We compute the electromagnetic fluctuations due to evanescent-wave Johnsonnoise in the vicinity of a thin conducting film, such as a metallic gate or a2-dimensional electron gas. This noise can decohere a nearby qubit and it isalso responsible for Casimir forces. We have improved on previous calculationsby including the nonlocal dielectric response of the film, which is animportant correction at short distances. Remarkably, the fluctuationsresponsible for decoherence of charge qubits from a thin film are greatlyenhanced over the case of a conducting half space. The decoherence times can bereduced by over an order of magnitude by decreasing the film thickness. Thisappears to be due to the leakage into the vacuum of modes that are welllocalized in the perpendicular direction. There is no corresponding effect forspin qubits (magnetic field fluctuations). We also show that a nonlocaldielectric function naturally removes the divergence in the Casimir force atvanishing separation between two metallic sheets or halfspaces. In theseparation regime where a local and nonlocal treatment are noticeably distinct,the Casimir attraction between two thin sheets and two halfspaces arepractically indistinguishable for any physical film thickness.
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