Emergent Dynamics of Noise and Loss-Generating Paramagnetic Spins in Superconducting Circuits*

The emergent mesoscale dynamics of noise and loss-generating degrees of freedom in superconducting qubits, such as surface adsorbates, defects, and impurities, ultimately determine the dynamics of the qubit. Paramagnetic O₂ has been identified experimentally as a likely flux noise source [Phys. Rev. Applied 6, 041001 (2016)], as well as atomic hydrogen [Phys. Rev. Lett. 118, 057703 (2017)]. In addition, computational studies [PRL 112, 017001 (2014)] of magnetic spins induced by molecules adsorbed on bare Al-terminated Al₂O₃ demonstrated the possibility of nearly degenerate adsorbate magnetic states. However, how these spins interact in real materials requires further theoretical study. We present results from Monte Carlo and Landau-Lifshitz-Gilbert equation simulations of spin systems relevant to superconducting qubits and resonators, as a function of applied field, temperature, and O₂ coverage. The spin anisotropy and interactions in these models are parameterized with density functional theory calculations of specific defects. The phase diagram of the system, spin vortices, spin-spin correlations, and 1/w^α flux noise will be discussed, as well as the integration of these results into macroscopic qubit device models.