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

Relaxation in quantum dots due to evanescent-wave Johnson noise from a metallic backgate