An experimental setup for quantum information processing using trapped Ba+ ions as qubit carriers has been designed and is under construction. Among various isotopes of barium, the isotope with a mass number A = 133 has the lowest non-zero nuclear spin I = 1/2, having several desired properties with regard to quantum information processing.† In this presentation, we show the setup for cooling and manipulationg 133Ba+ ions, as well as other isotopes.
An experimental setup for quantum information processing using trapped Ba+ ions as qubit carriers has been designed and is under construction. Among various isotopes of barium, the isotope with a mass number A = 133 has the lowest non-zero nuclear spin I = 1/2, having several desired properties with regard to quantum information processing.† In this presentation, we show the setup for cooling and manipulationg 133Ba+ ions, as well as other isotopes.
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, 133Ba+
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 ($\\tau\\sim 1min).Usingamicrogramsourceofradioactivematerial,wetrapandlaser−coolthesynthetictextitA=133radioisotopeofbariumII(\\tau_{1/2} \\approx 10.5yr)inaradio−frequencyiontrap.Todemonstratehighfidelityqubitoperations,anumberofunknownstateenergieswereneeded.Wemeasuretheisotopeshiftandhyperfinestructureofthe6^2P_{1/2}\\leftrightarrow 5^2D_{3/2}$ 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 62P3/2 and 52D5/2 hyperfine splittings, as well as the~62P3/2leftrightarrow62S1/2 and ~62P3/2leftrightarrow52D5/2 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), a factor of approx 2 improvement over the best reported performance of any qubit.Finally, we report the 62P1/2leftrightarrow52D3/2 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.
Trapped ions for quantum information processing have been an area of intense study due to the extraordinarily high fidelity operations that have been reported experimentally. Specifically, barium trapped ions have been shown to have exceptional state-preparation and measurement fidelities. The 133Ba+ (I = 1/2) isotope in particular is a promising candidate for large-scale quantum computing experiments. However, a major pitfall with this isotope is that it is radioactive and is thus generally used in microgram quantities to satisfy safety regulations. We describe a new method for creating microgram barium chloride (BaCl2) ablation targets for use in trapped ion experiments and compare our procedure to previous methods. We outline two recipes for the fabrication of ablation targets that increase the production of neutral atoms for isotope-selective loading of barium ions. We show that heat-treatment of the ablation targets greatly increases the consistency at which neutral atoms can be produced, and we characterize the uniformity of these targets using trap-independent techniques such as energy dispersive x-ray spectroscopy and neutral fluorescence collection. Our comparison between fabrication techniques and the demonstration of consistent neutral fluorescence paves a path toward reliable loading of 133Ba+ in surface traps and opens opportunities for scalable quantum computing with this isotope.
^{133}Ba^{+} has been identified as an attractive ion for quantum information processing due to the unique combination of its spin-1/2 nucleus and visible wavelength electronic transitions. Using a microgram source of radioactive material, we trap and laser cool the synthetic A=133 radioisotope of barium II in a radio-frequency ion trap. Using the same, single trapped atom, we measure the isotope shifts and hyperfine structure of the 6^{2}P_{1/2}↔6^{2}S_{1/2} and 6^{2}P_{1/2}↔5^{2}D_{3/2} electronic transitions that are needed for laser cooling, state preparation, and state detection of the clock-state hyperfine and optical qubits. We also report the 6^{2}P_{1/2}↔5^{2}D_{3/2} electronic transition isotope shift for the rare A=130 and 132 barium nuclides, completing the spectroscopic characterization necessary for laser cooling all long-lived barium II isotopes.
Tips and Tricks for Building a Quantum Computer with 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.
Microgram BaCl2 Ablation Targets for Trapped Ion Experiments
Trapped ions for quantum information processing has been an area of intense study due to the extraordinarily high fidelity operations that have been reported experimentally. Specifically, barium trapped ions have been shown to have exceptional state-preparation and measurement (SPAM) fidelities. The 133Ba+ (I=1/2) isotope in particular is a promising candidate for large-scale quantum computing experiments. However, a major pitfall with this isotope is that it is radioactive and is thus generally used in microgram quantities to satisfy safety regulations. We describe a new method for creating microgram barium chloride (BaCl2) ablation targets for use in trapped ion experiments and compare our procedure to previous methods. We outline two recipes for fabrication of ablation targets that increase the production of neutral atoms for isotope-selective loading of barium ions. We show that heat-treatment of the ablation targets greatly increases the consistency at which neutral atoms can be produced and we characterize the uniformity of these targets using trap-independent techniques such as energy dispersive x-ray spectroscopy (EDS) and neutral fluorescence collection. Our comparison between fabrication techniques and demonstration of consistent neutral fluorescence paves a path towards reliable loading of 133Ba+ in surface traps and opens opportunities for scalable quantum computing with this isotope.