  We compute the electromagnetic fluctuations due to evanescent-wave Johnson
noise in the vicinity of a thin conducting film, such as a metallic gate or a
2-dimensional electron gas. This noise can decohere a nearby qubit and it is
also responsible for Casimir forces. We have improved on previous calculations
by including the nonlocal dielectric response of the film, which is an
important correction at short distances. Remarkably, the fluctuations
responsible for decoherence of charge qubits from a thin film are greatly
enhanced over the case of a conducting half space. The decoherence times can be
reduced by over an order of magnitude by decreasing the film thickness. This
appears to be due to the leakage into the vacuum of modes that are well
localized in the perpendicular direction. There is no corresponding effect for
spin qubits (magnetic field fluctuations). We also show that a nonlocal
dielectric function naturally removes the divergence in the Casimir force at
vanishing separation between two metallic sheets or halfspaces. In the
separation regime where a local and nonlocal treatment are noticeably distinct,
the Casimir attraction between two thin sheets and two halfspaces are
practically indistinguishable for any physical film thickness.
