experiment at the Institut
Laue-Langevin in Grenoble, neutrons are reflected and trapped in a
gravitational cavity above a horizontal mirror. The population of the ground
state and the lowest states follows, step by step, the quantum mechanical
prediction. An efficient neutron absorber removes the higher, unwanted states.
The quantum states probe Newtonian gravity on the micrometer scale and we place
limits for gravity-like forces in the range between 1 micron and 10 microns.
