Developing a Large-Scale, Programmable Trapped Ion Quantum Simulator with In Situ Mid-Circuit Measurement and Reset

Abstract

Quantum simulators are a valuable resource for studying complex many-body systems. With their ability to provide near-term advantages, analog quantum simulators show great promise. In this thesis, my aim is to describe the construction of a large-scale trapped-ion based analog quantum simulator with several objectives in mind: controllability, minimal external decoherence, an expandable toolkit for quantum simulations, enhanced stability through robust design practices, and pushing the boundaries of error correction.

One of the techniques that significantly expands the range of simulations possible in trapped ions is site-selective mid-circuit measurements and resets. This technique enables the exploration of new classes of quantum simulations, including measurement-based phase transitions and reservoir engineering with controlled dissipation. Additionally, mid-circuit measurements play a crucial role and serve as one of the key factors in achieving error-correction in trapped ions. In this thesis, I address the challenges associated with implementing mid-circuit measurements and discuss strategies for achieving state-of-the-art fidelities in these processes within our simulator.

In this thesis, I present the results demonstrating the high-fidelity preservation of an "asset" ion qubit while simultaneously resetting or measuring a neighboring "process" qubit located a few microns away. The results show that we achieve a probability of accidental measurement of the asset qubit below 1×10⁻³ while resetting the process qubit. Similarly, when applying a detection beam on the same neighboring qubit to achieve fast detection times, the probability remains below 4×10⁻³ at a distance of 6 μm, which is four times the addressing Gaussian beam waist. These low probabilities correspond to the preservation of the quantum state of the asset qubit with fidelities above 99.9% for state reset and 99.6% for state measurement.

One of the many fascinating aspects of physics that can be explored through the use of site-selective mid-circuit resets is reservoir engineering. In this thesis, I discuss a protocol utilizing reservoir engineering to efficiently cool the spin state of a subsystem that is coupled to a reservoir with controlled dissipation. I demonstrate the effectiveness of this protocol through numerical simulations performed to optimize it for our experimental parameters. By leveraging the site-selective mid-circuit resets mentioned earlier, I successfully conduct a proof-of-principle dissipative many-body cooling experiment based on reservoir engineering. Through analog quantum simulation, I am able to demonstrate the lowering of energy within the system.

In addition to the above explorations, another important aspect of this thesis is the description of the design, fabrication, and assembly of a large-scale trapped ion quantum simulator called the Blade trap. I discuss the essential features of this system that include accommodating a significant number of ions, extending ion lifetimes and coherence times, enabling precise control of ion spins and interactions, implementing efficient detection methods, and incorporating mid-circuit measurement capabilities. I also provide a detailed account of our approach to optimizing the design and fabrication of the blade-trap system, covering the trap itself, mechanical components, system internals, optics, and the critical ultra-high vacuum (UHV) assembly. Furthermore, comprehensive testing procedures are presented to evaluate the performance and stability of the Blade trap.

I also describe the details about one of the significant challenges we faced in achieving and maintaining ultra-low pressure levels in the UHV system to ensure uninterrupted quantum simulation experiments with around 50 ions. High pressures can lead to detrimental collisions between ions and background atoms, necessitating re-trapping and introducing substantial overhead. Therefore, we aimed to achieve pressures in the range of 5×10⁻¹³ to 1×10⁻¹² mbar at room temperature, demanding meticulous optimization at the component level. Our journey to attain these pressures took approximately 2 to 3 years, complicated further by COVID-19 restrictions and equipment malfunctions. Despite the exhausting efforts, we successfully reached base pressures of less than 9×10⁻¹³ mbar in the vacuum chamber with partial internals. It is important to note that this measurement represents an upper limit due to gauge limitations.