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arXiv:physics/0608062

Rf-induced persistent long-range ordered structures in two-species ion Coulomb crystals in a linear Paul trap

A. Mortensen, Nielsen, E., Thierry Matthey, Michael Drewsen

Published Aug 7, 2006• 0 citations
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Abstract

We report on the observations of three-dimensional long-range ordered structures in the central $^{40}$Ca$^+$ ion component of $^{40}$Ca$^+$--$^{44}$Ca$^+$ two-species ion Coulomb crystals in a linear Paul trap. In contrast to long-range ordering previously observed in single species crystals, the structures observed are strikingly persistent and always of one specific type in one particular orientation. Molecular dynamics simulations strongly indicate that these characteristics are a hitherto unpredicted consequence of the co-axial cylindrical symmetry of the central ion component of the Coulomb crystal and the radio frequency quadrupole trapping field.

DOI
10.48550/arxiv.physics/0608062

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We report on the observations of three-dimensional long-range ordered structures in the central 40^{40}40Ca+^++ ion component of 40^{40}40Ca+^++--44^{44}44Ca+^++ two-species ion Coulomb crystals in a linear Paul trap. In contrast to long-range ordering previously observed in single species crystals, the structures observed are strikingly persistent and always of one specific type in one particular orientation. Molecular dynamics simulations strongly indicate that these characteristics are a hitherto unpredicted consequence of the co-axial cylindrical symmetry of the central ion component of the Coulomb crystal and the radio frequency quadrupole trapping field.
We report on the observations of strikingly persistent (lifetimes of ∼10 s) three-dimensional long-range ordered structures in the central 40 Ca + ion component of 40 Ca + – 44 Ca + two-species ion Coulomb crystals in a linear Paul trap. Molecular dynamics simulations strongly indicate that the observed structures are a hitherto unpredicted consequence of an effective anisotropy in the inter-particle interaction induced by the radio frequency quadrupole-trapping field. The results have implications for such diverse research fields as cold molecular ion studies and quantum information processing.
In this contribution, we present results on the structural properties of single‐component as well as two‐component ion Coulomb crystals confined in a linear Paul trap. Metastable long range ordered structures as the face‐centered cubic (fcc) and body‐centered cubic (bcc) lattices are observed in single‐component ion crystals of sizes down to ∼ 1000 ions. In the case of two‐component ion crystals we observe a stable long range ordered structure in one of the trapped components, which is compared with a Molecular Dynamics (MD) simulation of a two‐component crystal.
We report on structural properties of two-component Coulomb crystals in a linear Paul trap. The crystals consist of two laser cooled ion species, 24Mg+ and 40Ca+. The lighter 24Mg+ ions form an inner cylindrical crystal structure surprisingly similar to that of an infinitely long single component crystal, while the outermost shell of the surrounding 40Ca+ ions have a spheroidal shape, which is highly insensitive to the presence of the 24Mg+ ions. Observed changes in the radial separation of the two ion species with the radius of the inner cylindrical crystal is explained by a simple model.
Ion Coulomb crystals of laser-cooled Ca+{\text{Ca}}^{+}Ca+ ions have been observed in a cryogenic linear octupole rf ion trap for the first time. Due to the almost field-free central region, large ion crystals containing over 104{10}^{4}104 ions of millimeter size were successfully produced. The shapes of the crystals can be manipulated by changing the static voltages applied to the octupole rods and the endcap electrodes. Quasi-two-dimensional Coulomb crystals consisting of a small number of ions were also observed with single-ion sensitivity. When a large number of ions is crystallized in the octupole trap, a new kind crystal image with a dip along the axial direction was obtained. The results show that a linear octupole trap and multipole traps are available for producing large, cold, and novel ion Coulomb crystals. The planar ring-type crystal which has not been observed before may lead to future interesting applications.
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.
Linear strings of trapped atomic ions held in radio-frequency (rf) traps constitute one of the leading platforms for quantum simulation experiments, allowing the investigation of interacting quantum matter.However, linear ion strings have drawbacks, such as the difficulty of scaling beyond approximately 50 particles as well as the inability to naturally implement spin models with more than one spatial dimension.Here we present experiments with planar Coulomb crystals of up to 105 40 Ca + ions in a novel monolithic rf trap, laying the groundwork for quantum simulations of two-dimensional spin models with singleparticle control.We characterize the trapping potential by analysis of crystal images and compare the observed crystal configurations with numerical simulations.We further demonstrate stable confinement of large crystals, free of structural configuration changes, and find that rf heating of the crystal is not an obstacle for future quantum simulation experiments.Finally, we prepare the out-of-plane motional modes of planar crystals consisting of up to 105 ions close to their ground state by electromagnetically induced transparency cooling, an important prerequisite for implementing long-range entangling interactions.