Bulk and surface tunneling hydrogen defects in alumina

Area of Science:

• Quantum computing materials science
• Solid-state physics and chemistry

Background:

• Superconducting qubits are sensitive to noise from defects.
• Two-level systems (TLS) are a major source of decoherence in superconducting quantum bits.
• Alumina is a common dielectric material used in superconducting devices.

Purpose of the Study:

• To identify hydrogen-based defects in alumina that cause deleterious two-level systems (TLS).
• To compute the formation energies, dipole moments, and tunneling energies of these defects.
• To predict the density and frequency of TLS originating from these defects.

Main Methods:

• Ab initio calculations were used to model hydrogen defects in alumina.
• Formation energies were computed for Al-rich and O-rich environments.
• Potential energy surfaces and dipole moments were evaluated.
• Tunneling energies for hydrogen and deuterium defects were calculated.

Main Results:

• Analyzed bulk hydrogenated Al vacancies, interstitial defects, and surface OH rotors.
• Computed formation energies indicate likelihood of defect occurrence.
• Evaluated dipole moments to determine coupling to electric fields.
• Predicted hydrogenated cation vacancy defects generate GHz frequency TLSs.

Conclusions:

• Hydrogenated cation vacancy defects are a significant source of GHz frequency TLSs in alumina.
• These TLS can negatively impact the performance of superconducting qubits.
• Understanding and mitigating these defects is crucial for advancing quantum computing.