No abstract

The meaning of statistical experiments with single microsystems in quantum mechanics is discussed and a general model in the framework of non-relativistic quantum field theory is proposed, to describe both coherent and incoherent interaction of a single microsystem with matter.Compactly developing the calculations with superoperators, it is shown that the introduction of a time scale, linked to irreversibility of the reduced dynamics, directly leads to a dynamical semigroup expressed in terms of quantities typical of scattering theory. Its generator consists of two terms, the first linked to a coherent wavelike behaviour, the second related to an interaction having a measuring character, possibly connected to events the microsystem produces propagating inside matter. In case these events breed a measurement, an explicit realization of some concepts of modern quantum mechanics ("effects" and "operations") arises. The relevance of this description to a recent debate questioning the validity of ordinary quantum mechanics to account for such experimental situations as, e.g., neutron-interferometry, is briefly discussed.

A formalism developed in previous papers for the description of continual observations of some quantities in the framework of quantum mechanics is reobtained t. INTRODUCTIONIn a preceding paper m we have shown how, by a suitable limiting procedure, it is possible to introduce in the context of quantum mechanics the idea of the observation of a continuous trajectory for an appropriate quantity.The examples we have worked out until now are too simple indeed. However, the idea has interest for the light it can shed on the problem of the relation between the macroscopic description of a large body (in terms of a limited number of variables having definite values at any time) and its quantum mechanical treatment as an aggregate of particles. In particular, in his careful axiomatic analysis, Ludwig (2) has shown that an appropriate ~Dipartimento di Fisica delFUniversit/t, Milano. Italy~ and Istituto Nazionale di Fisica Nucleate, Sezione di Milano, Italy. 7790015-9018/83/0800-0779503.00/0