ABSTRACT
This chapter discusses the type of measurement in the framework of the path-integral method. It begins from the simplest case of measurement of the linear momentum of a free particle, which allows one to see all the typical features characteristic of quantum nondemolition (QND) measurements. The idea of QND measurement is to measure a variable such that the unavoidable disturbance of the complementary one does not disturb the evolution of the chosen variable. The weight factor describes monitoring of the momentum, which for a free particle turns out to be a QND variable. This can be seen from the formula for the probability distribution of the measurement results. Usual relation between linear momentum and velocity is violated in the modified dynamics. From the physical point of view this is a consequence of measurement, of the influence of a measuring device.
