A Simple Experiment for Discussion of Quantum Interference and Which-Way Measurement

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Motivated by a recent experiment involving which-way measurement in atom interference, we developed a completely analogous experiment using visible light. This simple experiment, easily accessible to undergraduate students and the resources of undergraduate departments, facilitates examination of the key elements of which-way measurement, quantum erasure and related mysteries of quantum measurement. The experiment utilizes a Mach-Zehnder interferometer, and visually demonstrates the loss of interference fringes when a which-way measurement is imposed, and the restoration of that pattern when the which-way information is destroyed. This device is also sensitive enough to observe interference fringes arising from single photons. At a level accessible to undergraduates, we present simple analyses of the interference appropriate for the coherent classical field limit, and the single photon limit. We briefly mention related issues of the nature of the photon, pointing to some useful references.