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Review of Scientific Instruments

Trapped Hg+ ion crystal generated by isotope-selective photoionization

Qu Liu, Hongxin Zou, Xiaokang He, Guozhu Chen, Yong Shen, Jianmin Yuan

Published Jan 1, 2019• 3 citations
Publisher

Abstract

We present studies on monoisotopic Hg+ ions generation and trapping at room temperature. An ultraviolet laser system, including a frequency-controlled 194 nm laser and a 254 nm laser, is constructed. Mercury atoms are isotope-selectively excited by the 254 nm laser. The spectrum of the 6s2–6s6p transition lines for natural mercury isotopes is measured, and the uncertainty is less than 100 MHz. Then, the selected mercury isotopes are ionized by the 194 nm laser and trapped in a linear Paul trap. The fluorescence of the ion crystal at 194 nm is detected, and the temperature of the ions is estimated to be less than 10 mK.

DOI
10.1063/1.5068692

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We present studies on monoisotopic Hg+ ions generation and trapping at room temperature. An ultraviolet laser system, including a frequency-controlled 194 nm laser and a 254 nm laser, is constructed. Mercury atoms are isotope-selectively excited by the 254 nm laser. The spectrum of the 6s2–6s6p transition lines for natural mercury isotopes is measured, and the uncertainty is less than 100 MHz. Then, the selected mercury isotopes are ionized by the 194 nm laser and trapped in a linear Paul trap. The fluorescence of the ion crystal at 194 nm is detected, and the temperature of the ions is estimated to be less than 10 mK.
Isotopically selective resonance ionization of mercury atoms has been accomplished by three-step excitation via the 6 3P1 and 8 1S0 excited states. Three collinear beams, respectively 254, 286, and 532 nm, from two dye lasers and a pulsed neodymium:yttrium aluminum garnet (ND:YAG) pump laser, were passed through a closed mercury vapor cell containing electrodes to which the ions were drawn by an electric field. The ion current and the 254-nm fluorescence radiation were measured as a function of laser wavelengths. Hyperfine structures for the 254- and 286-nm transitions were observed.
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.
Singly ionized mercury ions have been stored in a miniature rf (Paul) quadruple ion trap and observed by means of resonance fluorescence. The trap electrode spacings (~0.5 mm) and rf voltage (~500 V at 20 MHz) were chosen1 so that the Lamb-Dicke criterion could be satisfied for optical transitions on a single-laser-cooled ion. The presence of a single ion was determined by a statistical loading method2; that is, the loading conditions were scaled down to a point where only occasionally would any fluorescence be observed after a loading cycle. With high probability, this fluorescence was due to a single ion. The fluorescence observed was due to spontaneous scattering from a 194-nm cw source (~3-μW power focused to a beam waist of ~20 μm) directed at the ion and tuned near the strongly allowed first resonance line.
We present details on the ultraviolet lattice spectroscopy of the (6sss2{}^{2}2) 1{}^{1}1SSS${}_{0}\ensuremath{\leftrightarrow}$ (6$s666p$) 3{}^{3}3PPP0{}_{0}0​ transition in neutral mercury, specifically 199{}^{199}199Hg. Mercury atoms are loaded into a one-dimensional vertically aligned optical lattice from a magneto-optical trap with an rms temperature of $\ensuremath{\sim}$60 $\ensuremath{\mu}$K. We describe aspects of the magneto-optical trapping, the lattice cavity design, and the techniques employed to trap and detect mercury in an optical lattice. The clock-line frequency dependence on lattice depth is measured at a range of lattice wavelengths. We confirm the magic wavelength to be 362.51(0.16) nm. Further observations to those reported by Yi et al. [Phys. Rev. Lett. 106, 073005 (2011)] are presented regarding the laser excitation of a Wannier-Stark ladder of states.
Abstract Trapped ions have proven to be a powerful instrument in quantum logic, precision metrology and tests of fundamental theories. Many applications require isotopically pure ion crystals to be trapped; however, selective loading methods require either isotopically pure atom sources or dedicated ultraviolet laser systems. Here we investigate the loading dynamics of the isotopically pure magnesium ion crystals photoionized and Doppler-cooled with a two-color UV laser system at 285 nm and 280 nm. The system shares the same tapered amplifier and the first stage of a second-harmonic generation for both wavelengths, which significantly reduces the complexity and cost of the experiment. A selective loading rate of 0.05 ions s −1 was achieved, supporting theoretical evaluations.