Superconducting circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the coherence of superconducting quantum bits is severely reduced by atomic-scale material defects which provide a bath of parasitic two-state tunnelling systems, so-called TLS.

We review experiments where a qubit is operated as a sensitive detector to probe the quantum properties of individual TLS defects while they are tuned via applied mechanical strain and electric fields. This provides expressive spectroscopic data revealing coherent TLS interactions and the fluctuation dynamics of thermally activated TLS at low energies.

A new method is presented to determine whether a given defects resides within the tunnel barrier of a qubit’s Josephson junctions or on an interface of the qubit electrodes. Using spatially tailored electric fields, we can also distinguish on which thin-film circuit interface a defect is located. These techniques yield valuable information to guide improvements in sample fabrication that are urgently needed to obtain higher coherence in qubits and other micro-fabricated quantum devices by learning how to avoid the formation of material defects.