dsaun777 said:
But the hup puts a limit on measurements. Forget about mathematical derivation of hup, I am interested in how the hup influence measurements.
The HUP is often misinterpreted, which has historical reasons. The funny point is that Heisenberg published his first paper on the HUP without discussing its contents with Bohr, and there he interpreted it as if it would mean that you couldn't measure accurately momentum without disturbing the system in such a way that momentum gets more uncertain and vice versa. This is, however, not what the HUP, derived from QM, is saying, and that was corrected by Bohr. Unfortunately the first interpretation somehow stuck within the literature.
The point is, as was pointed out before in this thread, that the usual HUP,
$$\Delta A \Delta B \geq \frac{1}{2} |\langle \mathrm{i} [\hat{A},\hat{B}] \rangle|,$$
is about the statistical properties of measurement results when measuring accurate two observables ##A## and ##B## on ensembles of equally prepared quantum systems. Indeed ##\Delta A## and ##\Delta B## are standard deviations. The standard HUP thus refers not to measurements but to the preparability of the system in a way that the one or the other observable are accurately determined.
In the case of position and momentum it's saying
$$\Delta x_j \Delta p_j \geq \frac{\hbar}{2},$$
i.e., the right-hand side is independent of the prepared quantum state, and that makes this case easy to interpet: If you prepare a particle such that the ##j##-component of its position vector, ##x_j##, is pretty well determined (##\Delta x_j## small), then necessarily the ##j##-component of its momentum vector ##p_j## cannot be so well prepared (##\Delta p_j## gets necessarily large).
As any probabilistic statement, from a practical point of view, also the HUP refers to ensembles of equally prepared particles. The preparation in the corresponding state has nothing to do with the possibility to measure either ##x_j## or ##p_j## accurately. Of course such measurements usually need different setups of measurements. You can measure ##x_j## with one setup very accurately on many equally prepared particles. To check the uncertainty relation, the position resolution of the appartus must be much better than ##\Delta x_j##, such that you get random positions for any single measurement that you can then analyze statistically and determine ##\Delta x_j##. The same you can do for ##p_j##. Again your momentum resolution must be much better than ##\Delta p_j## to get significant statistics do determine ##\Delta p_j## in the statistical measurment analysis.
Of course, it's also true that any measurement has an unavoidable influence on the measured system, i.e., indeed there is some disturbance of the system involved in any measurement, and also this can be described by quantum theory, but it's not described by the usual HUP. Rather you have to make a detailed analysis of the specific experiment. Then you get also measurement-disturbance uncertainty relations, but this is not the usual HUP, which is pretty easily derived in any textbook on QM.
A very concise discussion about the important difference between the usual HUP and disturbance through measurement, see
https://arxiv.org/abs/quant-ph/0510083
https://doi.org/10.1088/1464-4266/7/12/033