Ion strings for quantum gates

Crystal structures of calcium ions have been prepared in a linear Paul trap. The trapped ions are laser-cooled by simultaneous resonant excitation near 397 nm and 866 nm. Images of the fluorescing ions are obtained with a CCD camera and show the individual ions spatially resolved. Complex crystal structures of more than 60 ions have been observed whereas smaller crystals with up to 10 ions arrange in a linear string. Measured distances between the ions in strings of different lengths are in good agreement with expected values obtained from a harmonic trap potential. The application of a calcium ion string for quantum computation is discussed.