Measurement process in a variable-barrier system

The description of a measuring process, such as that which occurs when a qu antum point contact (QPC) detector is influenced by a nearby external elect ron taking up different positions, provides a interesting application of th e method of quantum damping. We find a number of new effects, due to the co mplete treatment of phases afforded by the fonnalism, although our results are generally similiar to those of related treatments, particularly to thos e of Buks et al. These are effects depending on the phase shift in the dete ctor, effects which depend on the direction of the measuring current, and i n addition to damping or dissipative effects, an energy shift of the measur ed system. In particular, the phase shift effect leads to the conclusion th at there can be effects of "observation" even when the barriers in question pass the same current. Secondly, the nature of the current through the barriers and its statistics is discussed, and a description of the correlations in the current due to ''measurement'' is obtained. In particular it is possible to see the origin of "telegraphic" signals, which in a certain limit give behavior resemblin g a "collapse of the wavefunction". (C) 1999 Published by Elsevier Science B.V. All rights reserved.