We present the realisation of a matter wave interferometer which measures the accelerationof gravity. The wave-packets of 87Rb atoms are manipulated with two counter-propagatinglaser inducing stimulated Raman transitions. These transitions are used to create an interferometerand the interferometric phase difference depends on the acceleration of the atoms in the laboratory'sreferential. When the Raman lasers are applied vertically, the interferometer is sensitive to the Earthacceleration g. In the context of the watt balance experiment, realised at LNE, the knowledge of theEarth acceleration is required with an uncertainty of g/g = 10−9. The goal of our gravimeter is toobtain a sensitivity of 10−9g in less than one minute of integration, and an accuracy better than 10−9g.A particular care was brought to conceive a compact experimental set-up so that it could be moved. Itwill then be placed close to the balance or in different measure sites to compare it with other absolutegravimeters. We have reached a sensitivity of 7 × 10−8g.Hz−1/2, enabling to get a statistical uncertaintyof 3 × 10−9g after 1000 seconds of integration. A first analysis of the noise sources enabled usto identify the improvements required on the set-up and in particular those concerning the vibrationswhich currently limit the short term sensitivity.
