Resolving the Location of Parasitic Defects in Superconducting Qubits

New techniques to identify the location of decoherence-inducing material defects known as Two-Level-Tunneling systems (TLS) in superconducting qubits are demonstrated. We expose a transmon qubit circuit to a DC-electric field generated by electrodes surrounding the sample chip, and study the TLS response by monitoring their resonance frequencies using qubit swap spectroscopy. We find that about 50% of all detectable TLS do not couple to the applied E-field, as it is expected from TLS hosted in the Josephson junction tunnel barrier, but unlikely for TLS residing in surface oxides or at substrate interfaces. In contrast, all TLS respond to the mechanical strain generated by a piezo actuator. This indicates that surface TLS contribute about equally to qubit decoherence as those in qubit junctions. Moreover, by comparing measured and simulated coupling strengths to each DC-electrode, we obtain information about the possible locations and hosting interfaces of the observed surface TLS. This analysis directly indicates which circuit interfaces must be improved in order to enhance qubit coherence.