Raman cooling and heating of two trapped Ba+ ions

We study cooling of the collective vibrational motion of two 1 3 8 Ba + ions confined in an electrodynamic trap and irradiated by laser light close to the resonances S 1 / 2 -P 1 / 2 (493 nm) and P 1 / 2 -D 3 / 2 (650 nm). The motional state of the ions is monitored by a spatially resolving photomultiplier. Depending on detuning and intensity of the cooling lasers, macroscopically different motional states corresponding to different ion temperatures are observed. We also derive the ions' temperature from detailed analytical calculations of laser cooling taking into account the Zeeman structure of the energy levels involved. The observed motional states perfectly match the calculated temperatures. Significant heating is observed in the vicinity of the dark resonances of the Zeeman-split S 1 / 2 -D 3 / 2 Raman transitions. Here two-photon processes dominate the interaction between lasers and ions. Parameter regimes of laser light are identified that imply most efficient laser cooling.