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arXiv:2512.12191 · Physical Review A

Barium autoionization for efficient ion trap loading

Zachary J. Wall, J. Piel, Samuel R. Vizvary, Michael Bareian, Steven Diaz, Elijah Mossman, Anthony Ransford, Chris H. Greene, Eric R. Hudson, Wesley C. Campbell

Published May 26, 2026• 0 citations
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Abstract

We report a theoretical and experimental investigation of autoionizing resonances from the $5d6p\,{}^3\mathrm{D}_1^o$ 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 $531\,\mathrm{nm}$, and show that it furnishes an order-of-magnitude increase in trap loading efficiency compared to other demonstrated resonances.

DOI
10.1103/c5m6-kt93

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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.
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We report the direct measurement of isotope shifts of the barium 6s26{s}^{2}6s2 1S0–5d6p3D1o^{1}S_{0}\text{--}5d6p^{3}D_{1}^{\mathrm{o}}1S0​–5d6p3D1o​ 413-nm electric quadrupole transition, which is utilized for efficient barium ion trapping via photoionization using a single coherent light source. The measured isotope shifts relative to 138Ba^{138}\mathrm{Ba}138Ba are $392.9\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$, $178.1\ifmmode\pm\else\textpm\fi{}0.8\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$, $401.4\ifmmode\pm\else\textpm\fi{}1.2\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$, and $124.3\ifmmode\pm\else\textpm\fi{}1.3\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$ for isotopes with atomic numbers 137, 136, 135, and 134, respectively. We verify the measured isotopes with King plot analysis and compare the result with the formerly known shifts inferred from previous studies on neighboring transitions. The results can be used for efficient isotope selective loading of low-abundant barium ions, while careful suppression of line broadening is required for successful isotopic selectivity.
Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned 6s6p 1P16s6p\text{ }{^{1}P}_{1}6s6p 1P1​ state of barium in the energy range between the 5d3/25{d}_{3/2}5d3/2​ and 5d5/25{d}_{5/2}5d5/2​ states of Ba+{\text{Ba}}^{+}Ba+. We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel RRR-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.
Trapped-ion quantum information processing may benefit from qubits encoded in isotopes that are practically available in only small quantities, e.g., due to low natural abundance or radioactivity. Laser ablation provides a method of controllably liberating neutral atoms or ions from low-volume targets, but energetic ablation products can be difficult to confine in the small ion-electrode distance, micron-scale microfabricated traps amenable to high-speed, high-fidelity manipulation of ion arrays. Here, we investigate ablation-based ion loading into surface-electrode traps of different sizes to test a model describing ion loading probability as a function of effective trap volume and other trap parameters. We characterize loading of ablated barium from a metallic source in two cryogenic surface-electrode traps with 730 and 50 μm ion-electrode distances. Our loading rate agrees with a predictive analytical model, providing insight for the confinement of limited-quantity species of interest for quantum computing, simulation, and sensing.