Relevance of sub-surface chip layers for the lifetime ofmagnetically trapped atoms

We investigate the lifetime of magnetically trappedatoms above a planar, layered atom chip structure. Numericalcalculations of the thermal magnetic noise spectrum are performed,based on the exact magnetic Green function and multi layerreflection coefficients. We have performed lifetime measurementswhere the center of a side guide trap is laterally shifted withrespect to the current carrying wire using additional bias fields.Comparing the experiment to theory, we find a fair agreement anddemonstrate that for a chip whose topmost layer is metallic, themagnetic noise depends essentially on the thickness of that layer,as long as the layers below have a much smaller conductivity;essentially the same magnetic noise would be obtained with ametallic membrane suspended in vacuum. Based on our theory wegive general scaling laws of how to reduce the effect of surfacemagnetic noise on the trapped atoms.