The main principle of GBAR is the use of antihydrogen H+ ions (the antimatter equivalent of H- ions) to more easily manipulate the atoms before measurement. Once produced, the H+ ions are cooled in two stages with lasers and Paul traps to micro-kelvin temperatures, i.e. a 1 m/s velocity. The ions are then neutralized by photo-detachment and the neutral H atoms fall (see figure). From a 20 cm height the time of fall is 200 milliseconds (assuming g = g), which is easily measured.
This method is complementary to the one used by the AEGIS experiment. Both experiments aim at a 1% precision in the first phase.
In addition, the GBAR way opens the possibility of performing a spectroscopy of H quantum states. Indeed, an experiment performed ten years ago at ILL Grenoble with ultra-cold neutrons by GBAR collaborators showed that these neutrons, launched a few tens of micrometers above a plate, were reflected from this surface by the Casimir effect, and even get trapped in the gravity potential. In such a potential well the energy levels accessible to the trapped particle are quantized, i.e. the distribution of the altitudes reached after bouncing on the surface by the particles is quantized. The separation between these altitude levels is proportional to the acceleration due to gravity. It was calculated that antihydrogen atoms of low vertical velocity when arriving on such a plate, i.e. launched a few tens of microns above it, would be reflected with very high probability, allowing thus to use the same technique to measure g with much higher precision.