Here we study the loss of spatial coherence in the extended wave function of fullerenes due to collisions with background gases. From the gradual suppression of quantum interference with increasing gas pressure we can confirm the predictions of decoherence theory and our picture of the interaction process. The matter-wave fringe contrast decays exponentially with increasing residual gas pressure. We explore the practical limits of matter wave interferometry at finite gas pressures and estimate the required experimental conditions for interferometry with large objects.
The vanishing of the interference contrast can be understood using three different pictures which all lead to the same quantiative prediction - in agreement with our observations: