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In 1801, Thomas Young, an English physician and polymath, performed what has been called "the most beautiful experiment" in physics. He shone a beam of light at two narrow, closely-spaced slits and showed that the beam's image on the other side was not two bright lines but a whole sequence of alternating light and dark bands or interference fringes. Later, physicists discovered that particles of matter, such as electrons, behave in exactly the same way. Wave-particle duality—the idea that particles can sometimes behave as waves, or vice versa—thus became a central postulate of quantum mechanics, often linked to Heisenberg's uncertainty principle. In the article, the authors describe a new version of Young's experiment, in which the interference fringes can be switched off or on by inserting different kinds of polarizing plates. These plates have the effect of either erasing or revealing information about which way the photons traveled through the slits. The authors argue that quantum entanglement, rather than the uncertainty principle, is the best way of accounting for the results.