Quantum mechanics has so many counterintuitive features that it seems possible to learn a new one every month. Today's lesson involves particles that are set into the same quantum state, which effectively become indistinguishable. Once they are indistinguishable, they start behaving that way, showing up in the same place even when we'd expect to see them distributed at random. In today's issue of Nature, a paper describes getting atoms to behave this way, blurring the lines between a quantum probability function and what we think of as a physical object.
The original issue of indistinguishability was highlighted in an experiment done decades ago using photons. Called the Hong, Ou, and Mandel experiment, it involved sending photons in the same quantum state into a partial mirror along two different paths. The partial mirror, called a beam splitter, has a 50/50 chance of reflecting a photon, shifting it from one path to the second.
Based on the 50/50 chance, you'd expect three different outcomes. Half the time, when the beamsplitter reflects neither or both of the photons, you'd expect one photon in each of the output paths. When the beamsplitter reflects only one photon, you'd see both photons in one or the other path (with a 25 percent chance for each).
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