Parasitic two-level tunneling systems (TLS) emerge in amorphous dielectricsand constitute a serious nuisance for various microfabricated devices, wherethey act as a source of noise and decoherence. Here, we demonstrate a new testbed for the study of TLS in various materials which provides access toproperties of individual TLS as well as their ensemble response. We terminate asuperconducting transmission-line resonator with a capacitor that hosts TLS inits dielectric. By tuning TLS via applied mechanical strain, we observe thesignatures of individual TLS strongly coupled to the resonator in itstransmission characteristics and extract the coupling components of theirdipole moments and energy relaxation rates. The strong and well-definedcoupling to the TLS bath results in pronounced resonator frequency fluctuationsand excess phase noise, through which we can study TLS ensemble effects such asspectral diffusion, and probe theoretical models of TLS interaction.
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