A versatile Hong-Ou-Mandel interference experiment in optical fiber for the undergraduate laboratory

Editor's Note: The Hong-Ou-Mandel (HOM) experiment provides an intuitive demonstration of quantum interference and particle indistinguishability. However, the original free-space version of this experiment is notoriously difficult to align, requiring excellent spatial and temporal overlap of two identical photons. This paper presents a simplified version of the HOM experiment that is suitable for the undergraduate instructional laboratory, enabling students to gather data on the signature “HOM dip.” The use of a commercial photon-pair source and fused-fiber coupler greatly reduces the required optical alignments, while students gain hands-on experience by constructing a free-space propagation section. This versatile system allows students to explore the consequences of degrading the indistinguishability of the two interfering photons through manipulating their path length differences, by introducing slightly nondegenerate frequencies, and varying polarizations. Typical experimental results are presented, along with the theory needed to analyze those data.