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Physical Review A

Laser cooling and imaging of individual radioactive Sr+90 ions

Kyunghun Jung, Yoshihiro Iwata, Masabumi Miyabe, Kazuhiro Yamamoto, Tomohisa Yonezu, Ikuo Wakaida, Shuichi Hasegawa

Published Oct 30, 2017• 12 citations
Publisher

Abstract

We have developed an apparatus integrating resonance-ionization, ion-trap, and laser-cooling techniques for an ultratrace radioactive isotope $^{90}\mathrm{Sr}$ analysis. Trapped $^{90}\mathrm{Sr}^{+}$ isotope ions were laser cooled, and their $4d\phantom{\rule{0.16em}{0ex}}^{2}D_{3/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$ transition isotope shift was experimentally measured to be $\ensuremath{-}281(17)$ MHz by comparing individual spectra of $^{88}\mathrm{Sr}^{+}$ and $^{90}\mathrm{Sr}^{+}$ ions. Crystallization of $^{90}\mathrm{Sr}^{+}$ was carried out using the resonance frequency value confirmed in our experiment, and then $^{90}\mathrm{Sr}^{+}$ individual ions were successfully observed.

DOI
10.1103/physreva.96.043424

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We report laser cooling and trapping of 224Ra+{}^{224}{\mathrm{Ra}}^{+}224Ra+ ions. This was realized via two-step photoionization loading of radium into an ion trap. A robust source for 224Ra^{224}\mathrm{Ra}224Ra atoms, which have a 3.6-day half-life, was realized with an effusive oven containing 228Th^{228}\mathrm{Th}228Th, which has a 1.9-yr half-life, which continuously generates 224Ra^{224}\mathrm{Ra}224Ra via its $\ensuremath{\alpha}$-decay. We characterized the efficacy of this source and found that after depleting built-up radium the thorium decay provides a continuous source of radium atoms suitable for ion trapping. The vacuum system has been sealed for more than 6 months and continues to trap ions on demand. We also report a measurement of the $^{224}\mathrm{Ra} 7{s}^{2}{\phantom{\rule{4pt}{0ex}}}^{1}{\mathrm{S}}_{0}\ensuremath{\rightarrow}7s7p{\phantom{\rule{4pt}{0ex}}}^{1}{\mathrm{P}}_{1}$ transition frequency: $621\phantom{\rule{0.16em}{0ex}}043\phantom{\rule{0.16em}{0ex}}830\ifmmode\pm\else\textpm\fi{}60$ MHz, which is helpful for efficient photoionization. With this measurement and previous isotope shift measurements we find that the frequency of the same transition in 226Ra^{226}\mathrm{Ra}226Ra is $621\phantom{\rule{0.16em}{0ex}}037\phantom{\rule{0.16em}{0ex}}830\ifmmode\pm\else\textpm\fi{}60$ MHz, which disagrees with the most precise measurement, $621\phantom{\rule{0.16em}{0ex}}038\phantom{\rule{0.16em}{0ex}}489\ifmmode\pm\else\textpm\fi{}15$ MHz, which is used for the recommended value in the National Institute of Standards and Technology Atomic Spectra Database.
We measured by laser spectroscopy the isotope shifts between naturally occurring even isotopes of strontium ions for both the $5s\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2}$ (violet) and the $4d\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{D}_{3/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2}$ (infrared) dipole-allowed optical transitions. Fluorescence spectra were taken by simultaneous measurements on a two-component Coulomb crystal in a linear Paul trap containing 103{10}^{3}103--104{10}^{4}104 laser-cooled Sr+{\mathrm{Sr}}^{+}Sr+ ions. The isotope shifts are extracted from the experimental spectra by fitting the data with the analytical solution of the optical Bloch equations describing a three-level atom interacting with two laser beams. This technique allowed us to increase the precision with respect to previously reported data obtained by optogalvanic spectroscopy or fast atomic-beam techniques. The results for the $5s\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2}$ transition are ${\ensuremath{\nu}}_{88}\ensuremath{-}{\ensuremath{\nu}}_{84}=+378(4)$ MHz and ${\ensuremath{\nu}}_{88}\ensuremath{-}{\ensuremath{\nu}}_{86}=+170(3)$ MHz, in agreement with previously reported measurements. In the case of the previously unexplored $4d\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{D}_{3/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2}$ transition we find ${\ensuremath{\nu}}_{88}\ensuremath{-}{\ensuremath{\nu}}_{84}=\ensuremath{-}828(4)$ MHz and ${\ensuremath{\nu}}_{88}\ensuremath{-}{\ensuremath{\nu}}_{86}=\ensuremath{-}402(2)$ MHz. These results provide more data for stringent tests of theoretical calculations of the isotope shifts of alkali-metal-like atoms. Moreover, they simplify the identification and the addressing of Sr+{\mathrm{Sr}}^{+}Sr+ isotopes for ion frequency standards or quantum-information-processing applications in the case of multi-isotope ion strings.
This paper reports on trapping and laser-cooling of singly-ionized strontium ions in a linear Paul trap. We demonstrate loading of large ion clouds containing as much as 106 ions and laser cooling down to the Coulomb crystal transition. We observe the spatial segregation of the different Sr+ isotopes due to the mass-dependent Paul trap stiffness. Sympathetic cooling of the different isotopes is demonstrated, either by laser-cooling of 88Sr+ (83% abundance) or of 86Sr+ (10% abundance). These demonstrations open the way to the use of a large ion Coulomb crystal for quantum optics and quantum information experiments, where the strong confinement, the long lifetime, and the absence of perturbation by cooling lasers are crucial.
This paper reports on trapping and laser cooling of singly ionized strontium ions in a linear Paul trap. We describe the loading technique based on two-photon absorption of femtosecond pulses at a wavelength of 431 nm and we compare this technique to electron-bombardment ionization. We perform Doppler cooling of the ions in the few-ions regime as well as in the case of large clouds. The analysis of the fluorescence spectra and direct imaging of the cloud allows us to identify the Coulomb-crystal regime. These experiments open the way to the use of a large cold trapped-ion cloud as a medium for quantum optics and quantum information experiments.
The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation, and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a tabletop experiment with a low-activity (∼10 μCi) source where we laser-cool trapped radium ions. With a laser-cooled radium ion we measured the 7p^{2}P_{1/2}^{o} state's branching fractions to the ground state, 7s^{2}S_{1/2}, and a metastable excited state, 6d^{2}D_{3/2}, to be p=0.9104(7) and 0.0896(7), respectively. With a nearby tellurium reference line we measured the 7s^{2}S_{1/2}→7p^{2}P_{1/2}^{o} transition frequency, 640.096 63(6) THz.