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Fleet Captain
YellowSubmarine, “precisely measured” means that you "just read the exact momentum of the particle".
Once you do that, the particle's momentum will become determined, precise on its own (wave function collapse).
Well then, what you say is that a “precisely measured momentum” is a function that maps all possible values for the momentum to a certain probability amplitude? Since that's what an exact reading of the momentum would be.
First - What do the wave function's solutions represent?
As I said, a quantum particle is, simultaneously, in more places at once/has more momentum values at once.
But which places/which momenta does the particle have?
Quantum particles are 'jumpy', the values for position/momentum can be probable (no large 'jumps') or improbable (the tunneling effect; a non-zero, but really small chance for the particle to 'jump' on the other side of the universe).
The wave function will give you both the probable values and the improbable ones.
Now - All non-measured particle's possible momenta will be given by its wave function.
Among those, the particle will have, simultaneously, only a small subset of momenta. Which subset? The probabilities are given by the wave function.
When you measure the particle's momentum, its wave function will collapse - it will name far fewer possible momenta for the particle.
How few? Depends on the precision of the measurement.
All you need to do to in order to restrict/determine a particle's momentum is to measure the particle.