Realization of a gravity-resonance-spectroscopy technique

Spectroscopic techniques are mostly used to study the interaction between matter and electromagnetic fields. Here, an experiment that probes the transitions between quantum states of neutrons in the Earth’s gravitational field demonstrates an exotic variant of spectroscopy, and one that might lead to sensitive fundamental tests of gravity laws. Spectroscopy is a method typically used to assess an unknown quantity of energy by means of a frequency measurement. In many problems, resonance techniques 1 , 2 enable high-precision measurements, but the observables have generally been restricted to electromagnetic interactions. Here we report the application of resonance spectroscopy to gravity. In contrast to previous resonance methods, the quantum mechanical transition is driven by an oscillating field that does not directly couple an electromagnetic charge or moment to an electromagnetic field. Instead, we observe transitions between gravitational quantum states when the wave packet of an ultra-cold neutron couples to the modulation of a hard surface as the driving force. The experiments have the potential to test the equivalence principle 3 and Newton’s gravity law at the micrometre scale 4 , 5 .