Bulk and Surface Tunneling Hydrogen Defects in Alumina

TL;DR

This study uses ab initio calculations to analyze hydrogen-related tunneling defects in alumina, identifying potential two-level systems that could impact superconducting qubit performance.

Contribution

It provides a detailed computational analysis of hydrogen defects in alumina, predicting their formation likelihood and their role as GHz-frequency TLSs affecting qubit coherence.

Findings

Hydrogenated cation vacancy defects are likely to form significant GHz-frequency TLSs.

The defects exhibit specific tunneling energies and dipole moments relevant for qubit decoherence.

Formation energies suggest defect occurrence depends on growth environment.

Abstract

We perform ab initio calculations of hydrogen-based tunneling defects in alumina to identify deleterious two-level systems (TLS) in superconducting qubits. The defects analyzed include bulk hydrogenated Al vacancies, bulk hydrogen interstitial defects, and a surface OH rotor. The formation energies of the defects are first computed for an Al- and O-rich environment to give the likelihood of defect occurrence during growth. The potential energy surfaces are then computed and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. Finally, the tunneling energy is computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz-frequency TLSs in alumina.