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IEEE International Conference on Quantum Computing and Engineering (QCE)

High fidelity state preparation and measurement of ion qubits with spin I > ½

Andrew Schaffer, Fangzhao Alex An, Anthony Ransford, Lucas R. Sletten, John Gaebler, James Hostetter, Grahame Vittorini

Published Sep 1, 2022• 0 citations
Publisher

Abstract

We present a method of state preparation and measurement (SPAM) that is amenable to trapped ion hyperfine qubits with nuclear spins higher than I = 1/2. The ground states of these higher nuclear spin isotopes do not afford a simple frequency selective preparation scheme on a strong transition, this limitation is circumvented by stroboscopically driving strong and weak transitions. We demonstrate this method achieving a SPAM infidelity of137Ba+qubit used in this work. State preparation is performed by blending standard, fast optical pumping using E1 transitions and narrow microwave or optical E2 transitions addressing the extra states in the I=3/2 isotope of137Ba+. Measurement is performed with repeated coherent shelving to multiple metastable states.

DOI
10.1109/qce53715.2022.00120

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We present a method of state preparation and measurement (SPAM) that is amenable to trapped ion hyperfine qubits with nuclear spins higher than I = 1/2. The ground states of these higher nuclear spin isotopes do not afford a simple frequency selective preparation scheme on a strong transition, this limitation is circumvented by stroboscopically driving strong and weak transitions. We demonstrate this method achieving a SPAM infidelity of137Ba+qubit used in this work. State preparation is performed by blending standard, fast optical pumping using E1 transitions and narrow microwave or optical E2 transitions addressing the extra states in the I=3/2 isotope of137Ba+. Measurement is performed with repeated coherent shelving to multiple metastable states.
We present a method for achieving high fidelity state preparation and measurement (SPAM) using trapped ion hyperfine qubits with nuclear spins higher than I=1/2I=1/2I=1/2. The ground states of these higher nuclear spin isotopes do not afford a simple frequency-selective state preparation scheme. We circumvent this limitation by stroboscopically driving strong and weak transitions, blending fast optical pumping using dipole transitions, and narrow microwave or optical quadrupole transitions. We demonstrate this method with the I=3/2I=3/2I=3/2 isotope 137Ba+^{137}{\mathrm{Ba}}^{+}137Ba+ to achieve a SPAM infidelity of $(9.0\ifmmode\pm\else\textpm\fi{}1.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ ($\ensuremath{-}40.5\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{ }\mathrm{dB}$), facilitating the use of a wider range of ion isotopes with favorable wavelengths and masses for quantum computation.
We demonstrate a method of qubit state preparation and measurement (SPAM) based on a weakly open quantum channel that achieves a higher fidelity than previous demonstrations, even with low detection efficiency. The trapped-ion qubit SPAM infidelity ($2.{1}_{\ensuremath{-}0.4}^{+0.5}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$) is limited by setup-specific errors from ion loss and imperfect population transfer between qubit eigenstates, and we show that full transfer would yield an inaccuracy less than $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. The high precision of this method revealed errors caused by a rare ($\ensuremath{\approx}{10}^{\ensuremath{-}4}$ likelihood) magnetic-dipole decay that we measure and correct by driving an additional transition. Since this scheme allows fluorescence collection for effectively unlimited periods, high fidelity is achievable even with limited optical access and low quantum efficiency photon detectors.
