  We report on the measurement of the time required for a wave packet to tunnel
through the potential barriers of an optical lattice. The experiment is carried
out by loading adiabatically a Bose-Einstein condensate into a 1D optical
lattice. A sudden displacement of the lattice by a few tens of nm excites the
micromotion of the dipole mode. We then directly observe in momentum space the
splitting of the wave packet at the turning points and measure the delay
between the reflected and the tunneled packets for various initial
displacements. Using this atomic beam splitter twice, we realize a chain of
coherent micron-size Mach-Zehnder interferometers at the exit of which we get
essentially a wave packet with a negative momentum, a result opposite to the
prediction of classical physics.
