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Photoniques

Two-photon interference and the Hong-Ou-Mandel effect

Benjamin Vest, Lionel Jacubowiez

Published Jan 1, 2024• 0 citations
Publisher

Abstract

The Hong-Ou-Mandel (HOM) experiment is a landmark in quantum optics. A labwork version of this famous twophoton interference effect was developed at Institut d’Optique for students in engineering and MSc tracks. The setup enables the observation of the iconic HOM “dip” and the measurement of photon indistinguishability.
DOI
10.1051/photon/202312524

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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.
Abstract We present a simple setup allowing undergraduate students to reproduce the Hong–Ou–Mandel experiment during a half-day labwork session and observe the coalescence of two indistinguishable photons merging on a balanced beamsplitter. This two-photon interference effect, fundamentally related to the bosonic character of the photons, is commonly used in the fields of quantum communication and computing to test the indistinguishability of two single-photon wavepackets. The setup makes use of very few optical elements and requires little alignement that can be performed by students themselves. It allows them to gather essential experimental skills related to parametric crystals, fibre optics and single-photon detection, and to transpose abstract concepts of quantum physics to a hands-on experiment in the lab.
Controlling light at the level of individual photons has led to advances in fields ranging from quantum information and precision sensing to fundamental tests of quantum mechanics. A central development that followed the advent of single-photon sources was the observation of the Hong–Ou–Mandel (HOM) effect, a novel two-photon path interference phenomenon experienced by indistinguishable photons. The effect is now a central technique in the field of quantum optics, harnessed for a variety of applications such as diagnosing single-photon sources and creating probabilistic entanglement in linear quantum computing. Recently, several distinct experiments using atomic sources have realized the requisite control to observe and exploit Hong–Ou–Mandel interference of atoms. This chapter provides a summary of this phenomenon and discusses some of its implications for atomic systems. Transitioning from the domain of photons to atoms opens new perspectives on fundamental concepts, such as the classification of entanglement of identical particles. It aids in the design of novel probes of quantities such as entanglement entropy by combining well-established tools of AMO physics—unity single-atom detection, tunable interactions, and scalability—with the Hong–Ou–Mandel interference. Furthermore, it is now possible for established protocols in the photon community, such as measurement-induced entanglement, to be employed in atomic experiments that possess deterministic single-particle production and detection. Hence, the realization of the HOM effect with atoms represents a productive union of central ideas in quantum control of atoms and photons.
The dip in the Hong–Ou–Mandel (HOM) experiment is frequently attributed to the interference of two indistinguishable photons. The width of the dip is inversely proportional to the spectral bandwidth of the light. We confirm that this width is unchanged regardless of where the bandwidth selection happens, i.e. prior to the photons combining in the beam splitter or just before the detectors; the latter case seemingly after the interference of the photons. The equivalence of the results from these two cases is a simple demonstration that the interference in the HOM experiment is not between photons, but rather between probability amplitudes.