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arXiv:0712.2834 · Physical Review Letters

Microscopic Origin of Low-Frequency Flux Noise in Josephson Circuits

Lara Faoro, L. B. Ioffe

Published Jun 5, 2008• 137 citations
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Abstract

We analyze the data and discuss their implications for the microscopic origin of the low-frequency flux noise in superconducting circuits. We argue that this noise is produced by spins at superconductor insulator boundary whose dynamics is due to RKKY interaction. We show that this mechanism explains size independence of the noise, different frequency dependences of the spectra reported in large and small SQUIDs, and gives the correct intensity for realistic parameters.

DOI
10.1103/physrevlett.100.227005

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We analyze the data and discuss their implications for the microscopic origin of the low-frequency flux noise in superconducting circuits. We argue that this noise is produced by spins at superconductor insulator boundary whose dynamics is due to RKKY interaction. We show that this mechanism explains size independence of the noise, different frequency dependences of the spectra reported in large and small SQUIDs, and gives the correct intensity for realistic parameters.
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The magnetic flux noise caused by surface spin fluctuations in superconducting quantum interference devices (SQUIDs) limits their development. In this work, we report that different adsorbents such as H, O 2, NO, and NO 2 that adsorb on the surfaces of Mg-based and Pb-based SQUIDs, respectively, producing large local magnetic moments ranging from 0.7–1.6 μ B, with energy barriers for thermal spin fluctuation as low as 10–30 mK. Moreover, we observe that the presence of H atoms on the surface of MgO can cause the coadsorption of other molecules, which generates additional spin sources. Monte Carlo simulations of the weakly coupled spin on a two-dimensional square lattice produce a low-frequency flux noise spectrum. We suggest eliminating the surface magnetism by coating the surface with monolayer indium phosphide or protecting the surface from other molecules by nonmagnetic preoccupants with a larger adsorption energy. The work provides important physical insights and feasible strategies for reducing magnetic noise sources in superconducting circuits.
We have studied low frequency (1/f) noise of YBa2Cu3O7−δ dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates. 1/f flux noise, either measured at different temperatures for optimized bias current or measured at 77 K for different bias currents, is almost constant. These facts imply that 1/f noise mainly comes from fluctuations of the critical current of the Josephson junction that form the SQUID. Also, we explain the critical current fluctuations in the junction by an equilibrium temperature fluctuation model.
The comprehensive analysis of low-frequency excess flux noise both in terms of magnetic flux noise SΦ,1/f and energy sensitivity ϵ1∕f of 84 superconducting quantum devices studied at temperatures below 1 K reveals a universal behavior. When analyzing data in terms of ϵ1∕f, we find that noise spectra of independent devices cross each other all at certain crossing frequencies fc. Besides this main result of our paper, we further show that superconducting quantum interference device (SQUID) arrays systematically feature higher noise exponents than single SQUIDs and give evidence for a material and device type dependence of low-frequency excess flux noise. The latter results facilitate to engineer the shape of magnetic flux noise spectra and thus to experimentally modify key properties such as coherence or measurement times of superconducting quantum devices.
The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is 1/f1/f1/f flux noise. To understand its origin and find ways to minimize its impact, we systematically study flux noise amplitudes in more than 50 flux qubits with varied superconducting quantum interference device (SQUID) geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.
The origin of magnetic flux noise in superconducting quantum interference devices with a power spectrum scaling as 1/f (f is frequency) has been a puzzle for over 20 years. This noise limits the decoherence time of superconducting qubits. A consensus has emerged that the noise arises from fluctuating spins of localized electrons with an areal density of 5x10(17) m(-2). We show that, in the presence of potential disorder at the metal-insulator interface, some of the metal-induced gap states become localized and produce local moments. A modest level of disorder yields the observed areal density.