Abstract

Single ions in Paul traps are investigated for quantum information processing. Superpositions of the S_1/2 and the D_5/2 states are used to implement a qubit. Single ⁴⁰Ca⁺ 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_1/2 - P_1/2 transition is monitored for a state measurement by the “electron shelving” technique. The S_1/2 - D_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 [1]. In the spherical Paul trap (4.5 MHz trap frequency) a single Ca⁺ ion has been cooled to the ground state of vibration with up to 99.9% probability [2]. Cooling is achieved in all three dimensions simultaneously as well as ground state cooling of two ions. In the latter case, all modes were cooled resulting in a ground state population of more than 95% [3].

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