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arXiv:1106.1708 · New Journal of Physics

Millikelvin spatial thermometry of trapped ions

B. G. Norton, Erik W. Streed, M J Petrasiunas, Andreas Jechow, D. Kielpinski

Published Nov 17, 2011• 19 citations
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Abstract

We demonstrate millikelvin thermometry of laser-cooled trapped ions with high-resolution imaging. This equilibrium approach is independent of the cooling dynamics and has lower systematic error than Doppler thermometry, with ±5 mK accuracy and ±1 mK precision. We used it to observe the highly anisotropic dynamics of a single ion, finding temperatures of <60 mK and >15 K simultaneously along different directions. This thermometry technique can offer new insights into quantum systems sympathetically cooled by ions, including atoms, molecules, nanomechanical oscillators and electric circuits.

DOI
10.1088/1367-2630/13/11/113022

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We report on observations of the thermal motion of a single, Doppler-cooled ion along the axis of a linear radio-frequency quadrupole trap. We show that for a harmonic potential the thermal occupation of energy levels leads to a Gaussian distribution of the ion's axial position. The dependence of the spatial thermal spread on the trap potential is used for precise calibration of our imaging system's point spread function and sub-millikelvin thermometry. We employ this technique to investigate the laser detuning dependence of the Doppler temperature.
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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 , allowing for rapid determination of the ion temperature. In the measurement range of 10 −1 – mK we reach better than accuracy. Tuning the cooling laser to selected resonance features allows us to control the ion temperatures between mK and more than 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.
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