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arXiv:2406.14448 · Physical Review A

Polarization-insensitive state preparation for trapped-ion hyperfine qubits

A. D. Leu, Matthew C. Smith, Mario F. Gely, D. M. Lucas

Published Oct 16, 2024• 2 citations
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Abstract

Quantum state preparation for trapped-ion qubits often relies on high-quality circularly polarized light, which may be difficult to achieve with chip-based integrated optics technology. We propose and implement a hybrid optical and microwave scheme for intermediate-field hyperfine qubits which instead relies on frequency selectivity. Experimentally, we achieve 99.94 % fidelity for linearly polarized ( σ + / σ − ) light, using Ca + 43 at 28.8 mT. We find that the fidelity remains above 99.8 % for a mixture of all polarizations ( σ + / σ − / π ) . We calculate that the method is capable of 99.99 % fidelity in Ca + 43 , and even higher fidelities in heavier ions such as Ba + 137 . Published by the American Physical Society 2024

DOI
10.1103/physreva.110.l040402

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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.
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Microwave-driven logic is a promising alternative to laser control in scaling trapped-ion based quantum processors. We implement Mølmer-Sørensen two-qubit gates on 43 Ca + hyperfine clock qubits in a cryogenic ( ≈ 25 K ) surface trap, driven by near-field microwaves. We achieve gate durations of 154 µs [with 1.0(2)% error] and 331 µs [0.5(1)% error], which approaches the performance of typical laser-driven gates. In the 331 µs gate, we demonstrate a Walsh-modulated dynamical decoupling scheme which suppresses errors due to fluctuations in the qubit frequency as well as imperfections in the decoupling drive itself. Published by the American Physical Society 2024
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 demonstrate the use of trapped ytterbium ions as quantum bits for quantum information processing. We implement fast, efficient state preparation and state detection of the first-order magnetic field-insensitive hyperfine levels of 171Yb+^{171}\mathrm{Yb}^{+}171Yb+, with a measured coherence time of 2.5s2.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}2.5s. The high efficiency and high fidelity of these operations is accomplished through the stabilization and frequency modulation of relevant laser sources.
State preparation, qubit rotation, and high fidelity readout are demonstrated for two different 137Ba+^{137}\mathrm{Ba}{}^{+}137Ba+ qubit types. First, an optical qubit on the narrow 6S1/26{S}_{1/2}6S1/2​ to 5D5/25{D}_{5/2}5D5/2​ transition at 1.76 $\ensuremath{\mu}$m is implemented. Then, leveraging the techniques developed there for readout, a ground-state hyperfine qubit using the magnetically insensitive transition at 8 GHz is accomplished.