Control of matter wave tunneling in an optical lattice

Bose Einstein condensation is a phase transition emerging in systems of integer-spin particles whose temperature is lowered under a critical value. One of the signatures of this phenomenon is the emergence of a phase coherence through the whole system, so that its behaviors can be de- scribed by single particle wavefunctions. After two-decades-long efforts in the development of laser cooling techniques, Bose-Einstein conden- sation was achieved in dilute gases of neutral atoms. Apart from its fundamental interest, this experimental result opened the way to the study of the quantum world with macroscopic samples. In parallel with the research on cooling, the developments on laser physics led to the creation of artificial atomic crystals by use of light-induced periodic potentials, so-called optical lattices. These potentials were applied to Bose-Einstein condensates shortly after their discovery. In the last decade, a large part of the BEC community showed a strong interest in ultra-cold atoms loaded into optical lattices. The periodic potentials proved to be an exceptional tool for manipulating BECs, because of their feasibility in the laboratory with the present technology, and be- cause only few parameters govern the behavior of the sample. In fact, this is described by the interplay between two fundamental physical processes: atom-atom interactions and quantum tunneling. The unifying theme of this thesis is the quantum tunneling in an ultra-cold gas loaded into an optical lattice. In the experiments that we performed we were able to observe effects due to quantum tunneling as well as to develop experimental techniques to control it.