  The performance of modern quantum devices in communication, metrology or
microscopy relies on the quantum-classical interaction which is generally
described by the theory of decoherence. Despite the high relevance for long
coherence times in quantum electronics, decoherence mechanisms mediated by the
Coulomb force are not well understood yet and several competing theoretical
models exist. Here, we present an experimental study of the Coulomb-induced
decoherence of free electrons in a superposition state in a biprism electron
interferometer close to a semiconducting and metallic surface. The decoherence
was determined through a contrast loss at different beam path separations,
surface distances and conductibilities. To clarify the current literature
discussion, four theoretical models were compared to our data. We could rule
out three of them and got good agreement with a theory based on macroscopic
quantum electrodynamics. The results will enable the determination and
minimization of specific decoherence channels in the design of novel quantum
instruments.
