Probing hundreds of individual quantum defects in polycrystalline and amorphous alumina

Abstract

Quantum two-level systems (TLSs) are present in the materials of qubits and are considered defects because they limit qubit coherence. For superconducting qubits, the quintessential Josephson junction barrier is made of amorphous alumina, which hosts TLSs. However, TLSs are not understood generally -- either structurally or in atomic composition. In this study, we greatly extend the quantitative data available on TLSs by reporting on the physical dipole moment in two alumina types: polycrystalline ${\gamma -\mathrm{Al}}_2{\mathrm{O}}_3$ and amorphous $\mathrm{a}-\mathrm{Al}{\mathrm{O}}_{\mathrm{x}}$. To obtain the dipole moments $p_z$, rather from the less-structural coupling parameter g, we tune individual TLSs with an external electric field to extract the $p_z$ of the TLSs in a cavity QED system. We find a clear difference in the dipole moment distribution from the film types, indicating a difference in TLS structures. A large sample of approximately 400 individual TLSs are analyzed from the polycrystalline film type. Their dipoles along the growth direction $p_z$ have a mean value of 2.6$\pm$0.3 Debye (D) and standard deviation $\sigma$ = 1.6$\pm$0.2 D . The material distribution fits well to a single Gaussian function. Approximately 200 individual TLSs are analyzed from amorphous films. Both the mean $p_z$ =4.6$\pm$0.5 D and $\sigma$ =2.5$\pm$0.3 D are larger. Amorphous alumina also has some very large $p_z$, > 8.6 D, in contrast to polycrystalline which has none of this moment. These large moments agree only with oxygen-based TLS models. Based on data and the candidate models (delocalized O and hydrogen-based TLSs), we find polycrystalline alumina has smaller ratio of O-based to H-based TLS than amorphous alumina.

14 pages, 13 figures

References in corpus (13)

• Supplementary information for "Quantum supremacy using a programmable superconducting processor"
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• Microwave Dielectric Loss at Single Photon Energies and milliKelvin Temperatures
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• Generalized Tunneling Model for TLS in amorphous materials and its predictions for their dephasing and the noise in superconducting microresonators
• Measuring the temperature dependence of individual two-level systems by direct coherent control
• Delocalised oxygen as the origin of two-level defects in Josephson junctions
• Quantum Sensors for Microscopic Tunneling Systems
• Identification of structural motifs as tunneling two-level systems in amorphous alumina at low temperatures
• Transmission-line resonators for the study of individual two-level tunneling systems
• An argon ion beam milling process for native ${\text{AlO}}_{\text{x}}$ layers enabling coherent superconducting contacts
• Structural and Nanochemical Properties of AlOx Layers in $Al/Al{O}_x/Al$-Layer Systems for Josephson Junctions
• Atomic delocalisation as a microscopic origin of two-level defects in Josephson junctions

Cited by in corpus (12)

• Quantum bath suppression in a superconducting circuit by immersion cooling
• Enhancing the Coherence of Superconducting Quantum Bits with Electric Fields
• Infrared spectra in amorphous alumina: a combined ab initio and experimental study
• Modelling dielectric loss in superconducting resonators: Evidence for interacting atomic two-level systems at the Nb/oxide interface
• Compact vacuum gap transmon qubits: Selective and sensitive probes for superconductor surface losses
• Simulating noise on a quantum processor: interactions between a qubit and resonant two-level system bath
• Experimentally revealing anomalously large dipoles in a quantum-circuit dielectric
• Observation of discrete charge states of a coherent two-level system in a superconducting qubit
• Anomalous Loss Reduction Below Two-Level System Saturation in Aluminum Superconducting Resonators
• Quantum defects from single surface exhibit strong mutual interactions
• Identification of soft modes in amorphous Al${}_2$O${}_3$ via first-principles
• Non-equilibrium Dynamics of Two-level Systems directly after Cryogenic Alternating Bias