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Journal of Physics B Atomic Molecular and Optical Physics

Isotope-selective manipulation of Ca+using laser heating and cooling in a linear Paul trap

Masanori Kitaoka, Shuichi Hasegawa

Published Aug 8, 2012• 9 citations
Publisher

Abstract

We report on the isotope-selective manipulation of Ca + using the laser-heating and laser-cooling techniques in a linear Paul trap. Only 40 Ca + was selectively laser-heated using the optical isotope shift and eliminated from the trap while keeping the target isotope in the trap. The elimination process of the dominant isotope 40 Ca + was investigated and 42, 43, 44, 48 Ca + in the natural isotope mixture of Ca + generated by laser ablation were successfully observed by means of applying the appropriate dc voltage on the end-cap electrodes without changing the laser frequencies, the laser powers and the duration of the laser irradiation. This simplified method enables us to efficiently laser-cool the rare isotopes.

DOI
10.1088/0953-4075/45/16/165008

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We report on the isotope-selective manipulation of Ca + using the laser-heating and laser-cooling techniques in a linear Paul trap. Only 40 Ca + was selectively laser-heated using the optical isotope shift and eliminated from the trap while keeping the target isotope in the trap. The elimination process of the dominant isotope 40 Ca + was investigated and 42, 43, 44, 48 Ca + in the natural isotope mixture of Ca + generated by laser ablation were successfully observed by means of applying the appropriate dc voltage on the end-cap electrodes without changing the laser frequencies, the laser powers and the duration of the laser irradiation. This simplified method enables us to efficiently laser-cool the rare isotopes.
We present studies of resonance-enhanced photoionization for isotope-selective loading of Ca+{\mathrm{Ca}}^{+}Ca+ into a Paul trap. The ${4s}^{2}{}^{1}{S}_{0}\ensuremath{\leftrightarrow}4s4p{}^{1}{P}_{1}$ transition of neutral calcium is driven by a 423 nm laser and the atoms are photoionized by a second laser at 389 nm. Isotope selectivity is achieved by using crossed atomic and laser beams to reduce the Doppler width significantly below the isotope shifts in the 423 nm transition. The loading rate of ions into the trap is studied under a range of experimental parameters for the abundant isotope 40Ca+.{}^{40}{\mathrm{Ca}}^{+}.40Ca+. Using the fluorescence of the atomic beam at 423 nm as a measure of the Ca number density, we estimate a lower limit for the absolute photoionization cross section of 170(60) Mb. We achieve loading and laser cooling of all the naturally occurring isotopes, without the need for enriched sources. Laser heating/cooling is observed to enhance the isotope selectivity. In the case of the rare species 43Ca+{}^{43}{\mathrm{Ca}}^{+}43Ca+ and 46Ca+,{}^{46}{\mathrm{Ca}}^{+},46Ca+, which have not previously been laser cooled, the loading is not fully isotope selective, but we show that pure crystals of 43Ca+{}^{43}{\mathrm{Ca}}^{+}43Ca+ may nevertheless be obtained. We find that for loading 40Ca+{}^{40}{\mathrm{Ca}}^{+}40Ca+ the 389 nm laser may be replaced by an incoherent source.
In this paper, we present a detailed method for rapidly and effectively loading a single 40 Ca + ion into a miniature linear Paul trap. Calcium atoms are generated by laser ablation, and the single ion is loaded and specific isotopes are selected by a two-step photo-ionization method. Compared to the traditional photo-ionization method in which atoms are typically emitted from a resistively heated oven, the advantages of laser ablation are that it can be precisely controlled, it can effectively avoid the problem of fluxed calcium deposition on the trap electrodes, and it restricts the generation of dark ions, which can influence atomic-precision spectroscopy measurements. With its short loading time, it is possible to load an ion in few seconds with the appropriate parameters. The technique of laser ablation has been applied to generate several species of atoms and ions at present, it will potentially benefit in the studies of optical frequency standards, quantum simulation and quantum information processing in an ultra-high vacuum condition and cryogenic system.
Abstract We present a detailed method for accumulating Ca+ ions controllably in a linear Paul trap. The ions are generated by pulsed laser ablation and dynamically loaded into the ion trap by switching the trapping potential on and off. The loaded ions are precooled by buffer gas and then laser-cooled to form Coulomb crystals for verifying quantity. The number of ions is controlled by manipulating the trapping potential of the ion trap, partial pressure of buffer gas and turn-on time of the entrance end cap voltage. With single-pulse laser ablation, the number of trapped ions ranges from tens to ten thousand. The kinetic energy of loaded ions can be selected via the optimal turn-on time of the entrance end cap. Using multiple-pulse laser ablation, the number is further increased and reaches about 4 × 10 4 . The dynamic loading method has wide application for accumulating low-yielding ions via laser ablation in the ion trap.
Rare calcium isotope 43Ca+ ion (0.135%) is the only naturally abundant isotope with hyperfine structure. This isotope is considered an attractive candidate both for an optical frequency standard and for quantum information processing. We report a method to load 43Ca+ ions selectively into a linear Paul trap using a two-step, resonant photo-ionization process. Isotope selectivity is achieved by utilizing the isotope shifts for the resonant 4s2 1S0-4s4p 1P1 transition of neutral calcium. The second non-resonant excitation is achieved by an ultraviolet light emitting diode (LED). The use of an LED is simpler and more economical than using a laser for the second transition. Additionally the laser cooling of 43Ca+ is demonstrated using two ultraviolet-diode lasers and two infrared-diode lasers. Due to the hyperfine structure, the laser requirements for Doppler cooling 43Ca+ are more demanding than for the other isotopes.
We have investigated the efficiency of ion trapping in a linear Paul trap. First, we measured the dependence of trapping efficiency on dc and rf voltages using a channel electron multiplier for the direct detection of the electric current of the trapped ions. The most efficient dc and rf voltages were 5.0 and 175 V, respectively. Second, we calculated the trapping efficiency using a simulation code and compared it with the experimental results. We determined that the efficiency of ion trapping decreased monotonically with increasing dc voltage and had an optimum rf voltage. From the numerical calculation, the dependences of trapping efficiency on dc and rf voltages and the initial kinetic energy of ions was derived and the optimum dc and rf voltages and the initial kinetic energy of ions were found to be 0, 210 V, and 1 eV, respectively.