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133Ba+: High fidelity goldilocks qubits (Conference Presentation)

David Hucul, Paige Haas, Harris J. Rutbeck-Goldman, Zachary S. Smith, Boyan Tabakov, James A. Williams, Carson Woodford, Kathy-Anne Soderberg, Justin E. Christensen, Eric R. Hudson, Wesley C. Campbell

Published Apr 27, 2020• 0 citations
Publisher

Abstract

Atomic ions can be isolated from their environment through laser-cooling and trapping, making them useful for quantum information processing, measurement, and sensing. A variety of atomic ion species have been used as qubits. Hyperfine qubits with nuclear spin I = 1/2 have demonstrated the long qubit coherence times with simple, robust laser manipulation. Other qubits (I ≠ 1/2) have easily-prepared, long-lived metastable electronic excited states, and simple discrimination between these states allows high fidelity readout. However, none of the naturally- occurring, atomic ions with nuclear spin I = 1/2 have these excited states that are simultaneously long-lived and easy to prepare. We demonstrate loading, cooling, and qubit manipulation of an artificial, I = 1/2 species of barium with visible wavelength lasers: 133Ba+. We achieved a single shot qubit state preparation and readout fidelity of F = 0.9997, the lowest error rate ever achieved by any qubit on any platform.

DOI
10.1117/12.2559370

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High-fidelity operation of a radioactive trapped-ion qubit, $^{133}$Ba$^+$

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•Jan 1, 2020•3 citations

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Tips and Tricks for Building a Quantum Computer with $^{133}\mathrm{Ba}^{+}$

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Quantum phenomena of superposition paved the way for the quantum computer demonstrations utilizing different approaches of qubit creation and manipulation, such as, based on trapped ions, neutral atoms, nitrogen vacancies, photonic integrated circuits, superconducting loops, etc. The trapped-ion-based platform demonstrated record long coherence times, high-fidelity gates, qubit connectivity and potential for fault-tolerant operation [1]. Number of atomic species have been investigated as basis for qubits and various qubit types along with state-preparation and measurement (SPAM) techniques have been developed. These qubit initialization and readout operations as well as atomic ion preparation stages of photoionization and atom cooling are implemented by single-frequency lasers with narrow emission linewidths at frequencies corresponding to electronic transitions in atomic ion specie. Therefore, several single-frequency lasers with well-defined parameters such as linewidth, output power, polarization, intensity noise are required for trapped-ion-based quantum computer. Considering that currently there are about 11 ion species being employed as qubits, thus more than 100 different laser wavelengths are required across the spectrum. For practical use, scalability of quantum computer is required, i.e., steady increase of qubits number without the loss of quantum system control. Therefore, availability and performance of single-frequency stabilized laser systems with tailorable emission parameters is one of pre-requirements for quantum information processing field development and commercialization success.

IEEE International Conference on Quantum Computing and Engineering (QCE)•Sep 17, 2023•0 citations
Atomic ions can be isolated from their environment through laser-cooling and trapping, making them useful for quantum information processing, measurement, and sensing. A variety of atomic ion species have been used as qubits. Hyperfine qubits with nuclear spin I = 1/2 have demonstrated the long qubit coherence times with simple, robust laser manipulation. Other qubits (I ≠ 1/2) have easily-prepared, long-lived metastable electronic excited states, and simple discrimination between these states allows high fidelity readout. However, none of the naturally- occurring, atomic ions with nuclear spin I = 1/2 have these excited states that are simultaneously long-lived and easy to prepare. We demonstrate loading, cooling, and qubit manipulation of an artificial, I = 1/2 species of barium with visible wavelength lasers: 133Ba+. We achieved a single shot qubit state preparation and readout fidelity of F = 0.9997, the lowest error rate ever achieved by any qubit on any platform.
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.
Tips and Tricks for Building a Quantum Computer with 133Ba+^{133}\mathrm{Ba}^{+}133Ba+
Building a fault-tolerant quantum computer requires more than improving physical hardware in isolation. Hardware-level design choices must be guided by the architecture of the full quantum computer and by the bottlenecks that ultimately limit its performance. The 133Ba+ ion is well suited for this kind of architecture-driven optimization. Its simple nuclear spin structure, with I = 1/2, enables fast, high-fidelity state preparation and measurement (SPAM). We leverage this advantage to demonstrate record SPAM fidelity, along with a state-preparation time more than three orders of magnitude faster than previous record-level SPAM demonstrations. This makes 133Ba+ especially promising as a reusable ancilla qubit for quantum error correction.The long-lived D5/2 state, with τ = 30 s, also makes 133Ba+ uniquely positioned to take advantage of the optical-metastable-ground (OMG) qubit protocol, which enables same-species sympathetic cooling. Within this protocol, we demonstrate a magic polarization condition that allows global gates with 532 nm light to be applied in one qubit subspace while leaving other subspaces largely unaffected. We also demonstrate stimulated Raman gates in both the ground and metastable qubit subspaces with errors below the fault-tolerance threshold, supported by a direct measurement of the spontaneous Raman scattering rate. The main practical drawback of 133Ba+ is its radioactivity. Here, we mitigate this challenge by demonstrating ion loading from a 133Ba target below the Nuclear Regulatory Commission exempt quantity of 10 µCi. This is enabled by a new autoionization loading scheme that substantially reduces loading times for barium ions.
Coherent storage of quantum information is crucial to many quantum technologies. Long coherence times have been demonstrated in trapped-ion qubits, typically using the hyperfine levels within the ground state of a single ion. However, recent research suggests qubits encoded in metastable states could provide architectural benefits for quantum information processing, such as the possibility of effective dual-species operation in a single-species system and erasure-error conversion for fault-tolerant quantum computing. Here we demonstrate long-lived encoding of a quantum state in the metastable states of a trapped ion. By sympathetically cooling with another ion of the same species and constantly monitoring for erasure errors, we demonstrate a coherence time of 136(42) seconds with a qubit encoded in the metastable 5D5/25D_{5/2}5D5/2​ state of a single 137^{137}137Ba+^++ ion. In agreement with a model based on empirical results from dynamical-decoupling-based noise spectroscopy, we find that dephasing of the metastable levels is the dominant source of error once erasure errors are removed.
Quantum phenomena of superposition paved the way for the quantum computer demonstrations utilizing different approaches of qubit creation and manipulation, such as, based on trapped ions, neutral atoms, nitrogen vacancies, photonic integrated circuits, superconducting loops, etc. The trapped-ion-based platform demonstrated record long coherence times, high-fidelity gates, qubit connectivity and potential for fault-tolerant operation [1]. Number of atomic species have been investigated as basis for qubits and various qubit types along with state-preparation and measurement (SPAM) techniques have been developed. These qubit initialization and readout operations as well as atomic ion preparation stages of photoionization and atom cooling are implemented by single-frequency lasers with narrow emission linewidths at frequencies corresponding to electronic transitions in atomic ion specie. Therefore, several single-frequency lasers with well-defined parameters such as linewidth, output power, polarization, intensity noise are required for trapped-ion-based quantum computer. Considering that currently there are about 11 ion species being employed as qubits, thus more than 100 different laser wavelengths are required across the spectrum. For practical use, scalability of quantum computer is required, i.e., steady increase of qubits number without the loss of quantum system control. Therefore, availability and performance of single-frequency stabilized laser systems with tailorable emission parameters is one of pre-requirements for quantum information processing field development and commercialization success.