Hamiltonian and materials engineering for superconducting qubit lifetime enhancement

Abstract

The potential for quantum computing to expand the number of solvable problems has driven researchers across academia and industry, in multiple disciplines, to develop a variety of different qubit platforms, algorithms, and scaling strategies. At its core, quantum computation relies on the robustness, or coherence, of its building blocks (``qubits"). In current small-scale superconducting qubit processors, the fidelity of operations is often limited by qubit coherence. The coherence time of a single qubit depends on its lifetime $T_1$ and pure dephasing time $T_{\phi}$. In this thesis, we focus on the problem of improving $T_1$. Strategies for improving lifetimes are informed by models for relaxation - specifically Fermi's Golden Rule. Relaxation rates depend on noise properties of the environment and on properties of the qubit states. This dependence suggests two strategies for engineering longer lifetimes: environment engineering involves mitigating or filtering the noise that the qubit sees, and Hamiltonian engineering refers to optimizing the qubit circuit and its resulting eigenstates to optimize $T_1$. Significant enhancements of qubit lifetimes will require paradigm shifts in our approaches to both environment and Hamiltonian engineering. First, I present a side-by-side study of transmon coherence and materials measurements of the constituent Nb films, including synchrotron x-ray spectroscopy and electron microscopy. We found correlations between qubit lifetimes and materials properties such as grain size, grain boundary quality, and surface suboxides. This study expands the scope of superconducting qubit research by presenting a broad set of materials analyses alongside device measurements. Second, I will give an overview of Hamiltonian engineering, including the concepts behind intrinsic protection against relaxation and dephasing processes. I'll describe the soft $\mathrm{0-\pi}$ qubit, which is the first experimentally realized superconducting qubit to show signatures of simultaneous $T_1$ and $T_2$ protection. We improved coherence in the soft $\mathrm{0-\pi}$ through optimized fabrication processes. We have also characterized the effects of non-computational levels on gate fidelity, specifically AC Stark shifts and leakage. From the results in this thesis, we have gained a deeper understanding of what limits qubit coherence, informing future directions on both the materials and Hamiltonian engineering fronts.