  We present our study of decoherence in charge (spin) qubits due to
evanescent-wave Johnson noise (EWJN) in a laterally coupled double quantum dot
(single quantum dot). The high density of evanescent modes in the vicinity of
metallic gates causes energy relaxation and a loss of phase coherence of
electrons trapped in quantum dots. We derive expressions for the resultant
energy relaxation rates of charge and spin qubits in a variety of dot
geometries, and EWJN is shown to be a dominant source of decoherence for spin
qubits held at low magnetic fields. Previous studies in this field approximated
the charge or spin qubit as a point dipole. Ignoring the finite size of the
quantum dot in this way leads to a spurious divergence in the relaxation rate
as the qubit approaches the metal. Our approach goes beyond the dipole
approximation and remedies this unphysical divergence by taking into account
the finite size of the quantum dot. Additionally, we derive an enhancement of
EWJN that occurs outside a thin metallic film, relative to the field
surrounding a conducting half-space.
