Matter wave interference of single trapped atoms

Single neutral atoms trapped in an optical lattice form an ideal system for the investigation of coherent matter wave phenomena in a well controlled way. For this it is necessary to coherently manipulate and detect the system with a spatial resolution better than the periodicity of the optical lattice (lambda/2 = 433 nm). We have realized such a system by trapping single laser cooled Caesium atoms in a spin dependent optical lattice. High resolution fluorescence imaging yields precise detection of atomic positions even down to nearest neighbors in the lattice [1]. The spin dependent lattice enables us to coherently manipulate the spatial wave function of atoms depending on their internal state. Using such potentials, we have developed and applied a novel cooling mechanism based on microwave radiation. This mechanism relies on engineering the spatial wave function overlap of initial and final quantum states by spatially displacing the spin dependent potentials (see Fig. la). We have thereby reached axial ground state population of more than 95% (See Fig. lb,c). Starting from such a quasi-degenerate initial state, we have induced controllable and coherent motional dynamics of single atoms in our system. The spin dependent lattice can further be used to transport atoms along the lattice in a very controlled way. We have transported atoms for more than ten lattice sites, directly observed by fluorescence imaging. Intriguing perspectives arise, however, if this transport is used to coherently split a single atom that was initialized in a coherent superposition of two internal states (See Fig. 2). If these states are transported coherently into opposite directions [2] and subsequently re-united at a common lattice site, the sequence parallels a Mach-Zehnder interferometer for light. We have realized such a single trapped atom interferometer and realized the resulting interference fringes. Our results pave the way for a controlled creation of entangled atomic states in our- system by coherent interaction between two atoms [3,4] as well as for precise analysis of the resulting quantum correlations.