  Single photon first order interferences of spatially separated regions from
the cone structure of spontaneous parametric down conversion allow for
analyzing the role of the mode function in quantum optics. In earlier
experiments the role of the vacuum fields could be demonstrated in induced
coherence experiments as the source of complementarity \cite{Heu14}. Here the
spatial coherence properties of these vacuum fields are measured and as the
physical reason for complementarity in single photon quantum optics
demonstrated.
