We destroy quantumness in proportion to the amount of information we import from the system into its environment. Zurek and his colleague Bill Wootters have shown that, in a double-slit experiment, it is possible to obtain some information about which path a photon took without losing all the quantum interference. While you’re not totally certain which path it took, but have reason to think one is more likely than the other, some interference remains. And it turns out that you can get a surprising amount of path information without making the photon completely ‘particle-like’ and losing all
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****** "observation" involves extracting information from the environment
