  We demonstrate a light-pulse atom interferometer based on the diffraction of
free-falling atoms by a picosecond frequency-comb laser. More specifically, we
coherently split and recombine wave packets of cold $^{87}$Rb atoms by driving
stimulated Raman transitions between the $|5s~^2S_{1/2},F=1\rangle$ and
$|5s~^2S_{1/2},F=2\rangle$ hyperfine states, using two trains of picosecond
pulses in a counter-propagating geometry. We study the impact of the pulses
length as well as of the interrogation time onto the contrast of the atom
interferometer. Our experimental data are well reproduced by a numerical
simulation based on an effective coupling which depends on the overlap between
the pulses and the atomic cloud. These results pave the way for extending
light-pulse interferometry to transitions in other spectral regions and
therefore to other species, for new possibilities in metrology, sensing of
gravito-inertial effects and tests of fundamental physics.

Atomic Physics;Quantum Physics