Mitigating losses of superconducting qubits coupled strongly to defect modes

Energy relaxation in superconducting circuits is typically attributed to the coupling of qubits to a bath of material defects, which vary in nature, location and time. The defect modes show a large range of coherence times and coupling rates to qubits. Here, we investigate strategies to mitigate losses to the defects that couple strongly to the qubits, as they are particularly detrimental to the fidelities of operations relying on frequency excursions, such as the two-qubit controlled-phase gates. We report on the time and thermal-cycling dynamics of defect-mode configurations tracked for over 400 days in a single device. We also explore methods for fabricating qubits with a reduced number of strongly-coupled defect modes by changing the Josephson junction dimensions and the surface cleaning methods employed. Our results provide new insights into the properties of strongly-coupled defect modes and on strategies to mitigate loss of qubit coherence induced by those defects. The gained insights are invaluable for scaling up the number of qubits realized on devices for quantum information processing.