Addressing and cooling of single ions in Paul traps

The use of cold trapped ions for quantum information processing requires th e preparation of linear strings of ions at low temperatures and the coheren t manipulation by laser light of the quantum state of individual ions in th e string. In our experiment, Ca-40(+) ions are trapped in a linear Paul tra p, forming crystallized linear strings when laser cooled. These strings are observed by fluorescence detection on the S-1/2-P-1/2 dipole transition at 397 nm using a photomultiplier and a CCD camera. The narrow S-1/2-D-5/2 qu adrupole transition at 729 nm is used to investigate and manipulate the vib rational motion of the ion in the trap. The spectral resolution obtained up to now on this transition is 2 . 10(-12), proving long coherence time of t he two-level system. Addressing of individual ions in the string is achieve d, using a tightly focused laser beam at 729 nm, and detected by the observ ation of quantum jumps from the S-1/2 to the D-5/2 level. These experimenta l techniques make ions in a superposition of their S-1/2 and D-5/2 states s uitable as qubits for quantum information processing. The realization of a two-ion quantum gate furthermore requires ground-state cooling of the strin g. The status of current experiments is reviewed and techniques to achieve ground-state cooling of ion strings are discussed.