Probing hundreds of individual quantum defects in polycrystalline and amorphous alumina

Two-level systems (TLSs) are present in the materials of qubits and when they exchange energy with the qubit they also limit their coherence. TLSs are described by two energy terms, tunneling and asymmetry energy, and an electrical dipole moment which couples to the ac-mode of quantum systems. There are several strategies to reduce TLS density: material optimization, surface treatments, etc. However, TLSs are not generally understood microscopically -- either structurally or in atomic composition.

 

We use resonator circuit that has a strong coupling strength, g, to TLSs and applies a uniform dc field, which we have designated an Electrical-Bridge Quantum-Defect Sensor (EBQuDS). The eletrical field of the EBQuDS is applied directly on TLS-hosting capacitors, and this degree of freedom only slightly perturbs the quality factor of the resonator. Previous studies with a weaker coupling g circuit allowed extraction of the average projected dipole moment of the TLSs using a finite sweep-rate (< few MHz) of the ac voltage. However, the method involves scaling of the data and we cannot observe TLSs individually. In comparison, in the EBQuDs (where we quasi-statically tune the dc-voltage), we can accurately extract individual TLSs' dipole moments. In this talk I will mainly present new results on the latter circuit and on two structural phases of aluminum oxide: polycrystalline and amorphous. From these films we extract the dipole moment of hundreds of TLSs and we find that polycrystalline aluminum oxide hosts a smaller average dipole moment than amorphous. The material distribution of the polycrystalline dipole moments fit to a Gaussian function with a mean value of 2.6 Debye (0.54 e-Angstrom) and the standard deviation is 1.6 Debye. This allows a comparison to recent theoretical proposals of H- and O-based TLSs.