  Parasitic two-level tunneling systems (TLS) emerge in amorphous dielectrics
and constitute a serious nuisance for various microfabricated devices, where
they act as a source of noise and decoherence. Here, we demonstrate a new test
bed for the study of TLS in various materials which provides access to
properties of individual TLS as well as their ensemble response. We terminate a
superconducting transmission-line resonator with a capacitor that hosts TLS in
its dielectric. By tuning TLS via applied mechanical strain, we observe the
signatures of individual TLS strongly coupled to the resonator in its
transmission characteristics and extract the coupling components of their
dipole moments and energy relaxation rates. The strong and well-defined
coupling to the TLS bath results in pronounced resonator frequency fluctuations
and excess phase noise, through which we can study TLS ensemble effects such as
spectral diffusion, and probe theoretical models of TLS interaction.
