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 S1/2​--D5/2​ transition of single 40Ca+ ions
in a linear Paul trap. We find coherence times of ≃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