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

Hong–Ou–Mandel (HOM) interference is a quantum optics laboratory experiment that has recently become more accessible to undergraduate students. The experiment consists of two identical photons simultaneously entering a nonpolarizing beamsplitter, where the photon wavefunctions interfere. As a result, the photon pair exits from the same beamsplitter output, whereas classically, the two photons are equally likely to exit from the two different outputs. This effect is demonstrated by observation of a dip in coincidence counts measured between the two beamsplitter outputs. Due to the precision needed to achieve photon indistinguishability, the setup and alignment of this experiment is often considered to be too difficult and time consuming to be appropriate for the undergraduate instructional laboratory. Here, we present an alternative optical-fiber-based apparatus that gives a consistently reproducible experiment. In our approach, quantum interference occurs within a fused-fiber coupler, instead of within a traditional beamsplitter. We use a commercially available fiber-coupled biphoton source that requires minimal alignment and increases the interference coherence length. In addition, our biphoton source provides direct temperature-based control of the frequency degeneracy of the photon pairs, allowing students to investigate physical properties of HOM interference such as coherence length and interference visibility. Through use of standard optomechanical parts, combined with the commercially available fiber-integrated biphoton source, our apparatus is positioned midway between a completely built-from-scratch and a pre-aligned setup, making it ideal for the advanced instructional laboratory.