Origin of flux noise in superconducting devices: Size distribution of spin species from phonon-assisted spin resonance

While it is universally accepted that the origin of flux noise in superconducting circuits is due to spin impurities at surfaces and interfaces, the identity of the spins remains a mistery. On-chip spin resonance experiments were able to identify atoms adsorbed to the devices' surfaces, but a broad absorption background of spin impurities remained unidentified [1]. Here we show that this background can be ascribed to phonon-assisted microwave absorption, and argue that the asymmetry of the one-phonon peak provides information on the size distribution of the spin-impurity wavefunction.

[1] S. E. de Graaf, A. A. Adamyan, T. Lindström, D. Erts, S. E. Kubatkin, A. Ya. Tzalenchuk, and A. V. Danilov, Phys. Rev. Lett. 118, 057703 (2017).