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I don't understand this. Of course, if the two observables are compatible you can (at least in principle) prepare the system in a common eigenstate, and then both observables have a determined value. If the two observables are not compatible generally you can't do that. That's the content of the uncertainty relation.A. Neumaier said:OK, so you talk about the preparation of a nonexistent object, not of a state with particular properties. Of course, nonexistent things cannot be prepared.
The next question then is, what do you define as "fluctuation", and I think the usual meaning of the word is that any quantity that has an undefined value due to the prepared state fluctuates, and the fluctuation is characterized by the standard deviation of the corresponding probability distribution.