We present a gravity–gradiometer based on atom interferometry, developed for a precise determination of the Newtonian  gravitational constant G.
The interferometer is realized applying a sequence of π/2 − π − π/2 vertical Raman pulses on free falling cold 87Rb atoms in an atomic fountain. The resulting phase contains information related to the local vertical acceleration. Two cold atomic clouds are launched upwards with a juggling scheme in order to have two displaced sensors in rapid sequence. The Raman pulses simultaneously act on both samples. Heavy source masses are placed around the gradiometer region and accurately moved in two different positions along the vertical direction. The double differential measurement drastically reduces common mode spurious signals. The apparatus has been already tested using preliminary lead masses providing first results.