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Where is the Quantum Particle between two Measurements?


Wheeler Delayed Choice experiment, Elitzur-Vaidman Interaction-free Measurement, and Hosten-Kwiat Counterfactual Computation will be discussed to answer Bohr's forbidden question: "Where is a quantum particle while it is inside a Mach-Zehnder Interferometer?" I will argue that the naive Wheeler's approach fails to explain  a weak trace left by the particle and that the two-state vector description is required.