ABSTRACT
This chapter reviews the main points in the theory of instantaneous measurements. A description of continuous measurements as limits of series of instantaneous measurements turns out to be very cumbersome and in fact inadequate. A characteristic feature of quantum systems is that measurements in them unavoidably affect their dynamics. The well known manifestation of this back reaction is the uncertainty principle, discussion of which can be found in any textbook on quantum mechanics. The operator plays the role of a propagator for a quantum system under continuous measurement. The quantum Zeno effect is an example of a continuous measurement giving a trivial result. However, this is a consequence of the very special character of the measurement procedure described by a discrete set of orthogonal projectors each of which is a bounded operator. The description of approximate measurements was derived for the case when precise measurement is a discrete-spectrum measurement.
