How does the demon change the spread in measurement? What does it really mean to measure something?
See above question. The demon must interact with the particle in order to measure its position; in doing so it gives it a momentum spread. This translates to an angular spread which destroys the interference. We will not be covering the details of measurement theory in this course. For this example, you can consider a measurement to be “an interaction to determine the position”. In the QM treatment of this course we will consider measurements abstractly, as determinations of the value corresponding to an operator- how exactly one makes the measurement is not important for the theory (at this level).
