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.