  We use Bloch oscillations in a horizontal moving standing wave to transfer a
large number of photon recoils to atoms with a high efficiency (99.5% per
cycle). By measuring the photon recoil of $^{87}Rb$, using velocity selective
Raman transitions to select a subrecoil velocity class and to measure the final
accelerated velocity class, we have determined $h/m_{Rb}$ with a relative
precision of 0.4 ppm. To exploit the high momentum transfer efficiency of our
method, we are developing a vertical standing wave set-up. This will allow us
to measure $h/m_{Rb}$ better than $10^{-8}$ and hence the fine structure
constant $\alpha$ with an uncertainty close to the most accurate value coming
from the ($g-2$) determination.
