Crystal Senko

AC mains power line-synchronous disturbances are a common source of coherent, time-dependent error in precision quantum-control experiments. We show that when these disturbances are reproducible with respect to the mains phase, their effect can be measured in a line-triggered frame and compensated through software updates to control sequences. In our system, the disturbances manifest as magnetic-field-induced shifts in the energy level structure of a trapped $^{137}\text{Ba}^+$ ion, resulting in time-dependent detunings between the ion transitions and a local oscillator, as well as additional phases accumulated on superpositions of energy levels. We demonstrate a compensation protocol that corrects for the instantaneous oscillator detuning during control pulses, and for the phase accumulated by the energy levels between pulses. The calibrated AC line contribution to the detuning is reduced by $21(9)\times$, while the fitted AC phase amplitude is reduced below the measurement uncertainty. We then study gate performance on a magnetic-field-sensitive qubit and find that uncompensated mains-synchronous errors produce time-dependent fluctuations that make the usual randomized-benchmarking decay model unreliable. With compensation enabled, these fluctuations are suppressed sufficiently to recover a standard benchmarking decay and extract an average gate fidelity of $99.93(1)\%$. Finally, we extend the framework to multilevel qudit control and apply it to a single-qudit Bernstein-Vazirani algorithm, where AC compensation increases the success probability on a 16-level qudit from $10(7)\%$ to $70(9)\%$. These results show that reproducible line-synchronous noise can be treated as a calibrated control-frame error and corrected without additional hardware.

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 $^{133}\mathrm{Ba}^+$ ($I = 1/2$) 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 ($\mathrm{BaCl}_2$) 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 $^{133}\mathrm{Ba}^+$ in surface traps and opens opportunities for scalable quantum computing with this isotope.

Trapped ions have emerged as a front runner in quantum information processing due to their identical nature, all-to-all connectivity, and high fidelity quantum operations. As current trapped ion technologies are scaled, it will be important to improve the efficiency of loading ions, which is currently the slowest process in operating a trapped ion quantum computer. Here, we compare two isotope-selective photoionization schemes for loading $^{138}\mathrm{Ba}^+$ ions. We show that a two-step photoionization scheme ending in an autoionizing transition increases the ion loading rate nearly an order of magnitude compared to an established technique which does not excite an autoionizing state. The only additional technology required to implement the autoionizing transition is a commercial diode laser. Our technique can be extended to all isotopes of barium, and autoionizing resonances exist in every species currently used for trapped ion quantum processing, making this a promising technique to drastically increase the loading rates for all trapped ion computers.

To implement useful quantum algorithms which demonstrate quantum advantage, we must scale currently demonstrated quantum computers up significantly. Leading platforms such as trapped ions face physical challenges in including more information carriers. A less explored avenue for scaling up the computational space involves utilizing the rich energy level structure of a trapped ion to encode multi-level qudits rather than two-level qubits. Here we show control and single-shot readout of qudits with up to 13 computational states, using protocols which can be extended directly to manipulate qudits of up to 25 levels in our chosen information host, $^{137}\text{Ba}^{+}$. This represents more than twice as many computational states per qudit compared with prior work in trapped ions. In addition to the preparation and readout protocols we demonstrate, universal quantum computation requires other quantum logic primitives such as entangling gates. These primitives have been demonstrated for lower qudit dimensions and can be directly generalized to the higher dimensions we employ. Hence, our advance opens an avenue towards using high-dimensional qudits for large-scale quantum computation. We anticipate efficiently utilizing available energy states in a trapped ion to play a significant and complementary role in tackling the challenge in scaling up the computational space of a trapped ion quantum computer. A qudit architecture also offers other practical benefits, which include affording relaxed fault tolerance thresholds for quantum error correction, providing an avenue for efficient quantum simulation of higher spin systems, and more efficient qubit gates.

Trapped ions are one of the leading platforms for quantum information processing, exhibiting the highest gate and measurement fidelities of all contending hardware. In order to realize a universal quantum computer with trapped ions, independent and parallel control over the state of each qubit is necessary. The manipulation of individual qubit states in an ion chain via stimulated Raman transitions generally requires light focused on individual ions. In this manuscript, we present a novel, guided-light individual addressing system for hyperfine Ba$^+$ qubits. The system takes advantage of laser-written waveguide technology, enabled by the atomic structure of Ba$^+$, allowing the use of visible light to drive Raman transitions. Such waveguides define the spatial mode of light, suppressing aberrations that would have otherwise accumulated in a free-space optics set up. As a result, we demonstrate a nearest neighbour relative intensity crosstalk on the order of 10$^{-4}$, without any active aberration compensation. This is comparable to or better than other previous demonstrations of individual addressing. At the same time, our modular approach provides independent and agile control over the amplitude, frequency, and phase of each channel; combining the strengths of previous implementations.

The $^{133}\mathrm{Ba}^+$ ion is a promising candidate as a high-fidelity qubit, and the $^{137}\mathrm{Ba}^+$ isotope is promising as a high-fidelity qudit ($d>2$). Barium metal is very reactive, and $^{133}\mathrm{Ba}^+$ is radioactive and can only be sourced in small quantities, so the most commonly used loading method, oven heating, is less suited for barium, and is currently not possible for $^{133}\mathrm{Ba}^+$.Pulsed laser ablation solves both of these problems by utilizing compound barium sources, while also giving some distinct advantages, such as fast loading, less displaced material, and lower heat load near the ion trap. Because of the relatively low abundances of the isotopes of interest, a two-step photoionization technique is used, which gives us the ability to selectively load isotopes. Characterization of the ablation process for our $\mathrm{BaCl}_2$ targets are presented, including observation of neutral and ion ablation-fluence regimes, preparation/conditioning and lifetimes of ablation spots, and plume velocity distributions.We show that using laser ablation on $\mathrm{BaCl}_2$ salt targets with a two-step photoionization method, we can produce and trap barium ions reliably. Further, we demonstrate that with our photoionization method, we can trap $^{137}\mathrm{Ba}^+$ with an enhanced selectivity compared to its natural abundance.

The notion of universal quantum computation can be generalized to multi-level qudits, which offer advantages in resource usage and algorithmic efficiencies. Trapped ions, which are pristine and well-controlled quantum systems, offer an ideal platform to develop qudit-based quantum information processing. Previous work has not fully explored the practicality of implementing trapped-ion qudits accounting for known experimental error sources. Here, we describe a universal set of protocols for state preparation, single-qudit gates, a new generalization of the Mølmer-Sørensen gate for two-qudit gates, and a measurement scheme which utilizes shelving to a meta-stable state. We numerically simulate known sources of error from previous trapped ion experiments, and show that there are no fundamental limitations to achieving fidelities above \(99\%\) for three-level qudits encoded in \(^{137}\mathrm{Ba}^+\) ions. Our methods are extensible to higher-dimensional qudits, and our measurement and single-qudit gate protocols can achieve \(99\%\) fidelities for five-level qudits. We identify avenues to further decrease errors in future work. Our results suggest that three-level trapped ion qudits will be a useful technology for quantum information processing.