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arXiv:2108.12052 · Physical Review A

Weak dissipation for high-fidelity qubit-state preparation and measurement

Anthony Ransford, Conrad Roman, Thomas Dellaert, Patrick McMillin, Wesley C. Campbell

Published Dec 29, 2021• 17 citations
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Abstract

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.

DOI
10.1103/physreva.104.l060402

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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×\else\texttimes\fi10\ensuremath−42.{1}_{\ensuremath{-}0.4}^{+0.5}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}2.1\ensuremath−0.4+0.5​\ifmmode×\else\texttimes\fi10\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×\else\texttimes\fi10\ensuremath−58\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}8\ifmmode×\else\texttimes\fi10\ensuremath−5. The high precision of this method revealed errors caused by a rare (\ensuremath≈10\ensuremath−4\ensuremath{\approx}{10}^{\ensuremath{-}4}\ensuremath≈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 a novel protocol for high-fidelity qubit state preparation and measurement (SPAM) that combines standard SPAM methods with a series of in-sequence measurements to detect and remove errors. The protocol can be applied in any quantum system with a long-lived (metastable) level and a means to detect population outside of this level without coupling to it. We demonstrate the use of the protocol for three different qubit encodings in a single trapped 137Ba+^{137}\mathrm{Ba}^+137Ba+ ion. For all three, we achieve the lowest reported SPAM infidelities of 7(4)×10−67(4) \times 10^{-6}7(4)×10−6 (optical qubit), 5(4)×10−65(4) \times 10^{-6}5(4)×10−6 (metastable-level qubit), and 8(4)×10−68(4) \times 10^{-6}8(4)×10−6 (ground-level qubit).
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.
Quantum error correction of a grid-state qubit with state preparation and measurement errors below 10−310^{-3}10−3
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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.
Qubit state detection is an important part of a quantum computation. As the number of qubits in a quantum register increases, it is necessary to maintain high-fidelity detection to accurately measure the multiqubit state. Here we present experimental demonstration of high-fidelity detection of a multiqubit trapped-ion register with an average single-qubit detection error of 4.2(1.5) ppm and a four-qubit state detection error of 17(2) ppm, limited by the decay lifetime of the qubit, using a single-photon-sensitive camera with fast data collection, excellent temporal and spatial resolution, and low instrumental crosstalk.