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

This article focuses on the development and implementation of a simplified, fiber-optic-based apparatus for demonstrating Hong–Ou–Mandel (HOM) interference in an undergraduate instructional laboratory. The apparatus replaces the traditional free-space beamsplitter with a fused-fiber coupler and uses a commercially available fiber-coupled biphoton source based on spontaneous parametric down conversion (SPDC) in a periodically poled potassium titanyl phosphate (PPKTP) crystal, enabling easier alignment and temperature-controlled tuning of photon frequency degeneracy. Experimental results show the characteristic HOM dip in coincidence counts as a function of optical path length difference and crystal temperature, illustrating quantum interference and photon indistinguishability with improved coherence length and interference visibility. This setup balances pre-aligned fiber components with student-aligned free-space sections, providing hands-on experience while reducing complexity, and offers opportunities to explore additional quantum optics phenomena such as coherence length and polarization effects.