  Superconducting integrated circuits have demonstrated a tremendous potential
to realize integrated quantum computing processors. However, the downside of
the solid-state approach is that superconducting qubits suffer strongly from
energy dissipation and environmental fluctuations caused by atomic-scale
defects in device materials. Further progress towards upscaled quantum
processors will require improvements in device fabrication techniques which
need to be guided by novel analysis methods to understand and prevent
mechanisms of defect formation. Here, we present a new technique to analyse
individual defects in superconducting qubits by tuning them with applied
electric fields. This provides a new spectroscopy method to extract the
defects' energy distribution, electric dipole moments, and coherence times.
Moreover, it enables one to distinguish defects residing in Josephson junction
tunnel barriers from those at circuit interfaces. We find that defects at
circuit interfaces are responsible for about 60% of the dielectric loss in the
investigated transmon qubit sample. About 40% of all detected defects are
contained in the tunnel barriers of the large-area parasitic Josephson
junctions that occur collaterally in shadow evaporation, and only about 3% are
identified as strongly coupled defects which presumably reside in the
small-area qubit tunnel junctions. The demonstrated technique provides a
valuable tool to assess the decoherence sources related to circuit interfaces
and to tunnel junctions that is readily applicable to standard qubit samples.
