  Raman laser pulses are used to induce coherent tunnelling between
neighbouring sites of a vertical 1D optical lattice. Such tunneling occurs when
the detuning of a probe laser from the atomic transition frequency matches
multiples of the Bloch frequency, allowing for a spectroscopic control of the
coupling between Wannier Stark (WS) states. In particular, we prepare coherent
superpositions of WS states of adjacent sites, and investigate the coherence
time of these superpositions by realizing a spatial interferometer. This scheme
provides a powerful tool for coherent manipulation of external degrees of
freedom of cold atoms, which is a key issue for quantum information processing.
