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Laser Physics Letters

Photoionization dynamics of Mg atoms during Paul trap loading using a two-color UV laser system

Mikhail A. Aksenov, Ilia V. Zalivako, Nikita V. Semenin, Vasilii N. Smirnov, I. N. Zhuravlev, Pavel Vishnyakov, Pavel Sidorov, Alexander S. Borisenko, Ilia Semerikov, Ksenia Yu. Khabarova, N. Kolachevsky

Published Nov 18, 2020• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1612-202x/abc611

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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.
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.
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.
Experiments in atomic, molecular, and optical (AMO) physics rely on lasers at many different wavelengths and with varying requirements on spectral linewidth, power, and intensity stability. Vertical external-cavity surface-emitting lasers (VECSELs), when combined with nonlinear frequency conversion, can potentially replace many of the laser systems currently in use. Here we present and characterize VECSEL systems that can perform all laser-based tasks for quantum information processing experiments with trapped magnesium ions. For photoionization of neutral magnesium, 570.6 \,nm light is generated with an intracavity frequency-doubled VECSEL containing a lithium triborate (LBO) crystal for second harmonic generation. External frequency doubling produces 285.3 \,nm light for resonant interaction with the 1S0↔^{1}S_{0}\leftrightarrow1S0​↔ 1P1^{1}P_{1}1P1​ transition of neutral Mg. Using an externally frequency-quadrupled VECSEL, we implement Doppler cooling of 25^{25}25Mg+^{+}+ on the 279.6 \,nm 2S1/2↔^{2}S_{1/2}\leftrightarrow2S1/2​↔ 2P3/2^{2}P_{3/2}2P3/2​ cycling transition, repumping on the 280.4 \,nm 2S1/2↔^{2}S_{1/2}\leftrightarrow2S1/2​↔ 2P1/2^{2}P_{1/2}2P1/2​ transition, coherent state manipulation, and resolved sideband cooling close to the motional ground state. Our systems serve as prototypes for applications in AMO requiring single-frequency, power-scalable laser sources at multiple wavelengths.
We produce large numbers of low-energy ions by photoionization of laser-cooled atoms inside a surface-electrode-based Paul trap. The isotope-selective trap's loading rate of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{Yb}}^{+}\phantom{\rule{0.3em}{0ex}}\mathrm{ions}∕\mathrm{s}$ exceeds that attained by photoionization (electron-impact ionization) of an atomic beam by three (six) orders of magnitude. Traps as shallow as 0.13eV0.13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}0.13eV are easily loaded with this technique. The ions are confined in the same spatial region as the laser-cooled atoms, which will allow the experimental investigation of interactions between cold ions and cold atoms or Bose-Einstein condensates.
Simple and effective techniques for loading barium ions into linear Paul traps are demonstrated. Two-step photoionization of neutral barium is achieved using a weak intercombination line ($6{s}^{2}\phantom{\rule{0.2em}{0ex}}^{1}S_{0}\ensuremath{\leftrightarrow}6s6p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}$, $\ensuremath{\lambda}=791\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) followed by excitation above the ionization threshold using a nitrogen gas laser $(\ensuremath{\lambda}=337\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. Isotopic selectivity is achieved by using a near Doppler-free geometry for excitation of the triplet 6s6p3P16s6p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}6s6p3P1​ state. Additionally, we report a particularly simple and efficient trap loading technique that employs an inexpensive uv epoxy curing lamp to generate photoelectrons.
We demonstrate loading of ions into a surface-electrode trap (SET) from a remote, laser-cooled source of neutral atoms. We first cool and load $\ensuremath{\sim}$106{10}^{6}106 neutral 88{}^{88}88Sr atoms into a magneto-optical trap from an oven that has no line of sight with the SET. The cold atoms are then pushed with a resonant laser into the trap region where they are subsequently photoionized and trapped in an SET operated at a cryogenic temperature of 4.6 K. We present studies of the loading process and show that our technique achieves ion loading into a shallow (15 meV depth) trap at rates as high as 125 ions/s while drastically reducing the amount of metal deposition on the trap surface as compared with direct loading from a hot vapor. Furthermore, we note that due to multiple stages of isotopic filtering in our loading process, this technique has the potential for enhanced isotopic selectivity over other loading methods. Rapid loading from a clean, isotopically pure, and precooled source may enable scalable quantum-information processing with trapped ions in large, low-depth surface-trap arrays that are not amenable to loading from a hot atomic beam.