Localization of Metal-Induced Gap States at the Metal-Insulator Interface: Origin of Flux Noise in SQUIDs and Superconducting Qubits

ORAL

Abstract

The origin of magnetic flux noise in dc Superconducting Quantum Interference Devices (SQUIDs) with a power spectrum scaling as 1/$f$ ($f$ is frequency) has been a puzzle for over 25 years. This noise limits both the low frequency performance of SQUIDs and the decoherence time of flux-sensitive superconducting qubits, making scaling-up for quantum computing problematic. Recent calculations and experiments indicate that the noise is generated by electrons that randomly reverse their spin directions. Their areal density of $\sim $ 5 $\times $ 10$^{17}$ m$^{-2}$ is relatively insensitive to the nature of the superconductor and substrate. Here, we propose that the local magnetic moments originate in metal-induced gap states (MIGSs) localized by potential disorder at the metal-insulator interface. MIGSs are particularly sensitive to such disorder, so that the localized states have a Coulomb repulsion sufficiently large to make them singly occupied. Our calculations demonstrate that a modest level of disorder generates the required areal density of localized moments. This result suggests that magnetic flux noise could be reduced by fabricating superconductor-insulator interfaces with less disorder. Support: NSF DMR07-05941, US DOE De-AC02-05CH11231, Samsung Foundation, Teragrid, NERSC.

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Authors

  • Sangkook Choi

    University of California at Berkeley and Lawrence Berkeley National Lab

  • Dung-Hai Lee

    University of California at Berkeley and Lawrence Berkeley National Lab

  • Steven G. Louie

    UC Berkeley and Lawrence Berkeley National Laboratory, Department of Physics, University of California, Berkeley. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, University of California at Berkeley and Lawrence Berkeley National Lab, University of California at Berkeley and Lawrence Berkeley National Laboratory, University of California, Berkeley and Lawrence Berkeley National Laboratory, UC Berkeley, Department of Physics, University of California, Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, UC Berkeley and Lawrence Berkeley National Lab, UC Berkeley and LBNL, Department of Physics, University of California, Berkeley, and Materials Science Division, Lawrence Berkeley National Laboratory, Dept. of Physics, University of California Berkeley and The Molecular Foundry, LBNL, Department of Physics, University of California at Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

  • John Clarke

    University of California at Berkeley and Lawrence Berkeley National Lab, University of California, Berkeley, UC Berkeley, Dept. of Physics, UC Berkeley, Department of Physics, University of California Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, Department of Physics, University of California, Berkeley, CA 94720