  We propose and study a realistic model for the decoherence of topological
qubits, based on Majorana fermions in one-dimensional topological
superconductors. The source of decoherence is the fluctuating charge on a
capacitively coupled gate, modeled by non-interacting electrons. In this
context, we clarify the role of quantum fluctuations and thermal fluctuations
and find that quantum fluctuations do not lead to decoherence, while thermal
fluctuations do. We explicitly calculate decay times due to thermal noise and
give conditions for the gap size in the topological superconductor and the gate
temperature. Based on this result, we provide simple rules for gate geometries
and materials optimized for reducing the negative effect of thermal charge
fluctuations on the gate.
