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Physics

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    $\begingroup$ Good question, +1. See the notion of "coarse grained measurements", "non-selective measurements" and the notion of "quantum instruments" in general. The overall long story short is that the post-measurement state depends on the measurement scheme, and there are generally infinitely many different measurement schemes associated to the same observable. $\endgroup$ Commented Dec 17, 2025 at 23:00
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    $\begingroup$ See also this excellent post. $\endgroup$ Commented Dec 17, 2025 at 23:43
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    $\begingroup$ @TobiasFünke Thank you for the references, and that is indeed a good post. $\endgroup$ Commented Dec 18, 2025 at 1:48