Direct observation of microscopic tow-level systems in granular aluminum films

Thin films of disordered superconductors are extensively studied due to their applicability in quantum circuits and detectors, where they can provide kinetic inductances orders of magnitude higher than their geometric inductance. While these films generally show very high quality factors in microwave measurements, their disordered microscopic structure favors the presence of numerous material defects behaving as two-level systems (TLS). TLS have been shown to be a major source of dielectric loss in microwave circuits and are limiting the coherence properties of modern superconducting qubits.
We present microwave spectroscopy measurements of resonators made from highly resistive granular aluminum films. By applying mechanical strain and electric fields, we observe several TLS strongly interacting with the resonator modes. We compare the measured data to an analytical model for the single-photon interaction with TLS and estimate relevant physical properties.