Spatial coherence effects on second- and fourth-order temporal interference

TL;DR

This paper investigates how spatial coherence influences second- and fourth-order temporal interference using two-photon light in a Mach-Zehnder interferometer, revealing that second-order interference depends on spatial coherence while fourth-order does not.

Contribution

The study demonstrates experimentally and theoretically that spatial coherence affects second-order but not fourth-order interference in two-photon experiments.

Findings

High-visibility fringes observed for both second- and fourth-order interferograms.

Removing a mirror affects second-order but not fourth-order interference.

A model incorporating spatial and temporal degrees-of-freedom explains the results.

Abstract

We report the results of two experiments performed with two-photon light, produced via collinear degenerate optical spontaneous parametric downconversion (SPDC), in which both second-order (one-photon) and fourth-order (two-photon) interferograms are recorded in a Mach-Zehnder interferometer (MZI). In the first experiment, high-visibility fringes are obtained for both the second- and fourth-order interferograms. In the second experiment, the MZI is modified by the removal of a mirror from one of its arms; this leaves the fourth-order interferogram unchanged, but extinguishes the second-order interferogram. A theoretical model that takes into consideration both the temporal and spatial degrees-of-freedom of the two-photon state successfully explains the results. While the temporal interference in the MZI is independent of the spatial coherence of the source, that of the modified MZI is not.