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 defect
s, 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.
The loss of amorphous hydrogenated silicon nitride (a-SiN$_{x}$:H) is measured at 30 mK and 5 GHz using a superconducting LC resonator down to energies where a single-photon is stored, and analyzed with an independent two-level system (TLS) defect mo
del. Each a-SiN$_{x}$:H film was deposited with different concentrations of hydrogen impurities. We find that quantum-regime dielectric loss tangent $tandelta_{0}$ in a-SiN$_{x}$:H is strongly correlated with N-H impurities, including NH$_{2}$. By slightly reducing $x$ we are able to reduce $tandelta_0$ by approximately a factor of 50, where the best films show $tandelta_0$ $simeq$ 3 $times$ 10$^{-5}$.
