  In many quantum computer architectures, the qubits are in close proximity to
metallic device elements. The fluctuating currents in the metal give rise to
noisy electromagnetic fields that leak out into the surrounding region. These
fields are known as evanescent-wave Johnson noise. The noise can decohere the
qubits. We present the general theory of this effect for charge qubits subject
to electric noise and for spin and magnetic qubits subject to magnetic noise. A
mapping of the quantum-mechanical problem onto a problem in classical
electrodynamics simplifies the calculations. The focus is on relatively simple
geometries in which analytical calculations can be done. New results are
presented for the local noise spectral density in the vicinity of cylindrical
conductors such as small antennae, noise from objects that can be treated as
dipoles, and noise correlation functions for several geometries. We summarize
the current state of the comparison of theory with experimental results on
decoherence times of qubits. Emphasis is placed on qualitative understanding of
the basic concepts and phenomena.
