According to the QM postulates, when we measure momentum, we get an eigenvalue of the momentum operator and the system takes on a state that is the corresponding eigenket. Now, as I understand it, the eigenkets of the momentum operator are all Dirac delta functions corresponding to plane waves. A particle whose state vector is like that has a uniform probability of being anywhere at all in the universe. My understanding from Shankar is that no system actually has a plane wave state vector, and that these are just devices for helping us calculate and think about simple systems. Shankar (2nd ed. p138) seems to imply that the state immediately after we take a momentum measurement is a vector whose representation in the momentum space is a very sharply peaked Gaussian-style curve centred around the result that we got. The representation in the coordinate space is then a broader Gaussian. The trouble is that interpretation doesn't seem consistent with the postulate, which says that the state changes to an eigenket of the momentum operator. No matter how narrow and sharply peaked the Gaussian, it is still not an eigenket of that operator. How can this puzzle be resolved?