Investigating a qubit candidate: Spectroscopy on theS1/2toD5/2transition of a trapped calcium ion in a linear Paul trap

A single ${}^{40}{\mathrm{Ca}}^{+}$ ion is confined in a linear Paul trap and Doppler-cooled on the $S_{1/2}$ to $P_{1/2}$ dipole transition. Then the narrow quadrupole $S_{1/2}$ to $D_{5/2}$ transition at 729 nm is probed. The observed spectrum is interpreted in terms of the Zeeman substructure superimposed with oscillation sidebands due to the harmonic motion in the trap. The height of the motional sidebands provides a sensitive method to determine the ion’s temperature and thus allows us to test sub-Doppler laser cooling schemes needed for quantum state preparation and quantum computation. We also observe the dynamics induced by Rabi oscillations on a carrier transition and interpret it in terms of the thermal state which is reached after Doppler cooling.