Ground state cooling, quantum state engineering and study of decoherence of ions in Paul traps

Summary form only given. Single ions in Paul traps are investigated for quantum information processing. Superpositions of the S/sub 1/2/ and the D/sub 5/2/ states are used to implement a qubit. Single /sup 40/Ca/sup +/ ions are either held in a spherical Paul trap or alternatively, in a linear Paul trap. The ions are optically cooled and fluorescence light at 397 nm on the S/sub 1/2/-P/sub 1/2/ transition is monitored for a state measurement by the electron shelving technique. The S/sub 1/2/-D/sub 5/2/ quadrupole transition at 729 nm is excited for sideband cooling and for quantum state preparation. Individual ions in a linear string are addressed by a tightly focused 729 nm laser beam. In the spherical Paul trap (4.5 MHz trap frequency) a single Ca/sup +/ ion has been cooled to the ground state of vibration with a probability of up to 99.9%. Starting from this Fock state |n=0> coherent quantum state manipulation on the S/sub 1/2/-D/sub 5/2/ transition is achieved.