  The anomalous low-temperature properties of glasses arise from intrinsic
excitable entities, so-called tunneling Two-Level-Systems (TLS), whose
microscopic nature has been baffling solid-state physicists for decades. TLS
have become particularly important for micro-fabricated quantum devices such as
superconducting qubits, where they are a major source of decoherence. Here, we
present a method to characterize individual TLS in virtually arbitrary
materials deposited as thin-films. The material is used as the dielectric in a
capacitor that shunts the Josephson junction of a superconducting qubit. In
such a hybrid quantum system the qubit serves as an interface to detect and
control individual TLS. We demonstrate spectroscopic measurements of TLS
resonances, evaluate their coupling to applied strain and DC-electric fields,
and find evidence of strong interaction between coherent TLS in the sample
material. Our approach opens avenues for quantum material spectroscopy to
investigate the structure of tunneling defects and to develop low-loss
dielectrics that are urgently required for the advancement of superconducting
quantum computers.
