Direct identification of dilute surface spins on Al${}_2$O${}_3$: Origin of flux noise in quantum circuits

Abstract

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.

6 pages, 2 figures (supplementary material 6 pages, 6 figures)

References in corpus (8)

• Strong Coupling of a Spin Ensemble to a Superconducting Resonator
• Experimental evidence for a surface distribution of two-level systems in superconducting lithographed microwave resonators
• Model for l/f Flux Noise in SQUIDs and Qubits
• Magnetism in SQUIDs at Millikelvin Temperatures
• Microscopic origin of low frequency flux noise in Josephson circuits
• Observation of classical-quantum crossover of 1/f flux noise and its paramagnetic temperature dependence
• Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory
• Generalized Tunneling Model for TLS in amorphous materials and its predictions for their dephasing and the noise in superconducting microresonators

Cited by in corpus (2)

• Observation of classical-quantum crossover of 1/f flux noise and its paramagnetic temperature dependence
• Inductive-detection electron-spin resonance spectroscopy with $65$spins$/\sqrt{\text{Hz}}$ sensitivity