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We present two novel approaches to state detection of qubits defined with trapped ions. The first uses simple pulse sequences from a mode-locked laser to induce state-dependent excitations in less than 1 ns. The resulting atomic fluorescence occurs in the dark, allowing the placement of non-imaging detectors right next to the atom to improve the qubit state detection efficiency and speed.The second employs the long lived F state in Yb+ which has been used in quantum information science almost exclusively for clocks and optical-frequency qubits. We describe how this resource can be used in conjunction with the ground state S1/2 manifold to aid in the scaling of trapped ion quantum information science. Narrow-band optical pumping into the 2F7/2 from one of the conventional 2S1/2 qubit states is projected to achieve a higher state preparation and measurement (SPAM) fidelity than any other demonstrated technique.
High-fidelity operation of a radioactive trapped-ion qubit, 133^{133}133Ba+^++
133^{133}133Ba+^++ has been identified as an attractive ion for quantum information processing due to the unique combination of its spin-1/2 nucleus, visible wavelength electronic transitions, and long D-state lifetimes ($\\tau\\sim 1 min).Usingamicrogramsourceofradioactivematerial,wetrapandlaser−coolthesynthetictextitA=133radioisotopeofbariumII(1~min). Using a microgram source of radioactive material, we trap and laser-cool the synthetic \\textit{A} = 133 radioisotope of barium II (1 min).Usingamicrogramsourceofradioactivematerial,wetrapandlaser−coolthesynthetictextitA=133radioisotopeofbariumII(\\tau_{1/2} \\approx 10.5 yr)inaradio−frequencyiontrap.Todemonstratehighfidelityqubitoperations,anumberofunknownstateenergieswereneeded.Wemeasuretheisotopeshiftandhyperfinestructureofthe6~yr) in a radio-frequency ion trap. To demonstrate high fidelity qubit operations, a number of unknown state energies were needed. We measure the isotope shift and hyperfine structure of the 6 yr)inaradio−frequencyiontrap.Todemonstratehighfidelityqubitoperations,anumberofunknownstateenergieswereneeded.Wemeasuretheisotopeshiftandhyperfinestructureofthe6^2PPP_{1/2}\\leftrightarrow 5^2DDD_{3/2}$ electronic transition needed for laser cooling, state preparation, and state detection of the clock-state hyperfine and optical qubits. For high-fidelity operations with electron shelving, we report measurements of the 62^22P3/2_{3/2}3/2​ and 525^252D5/2_{5/2}5/2​ hyperfine splittings, as well as the~62^22P3/2 leftrightarrow 62_{3/2}~\\leftrightarrow~6^23/2​ leftrightarrow 62S1/2_{1/2}1/2​ and ~62^22P3/2 leftrightarrow 52_{3/2}~\\leftrightarrow~5^23/2​ leftrightarrow 52D5/2_{5/2}5/2​ transition frequencies. Using these transitions, we demonstrate high-fidelity \\ba \\ hyperfine qubit manipulation with electron shelving detection to benchmark qubit state preparation and measurement (SPAM). Using single-shot, threshold discrimination, we measure an average SPAM fidelity of mathcalF=0.99971(3)\\mathcal{F} = 0.99971(3)mathcalF=0.99971(3), a factor of approx\\approxapprox 2 improvement over the best reported performance of any qubit.Finally, we report the 62^22P1/2leftrightarrow52_{1/2}\\leftrightarrow 5^21/2​leftrightarrow52D3/2_{3/2}3/2​ electronic transition isotope shift for the rare \\textit{A} = 130 and 132 barium nuclides, completing the spectroscopic characterization necessary for laser cooling all long-lived barium II isotopes.
Abstract The recently demonstrated trapping and laser cooling of 133Ba+ has opened the door to the use of this nearly ideal atom for quantum information processing. However, before high-fidelity qubit operations can be performed, a number of unknown state energies are needed. Here, we report measurements of the 2P3/2 and 2D5/2 hyperfine splittings, as well as the 2P3/2 ↔ 2S1/2 and 2P3/2 ↔ 2D5/2 transition frequencies. Using these transitions, we demonstrate high-fidelity 133Ba+ hyperfine qubit manipulation with electron shelving detection to benchmark qubit state preparation and measurement (SPAM). Using single-shot, threshold discrimination, we measure an average SPAM fidelity of F=0.99971(3){\mathcal{F}}=0.99971(3)F=0.99971(3) F = 0.99971 ( 3 ) , a factor of ≈2 improvement over the best reported performance of any qubit.