Quantum Circuit Studies with Two-Level Defects of Aluminum Oxide in a Polycrystalline Phase, Amorphous Phase, and at a Metal Surface

Abstract

This thesis reports on recent achievements toward understanding the nanoscale two-level systems (TLS) within aluminum oxide layers. I will discuss novel experimental and theoretical methods using superconducting resonator data to characterize the TLSs, which are deleterious to qubit coherence. This includes (1) a traditional power dependent loss, which provides the information of collective TLS effects, (2) spectroscopy of individual TLSs by DC-tuning, and (3) two-tone spectroscopy of ensemble TLSs by a second saturation tone. We find that the behaviors of TLSs in different structural phases have distinguishing features. Utilizing the DC-tuning feature of our sensor, we further extract dipole moments from individual TLSs and provide the moment histograms of the two aluminum oxide film types. We observe polycrystalline oxide has an average dipole moment = 2.6 Debye and a single-peak histogram consistent with a single TLS origin. On the other hand, TLSs in amorphous oxide have a wide spread of dipole moment values probably due to oxygen deficiency. Saturation slopes of TLSs in bulk films (polycrystalline and amorphous phases) show a square root dependence of power indicating an ignorable TLS-TLS interaction. Moreover, TLSs in the polycrystalline phase are more stable in the time domain than TLSs in the amorphous phase. Unlike the previous two bulk TLSs, TLSs at the metal-air interface require an explanation from the model assuming TLS frequencies are under stochastic fluctuations originating from TLS-TLS interaction since we find a weak power dependence. We also demonstrate the first published transmon qubits which are solely made from optical lithography. They have a comparable relaxation time and junction resistance to those made from e-beam lithography.