Coherence of qubits based on single Ca+^+ ions

Two-level ionic systems, where quantum information is encoded in long lived states (qubits), are discussed extensively for quantum information processing. We present a collection of measurements which characterize the stability of a qubit based on the $S_{1/2}$--$D_{5/2}$ transition of single $^{40}$Ca$^+$ ions in a linear Paul trap. We find coherence times of $\simeq$1 ms, discuss the main technical limitations and outline possible improvements.Comment: Proceedings of "Trapped charged particles and fundamental interactions" submitted to Journal of Physics B (IoP

Precise lifetime determination of the metastable 3d

$\rm Ca^+$ ions confined as a cloud in a Paul trap were excited and cooled by radiation from two lasers, tuned to the $\rm 4s^2S_{1/2}-4p^2P_{1/2}$ and $\rm 4p^2P_{1/2}-3d^2D_{3/2}$ transitions. The strong fluorescence of these lines disappears when we excite the ions simultaneously by a third laser on the $\rm 3d^2D_{3/2}-4p^2P_{3/2}$ line because of population trapping in the metastable $\rm 3d^2 D_{5/2}$ state, into which the $\rm 4p^2P_{3/2}$ level decays. From the time dependence of the fluorescence after blocking the third laser we determined the lifetime of the $\rm 3d^2D_{5/2}$ level. Our result of $\tau = 1064 (17)\;$ ms improves earlier results and agrees with the value recently computed by Liaw using the Brueckner approximation