Probing hundreds of individual two-level defects in polycrystalline and amorphous alumina.

Quantum two-level systems (TLSs) appear in the dielectrics of superconducting qubits and parasitically limit the qubit coherence time. The Josephson junction barrier, amorphous alumina, is known to host TLSs similar to other amorphous films, but TLSs are not microscopically understood in atomic composition. In this study we extend the quantitative data available on TLSs by using a superconducting resonator to characterize two alumina film types: polycrystalline and amorphous. We find a clear difference between the structure types when analyzing TLSs. A large sample of ~ 400 individual TLSs is analyzed from polycrystalline films. Their dipole moments directed vertically have a mean value of 2.6(0) Debye and a standard deviation of 1.6(5) Debye. They fit well to a single Gaussian distribution indicating a single defect type. In contrast, the amorphous film contains TLSs with the larger dipole moments but only few are observed due to larger frequency noise.