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Physical Review A

Spatial separation of ion clouds between sympathetically laser-cooled Cd + -ion isotopes in a Penning trap

Hidetsuka Imajo, Kazuhiro Hayasaka, R. Ohmukai, U. Tanaka, Masayoshi Watanabe, Shinji Urabe

Published Feb 1, 1997• 25 citations

Abstract

Natural ${\mathrm{Cd}}^{+}$ ions in a Penning trap are cooled sympathetically below 1 K by a Coulomb interaction with directly laser-cooled ${\mathrm{Be}}^{+}$ ions. Spatial separation of the ${\mathrm{Be}}^{+}$ and ${\mathrm{Cd}}^{+}$ ion clouds due to centrifugal force is observed directly. When the frequency of rotation of the ion clouds is accelerated, the separation between ${\mathrm{Cd}}^{+}$ -ion isotopes (A=110 and 116) can be clearly observed, though their mass difference is very small. The equations of motion for 100 ${\mathrm{Be}}^{+}$ ions and 50 ${\mathrm{Cd}}^{+}$ ions are solved to simulate the experiments, and the result shows the spatial separation between the two ion species. The separation between the ${\mathrm{Cd}}^{+}$ -ion isotopes is investigated by comparing the experimental results with numerical calculations.

DOI: 10.1103/physreva.55.1276

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Natural

{\mathrm{Cd}}^{+}

ions in a Penning trap are cooled sympathetically below 1 K by a Coulomb interaction with directly laser-cooled

{\mathrm{Be}}^{+}

ions. Spatial separation of the

{\mathrm{Be}}^{+}

and

{\mathrm{Cd}}^{+}

ion clouds due to centrifugal force is observed directly. When the frequency of rotation of the ion clouds is accelerated, the separation between

{\mathrm{Cd}}^{+}

-ion isotopes (A=110 and 116) can be clearly observed, though their mass difference is very small. The equations of motion for 100

{\mathrm{Be}}^{+}

ions and 50

{\mathrm{Cd}}^{+}

ions are solved to simulate the experiments, and the result shows the spatial separation between the two ion species. The separation between the

{\mathrm{Cd}}^{+}

-ion isotopes is investigated by comparing the experimental results with numerical calculations.

Sympathetic cooling of trapped ions has been demonstrated in an experiment where

^{198}\mathrm{Hg}^{+}

ions were confined in a Penning ion trap with laser-cooled

^{9}\mathrm{Be}^{+}

ions.

^{198}\mathrm{Hg}^{+}

temperatures below 1 K were achieved. Ion plasma sizes, shapes, and rotation frequencies were measured. Dramatic changes in the

^{9}\mathrm{Be}^{+}

plasma were observed when the

^{198}\mathrm{Hg}^{+}

ions were introduced into the trap. These observations are consistent with the prediction of centrifugal separation for ions of different charge-to-mass ratios.

Cadmium ions (

{\mathrm{Cd}}^{+}

) have been cooled sympathetically below 1 K by means of Coulomb interactions with laser-cooled

{\mathrm{Be}}^{+}

ions that are trapped simultaneously in a Penning trap. The ultraviolet resonance lines (5

{\mathit{s}}^{\mathrm{}2}

{\mathit{S}}_{1/2}

\ensuremath{\leftrightarrows}5

{\mathit{s}}^{\mathrm{}2}

{\mathit{S}}_{1/2}

, \ensuremath{\lambda}=214.5 nm) of the cooled

{\mathit{p}}^{\mathrm{}2}

ions were observed using a frequency-doubled cw dye laser. This radiation is based on efficient frequency doubling in a nonlinear crystal [\ensuremath{\beta}-

{\mathrm{BaB}}_{2}

{\mathrm{O}}_{4}

(BBO)] with an external enhancement cavity. The spectra of the six isotopes (A=110--114, and 116) were obtained using a natural isotope mixture, and the reliability of the measured isotope shifts is discussed. \textcopyright{} 1996 The American Physical Society.

We sympathetically cool a trapped

{}^{112}{\mathrm{Cd}}^{+}

ion by directly Doppler cooling a

{}^{114}{\mathrm{Cd}}^{+}

ion in the same trap. This is the first demonstration of optically addressing a single trapped ion being sympathetically cooled by a different species ion. Notably, the experiment uses a single laser source, and does not require strong focusing. This paves the way toward reducing decoherence in an ion trap quantum computer based on

{\mathrm{Cd}}^{+}

isotopes.

44Ca+, Zn+, or Ga+ ions are sympathetically cooled by Coulomb interaction with laser-cooled 40Ca+ ions, and motional resonances of these ions are observed by monitoring the fluorescence of the 40Ca+ ions. In the experiment of sympathetic cooling of 44Ca+, the ratio of the decrease in the fluorescence signal at the motional resonance of the sympathetically cooled species to that at the motional resonance of the laser-cooled species is almost proportional to the number ratio of the two species. In the case of Ga+ ions as the sympathetically cooled species, the change in the response of sympathetically cooled species becomes larger as the temperature of the ion cloud decreases. Crystallization of an ion cloud which contains both 40Ca+ and Zn+ (or Ga+) is observed.

We have studied the formation and properties of two-species ion Coulomb crystals in the Penning trap of the SpecTrap experiment. These crystals have been formed by injection of admixture ions from an external source into a previously confined and laser-cooled cloud of magnesium ions. This kind of study, performed over a range of the admixture ions’ charge-to-mass ratios, indicates the conditions for their sympathetic cooling and the formation of two-species ion crystals. This mechanism allows efficient cooling of the admixed species such as highly charged ions which do not feature suitable laser-cooling transitions, and thus make them accessible to high-resolution laser spectroscopy.

We have trapped and sympathetically cooled the molecular ions

{\mathrm{HCO}}^{+}

and

{\mathrm{N}}_{2}{\mathrm{H}}^{+}

in a Penning trap. Through their Coulomb interaction with laser-cooled

{\mathrm{Mg}}^{+}

ions the molecules were sympathetically cooled to cryogenic temperatures. We identify the molecules through a measurement of their characteristic mass-dependent breathing mode frequencies. From a measurement of the temperature of the

{\mathrm{Mg}}^{+}

ions we estimate that the final temperature of the sympathetically cooled molecules is 4 K.