Measurement process in a variable-barrier system

The description of a measuring process, such as that which occurs when a quantum point contact (QPC) detector is influenced by a nearby external electron taking up different positions, provides a interesting application of the method of quantum damping. We find a number of new effects, due to the complete treatment of phases afforded by the fonnalism, although our results are generally similiar to those of related treatments, particularly to those of Buks et al. These are effects depending on the phase shift in the detector, effects which depend on the direction of the measuring current, and in addition to damping or dissipative effects, an energy shift of the measured system. In particular, the phase shift effect leads to the conclusion that 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 resembling a "collapse of the wavefunction". (C) 1999 Published by Elsevier Science B.V. All rights reserved.