Heralded single photon partial coherence

Optical spatial coherence, characterizing correlations between optical fields at distinct points, plays a key role in the understanding of classical and quantum phenomena. Recent decades´ technological advances, such as powerful lasers and fast, efficient photon counters enabled the study of nonclassical optical fields. Here, we describe single photon partial coherence, providing a theoretical model from quantum coherence theory and experimental observation. In particular, we make clear the role of quantum birth-zones for SPDC sources. These birth-zones are the quantum analog of classical emitters; they characterize the heralded single photon´s coherence properties. We make the first observation of the predicted inversion of the single photon degree of coherence.