The new work by the ILL team has added what is known as a piezoelectric resonator to the bottom plate; its purpose is to jiggle the bottom plate at a very particular frequency.
The researchers found that as they changed the bottom plate's vibration frequency, there were distinct dips in the number of neutrons detected outside the plates - particular, well-spaced "resonant" frequencies that the neutrons were inclined to absorb.
These frequencies, then, are the gravitational quantum states of neutrons, essentially having energy bounced into them by the bottom plate, and the researchers were able for the first time to force the neutrons from one quantum state to another.
This then could be used as an extremely sensitive device to measure gravity at atomic scale.
 T. Jenke et al. Nature Physics doi:10.1038/nphys1970 (online publication - full citation not available yet).