Magnetically induced electron shelving in a trapped Ca^{+} ion

Atomic states are perturbed by externally applied magnetic fields (Zeeman effect). As well as the usual Zeeman splittings, the magnetic field leads to mixing of states with different values of the J quantum number. We report on the direct experimental measurement of this effect using the electron shelving technique (employed to great effect in single-ion spectroscopy and quantum-information processing). Specifically we observe shelving to the metastable (3p(6)3d) (2) D-5/2 state in a single Ca-40(+) ion, via spontaneous decay on the strongly forbidden 4p P-2(1/2) <-> 3d D-2(5/2) transition. The rate of this transition is shown to scale as the square of the magnetic-field strength. The scaling and magnitude of the effect is compared to the result derived from first-order perturbation theory. For applications in quantum-information processing the J-mixing effect causes a degradation of readout fidelity. We show that this degradation is at a tolerable level for Ca+ and is much less problematic for other trapped ionic species.