Sideband cooling of ions in radio-frequency traps

An experimental study of sideband laser cooling of trapped ions in miniature radio-frequency traps is presented. In+ ions are laser cooled by exciting the narrow intercombination line 5s(2) S-1(0) --> 5s5p (3)p(1), which shows resolved sidebands due to the vibrational motion of the ions in the trap. The influence of the micromotion at the frequency of the oscillating trapping field is investigated. Heating processes through coupling with the trapping held occur for certain regions of negative laser detuning. A method of bichromatic sideband cooling is introduced that combines efficient cooling from high temperatures with the possibility of reaching low mean vibrational quantum numbers [n]<1 by using narrow optical transitions. This technique allows recording of high-resolution excitation spectra of the cooling transition in the Lamb-Dicke regime and thus determination of [n]. Single ions as well as two-ion Coulomb crystals have been cooled to temperatures below 100 mu K, and population of the ground state of motion to more than 50% over long time scales is demonstrated. These values are close to the lower limit for the detection of vibrational excitation with the present method. [S1050-2947(99)08407-3].