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arXiv:2307.07627

Trapping $\mathbf{Ba}^+$ with Seven-fold Enhanced Efficiency Utilizing an Autoionizing Resonance

Noah Greenberg, Brendan White, Pei Jiang Low, Crystal Senko

Published Jul 14, 2023• 0 citations
arXivHTMLPDF

Abstract

Trapped ions have emerged as a front runner in quantum information processing due to their identical nature, all-to-all connectivity, and high fidelity quantum operations. As current trapped ion technologies are scaled, it will be important to improve the efficiency of loading ions, which is currently the slowest process in operating a trapped ion quantum computer. Here, we compare two isotope-selective photoionization schemes for loading $^{138}\mathrm{Ba}^+$ ions. We show that a two-step photoionization scheme ending in an autoionizing transition increases the ion loading rate nearly an order of magnitude compared to an established technique which does not excite an autoionizing state. The only additional technology required to implement the autoionizing transition is a commercial diode laser. Our technique can be extended to all isotopes of barium, and autoionizing resonances exist in every species currently used for trapped ion quantum processing, making this a promising technique to drastically increase the loading rates for all trapped ion computers.

DOI
10.48550/arxiv.2307.07627

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Trapping Ba+\mathbf{Ba}^+Ba+ with Seven-fold Enhanced Efficiency Utilizing an Autoionizing Resonance
Trapped ions have emerged as a front runner in quantum information processing due to their identical nature, all-to-all connectivity, and high fidelity quantum operations. As current trapped ion technologies are scaled, it will be important to improve the efficiency of loading ions, which is currently the slowest process in operating a trapped ion quantum computer. Here, we compare two isotope-selective photoionization schemes for loading 138Ba+^{138}\mathrm{Ba}^+138Ba+ ions. We show that a two-step photoionization scheme ending in an autoionizing transition increases the ion loading rate nearly an order of magnitude compared to an established technique which does not excite an autoionizing state. The only additional technology required to implement the autoionizing transition is a commercial diode laser. Our technique can be extended to all isotopes of barium, and autoionizing resonances exist in every species currently used for trapped ion quantum processing, making this a promising technique to drastically increase the loading rates for all trapped ion computers.
Abstract Trapped ions have emerged as a front runner in quantum information processing due to their identical nature, all-to-all connectivity, and high fidelity quantum operations. As current trapped ion technologies are scaled, it will be important to improve the efficiency of loading ions, especially when working with long chains of ions or rare isotopes. Here, we compare two different isotope-selective photoionization schemes for loading 138 Ba + ions. We show that a two-step photoionization scheme ending in an autoionizing transition increases the ion loading rate nearly an order of magnitude compared to an established technique which does not excite an autoionizing state. Our novel photoionization scheme can be extended to all isotopes of barium. Given that autoionizing resonances exist in every trapped ion species, exploitation of this process is a promising pathway to increase the loading rates for trapped ion computers.
The 133Ba+^{133}\mathrm{Ba}^{+}133Ba+ ion is a promising candidate as a high-fidelity qubit, and the 137Ba+^{137}\mathrm{Ba}^{+}137Ba+ isotope is promising as a high-fidelity qudit (d>2)(d>2)(d>2). Barium metal is very reactive, and 133Ba+^{133}\mathrm{Ba}^{+}133Ba+ is radioactive and can only be sourced in small quantities, so the most commonly used loading method, oven heating, is less suited for barium and is currently not possible for 133Ba+^{133}\mathrm{Ba}^{+}133Ba+. Pulsed laser ablation solves both of these problems by utilizing compound barium sources while also giving some distinct advantages, such as fast loading, less displaced material, and lower heat load near the ion trap. Because of the relatively low abundances of the isotopes of interest, a two-step photoionization technique is used, which gives us the ability to selectively load isotopes. Characterization of the ablation process for our BaCl2{\mathrm{BaCl}}_{2}BaCl2​ targets are presented, including observation of neutral and ion ablation-fluence regimes, preparation and conditioning, lifetimes of ablation spots, and plume velocity distributions. We show that by using laser ablation on BaCl2{\mathrm{BaCl}}_{2}BaCl2​ salt targets with a two-step photoionization method, we can produce and trap barium ions reliably. Furthermore, we demonstrate that with our photoionization method, we can trap 137Ba+^{137}\mathrm{Ba}^{+}137Ba+ with an enhanced selectivity compared to its natural abundance.
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 report a theoretical and experimental investigation of autoionizing resonances from the 5d6p 3D1o5d6p\,{}^3\mathrm{D}_1^o5d6p3D1o​ manifold in neutral barium for efficient loading of ion traps. Our calculations predict large resonant cross sections for many narrow autoionizing resonances, but we find experimentally that for most of these, Doppler broadening during trap loading depresses the effective cross sections that can be achieved in practice. We identify and demonstrate a strong, broad transition at 531 nm531\,\mathrm{nm}531nm, and show that it furnishes an order-of-magnitude increase in trap loading efficiency compared to other demonstrated resonances.
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.
Microgram BaCl2\mathrm{BaCl}_2BaCl2​ Ablation Targets for Trapped Ion Experiments
Trapped ions for quantum information processing has been an area of intense study due to the extraordinarily high fidelity operations that have been reported experimentally. Specifically, barium trapped ions have been shown to have exceptional state-preparation and measurement (SPAM) fidelities. The 133Ba+^{133}\mathrm{Ba}^+133Ba+ (I=1/2I = 1/2I=1/2) isotope in particular is a promising candidate for large-scale quantum computing experiments. However, a major pitfall with this isotope is that it is radioactive and is thus generally used in microgram quantities to satisfy safety regulations. We describe a new method for creating microgram barium chloride (BaCl2\mathrm{BaCl}_2BaCl2​) ablation targets for use in trapped ion experiments and compare our procedure to previous methods. We outline two recipes for fabrication of ablation targets that increase the production of neutral atoms for isotope-selective loading of barium ions. We show that heat-treatment of the ablation targets greatly increases the consistency at which neutral atoms can be produced and we characterize the uniformity of these targets using trap-independent techniques such as energy dispersive x-ray spectroscopy (EDS) and neutral fluorescence collection. Our comparison between fabrication techniques and demonstration of consistent neutral fluorescence paves a path towards reliable loading of 133Ba+^{133}\mathrm{Ba}^+133Ba+ in surface traps and opens opportunities for scalable quantum computing with this isotope.