  Non-equilibrium quasiparticles are possible sources for decoherence in
superconducting qubits because they can lead to energy decay or dephasing upon
tunneling across Josephson junctions (JJs). Here, we investigate the impact of
the intrinsic properties of two-dimensional transmon qubits on quasiparticle
tunneling (QPT) and discuss how we can use quasiparticle dynamics to gain
critical information about the quality of JJ barrier. We find the tunneling
rate of the nonequilibrium quasiparticles to be sensitive to the choice of the
shunting capacitor material and their geometry in qubits. In some devices, we
observe an anomalous temperature dependence of the QPT rate below 100 mK that
deviates from a constant background associated with non-equilibrium
quasiparticles. We speculate that this behavior is caused by high transmission
sites/defects within the oxide barriers of the JJs, leading to spatially
localized subgap states. We model this by assuming that such defects generate
regions with a smaller effective gap. Our results present a unique in situ
characterization tool to assess the uniformity of tunnel barriers in qubit
junctions and shed light on how quasiparticles can interact with various
elements of the qubit circuit.
