Realizing a practical quantum computer with superconducting qubits requires substantially higher gate fidelities, which necessitates further improvements in the coherence of superconducting quantum circuits. Steady improvements have been made over the past two decades, including substantial exploration into energy relaxation mechanisms in superconducting thin films. However, a comprehensive and quantitative understanding of the relative impact of these mechanisms does not yet exist. In this thesis, I utilize a multimode approach to systematically characterize microwave losses in the quantum regime, with the goals of understanding relaxation-limiting loss mechanisms and improving device coherence through materials, process, and circuit design optimization. Using this approach, we measure significant reductions in surface losses by employing a tantalum-based materials platform, and in bulk substrate loss by utilizing high-temperature annealing processes. With this knowledge we predict and experimentally verify the relaxation times of aluminum and tantalum-based transmon qubits. We additionally optimize device geometry to maximize coherence within a coaxial tunnel architecture, and realize on-chip quantum memories with single-photon Ramsey times of 2.0-2.7 ms, limited by their energy relaxation times of 1.0-1.4 ms. This demonstrates an important link between microwave loss characterization and improving coherence in superconducting qubits, and enables a more modular and compact coaxial circuit architecture for bosonic qubits with reproducibly high coherence.
Thesis committee: Robert Schoelkopf (advisor), Michel Devoret, Leonid Glazman, Peter Rakich