The coherence of qubits based on single Ca  ions

Two-level ionic systems, where quantum information is encoded in long-lived states (quantum bits, 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 sing le Ca-40(+) ions in a linear Paul trap. We find coherence times of similar or equal to1 ms, discuss the main technical limitations and outline possible improvements.