Direct identification of dilute surface spins on Al₂O₃: 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 Al₂O₃ - 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=2.2times 10¹⁷m^-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 Al₂O₃ surface captured at strong Lewis base defect sites, producing charged O₂^-. 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.