Direct identification of dilute surface spins on Al$_2$O$_3$: Origin of   flux noise in quantum circuits
  It is universally accepted that noise and decoherence affecting the performance of superconducting quantum circuits are consistent with the presence of spurious two-level systems (TLS). In recent years bulk defects have been generally ruled out as the dominant source, and the search has focused on surfaces and interfaces. Despite a wide range of theoretical models and experimental efforts, the origin of these surface TLS still remains largely unknown, making further mitigation of TLS induced decoherence extremely challenging. Here we use a recently developed on-chip electron spin resonance (ESR) technique that allows us to detect spins with a very low surface coverage. We combine this technique with various surface treatments specifically to reveal the nature of native surface spins on Al$_2$O$_3$ -- the mainstay of almost all solid state quantum devices. On a large number of samples we resolve three ESR peaks with the measured total paramagnetic spin density $n=2.2\times 10^{17}$m$^{-2}$, which matches the density inferred from the flux noise in SQUIDs. We show that two of these peaks originate from physisorbed atomic hydrogen which appears on the surface as a by-product of water dissociation. We suggest that the third peak is due to molecular oxygen on the Al$_2$O$_3$ surface captured at strong Lewis base defect sites, producing charged O$_2^-$. These results provide important information towards the origin of charge and flux noise in quantum circuits. Our findings open up a whole new approach to identification and controlled reduction of paramagnetic sources of noise in solid state quantum devices. 
S. E. de Graaf, A. A. Adamyan, T. Lindstr\"om, D. Erts, S. E.   Kubatkin, A. Ya. Tzalenchuk, A. V. Danilov
quant-ph cond-mat.mes-hall