Precise determination of h/m_Rb using Bloch oscillations and atomic interferometry: a mean to deduce the fine structure constant

We use Bloch oscillations to transfer coherently many photon momenta to atoms. Then we can measure accurately the ratio h/m_Rb and deduce the fine structure constant alpha. The velocity variation due to the Bloch oscillations is measured thanks to Raman transitions. In a first experiment, two Raman $\pi$ pulses are used to select and measure a very narrow velocity class. This method yields to a value of the fine structure constant alpha^{-1}= 137.035 998 84 (91) with a relative uncertainty of about 6.6 ppb. More recently we use an atomic interferometer consisting in two pairs of pi/2 pulses. We present here the first results obtained with this method.