  Parasitic two-level tunneling systems originating from structural material
defects affect the functionality of various microfabricated devices by acting
as a source of noise. In particular, superconducting quantum bits may be
sensitive to even single defects when these reside in the tunnel barrier of the
qubit's Josephson junctions, and this can be exploited to observe and
manipulate the quantum states of individual tunneling systems.
  Here, we detect and fully characterize a system of two strongly interacting
defects using a novel technique for high-resolution spectroscopy. Mutual defect
coupling has been conjectured to explain various anomalies of glasses, and was
recently suggested as the origin of low frequency noise in superconducting
devices. Our study provides conclusive evidence of defect interactions with
full access to the individual constituents, demonstrating the potential of
superconducting qubits for studying material defects. All our observations are
consistent with the assumption that defects are generated by atomic tunneling.
