Correlated and uncorrelated parts of scalar fields in two-beam optical interferometry.

We show that in the interference of two partially correlated scalar light beams, the fields can be divided into parts that are mutually completely correlated (coherent) and parts that are fully uncorrelated with the correlated parts and with each other. Such correlated and uncorrelated parts cannot, in general, be unambiguously specified, but with a certain additional constraint, the partition becomes unique and can be determined. We demonstrate experimentally that the uncorrelated contribution can be physically isolated with the help of a spatial unitary transformation, such as a nonabsorbing beam splitter. Our findings constitute foundational results on optical two-beam interferometry.