Controlled decoherence of free electrons due to Coulomb interaction with a truly macroscopic environment, the electron (and phonon) gas inside a semiconducting plate, is studied experimentally. The quantitative results are compared with different theoretical models. The experiment confirms the main features of the theory of decoherence and can be interpreted in terms of which-path information. In contrast to previous model experiments on decoherence, the obtained interferograms directly visualize the transition from quantum to classical.

An experiment in which decoherence, i.e. the transition from quantum to classical behaviour, can be studied in detail was proposed by Anglin and Zurek [1] and has now been realized. An electron beam in a biprism interferometer is split into two parts both of which travel over a plate made of a highly resistive material at the same, small height. The induced charges inside the plate move along with the beam electron, therefore a current results which encounters ohmic resistance. This process leads to a disturbance in the electron and phonon gas in the plate. As this disturbance is different for the two parts of the beam, entanglement between beam electron and plate is formed. The strength of decoherence, represented by the visibility of the interference fringes, varies as a function of two parameters, the height above the plate and the lateral separation of the beams. Allowing electrons of different height to reach the fluorescent screen successively, 'photos' of the quantum-classical border (continuous decrease of contrast with decreasing height above the plate) are built up.

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