  The dominant contribution to the energy relaxation of state-of-the-art
superconducting qubits is often attributed to their coupling to an ensemble of
material defects which behave as two-level systems. These defects have varying
microscopic characteristics which result in a large range of observable defect
properties such as resonant frequencies, coherence times and coupling rates to
qubits $g$. Here, we investigate strategies to mitigate losses to the family of
defects that strongly couple to qubits ($g/2\pi\ge$ 0.5 MHz). Such strongly
coupled defects are particularly detrimental to the coherence of qubits and to
the fidelities of operations relying on frequency excursions, such as
flux-activated two-qubit gates. To assess their impact, we perform swap
spectroscopy on 92 frequency-tunable qubits and quantify the spectral density
of these strongly coupled modes. We show that the frequency configuration of
the defects is rearranged by warming up the sample to room temperature, whereas
the total number of defects on a processor tends to remain constant. We then
explore methods for fabricating qubits with a reduced number of strongly
coupled defect modes by systematically measuring their spectral density for
decreasing Josephson junction dimensions and for various surface cleaning
methods. Our results provide insights into the properties of strongly coupled
defect modes and show the benefits of minimizing Josephson junction dimensions
to improve qubit properties.
