Fast thermometry for trapped ions using dark resonances

We experimentally demonstrate a method to determine the temperature of trapped ions which is suitable for monitoring fast thermalization processes. We show that observing and analyzing the lineshape of dark resonances in the fluorescence spectrum provides a temperature measurement which is accurate over a large dynamic range, applied to single ions and small ion crystals. Laser induced fluorescence is detected over a time of only 20 mu s, allowing for rapid determination of the ion temperature. In the measurement range of 10(-1)-10(2) mK we reach better than 15% accuracy. Tuning the cooling laser to selected resonance features allows us to control the ion temperatures between 0.7 mK and more than 10 mK. Experimental work is supported by a solution of the eight-level optical Bloch equations when including the ions' classical motion. This technique paves the way for many experiments, including heat transport in ion strings, heat engines, non-equilibrium thermodynamics or thermometry of large ion crystals.