Raman cooling and heating of two trapped Ba+ ions
We study cooling of the collective vibrational motion of two 138Ba+ ions conﬁned in an electrodynamic trap and irradiated by laser light close to the resonances S1/2-P1/2 (493 nm) and P1/2-D3/2 (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 Zeemansplit S1/2 -D3/2 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.