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.