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Chinese Physics Letters

Control of a Cloud of Laser-Cooled 40 Ca + in a Linear Ion Trap

Zhou Fei, Yi Xie, You-Yang Xu, Jiaomei Li, Huang Xue-Ren, Feng Mang

Published Apr 1, 2010• 7 citations
Publisher

Abstract

A cloud of laser-cooled 40 Ca + is successfully trapped and manipulated under well control in our home-built linear ion trap, which is designed and constructed solely for studying quantum information processing. By exploring the variation of the ion cloud with respect to the trap parameters, we have optimized the trapping condition and obtained very good fluorescence spectra. We observe the dynamics of the ion cloud, and estimate the temperature of the ion cloud to be of the order of milli-Kelvin.

DOI
10.1088/0256-307x/27/4/043201

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A cloud of laser-cooled 40 Ca + is successfully trapped and manipulated under well control in our home-built linear ion trap, which is designed and constructed solely for studying quantum information processing. By exploring the variation of the ion cloud with respect to the trap parameters, we have optimized the trapping condition and obtained very good fluorescence spectra. We observe the dynamics of the ion cloud, and estimate the temperature of the ion cloud to be of the order of milli-Kelvin.
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A laser-cooling experiment with Ca+{\mathrm{Ca}}^{+}Ca+ ions trapped in a linear rf octupole ion trap is presented. The phase transition of the laser-cooled Ca+{\mathrm{Ca}}^{+}Ca+ ions from the cloud to the crystal state is observed by an abrupt dip of the laser-induced fluorescence spectrum and indicates that mK temperatures are obtained. We have also performed molecular dynamics simulations under various conditions to confirm this property by deducing axially symmetric structures of Coulomb crystals and by evaluating the translational temperatures of the laser-cooled ions. The simulation results show that for small numbers of ions novel ring-shaped crystals are produced. As the number of ions is increased, cylindrical layers in the ring crystal are sequentially formed. For more than 100 ions, also hexagonal and spiral structures emerge in parts of the large-size ion crystal, which has a length on the order of millimeters for the present geometrical arrangement and voltages. An advantage of the linear rf octupole trap is its large almost-field-free region in the middle of the trap, where the micromotion amplitude is small for trapped ions. These results demonstrate that such a multipole trap has attractive features for quantum computing and ultracold ion-atom collision studies.
Trapped laser-cooled atomic ions are a new tool for understanding cold molecular ions. The atomic ions not only sympathetically cool the molecular ions to millikelvin temperatures, but the bright atomic ion fluorescence can also serve as a detector of both molecular reactions and molecular spectra. We are working towards the detection of single molecular ion spectra by sympathetic heating spectroscopy. Sympathetic heating spectroscopy uses the coupled motion of two trapped ions to measure the spectra of one ion by observing changes in the fluorescence of the other ion. Sympathetic heating spectroscopy is a generalization of quantum logic spectroscopy, but does not require ions in the motional ground state or coherent control of the ion internal states. We have recently demonstrated this technique using two isotopes of Ca+ [Phys. Rev. A, 81, 043428 (2010)]. Limits of the method and potential applications for molecular spectroscopy are discussed.