Observation of two-level systems with a giant dipole moment in superconducting granular aluminum films

Thin films of disordered superconductors are currently extensively studied, for instance due to their applicability in modern quantum circuits or kinetic inductance detectors. While a variety of materials are possible, especially the ones with low microwave loss and a high degree of disorder are of interest for high impedance circuits. The disordered microscopic structure also favors the presence of intrinsic material defects, with some of them behaving as two-level systems (TLS). Found in dielectrics like surface oxides or tunnelling barriers, TLS are a major source of electromagnetic loss and limit the coherence of superconducting qubits.

We present microwave spectroscopy measurements of ultra-compact, high impedance resonators made from granular aluminum films. By applying mechanical strain and electric fields, we observe numerous TLS strongly interacting with the resonator modes. Our analysis of the measured data shows the presence of TLS with a wide distribution of electrical dipole moments. In comparison with similar TLS studies on low impedance superconducting resonators, we observe in addition to the common surface TLS, also a variety of TLS with an unusual, order of magnitude larger dipole moments.