  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.
