Interaction-free measurements of the second type: Concepts and proposal for a feasible experiment

Wave aspects inherent in quantum physics lead to observable consequences, w hich are far removed from classical intuition. The possibility of interacti on-free measurement is one consequence of quantum wave nature at the single particle level. To date, all experiments, which have studied interaction-f ree measurements, have dealt with modification of the amplitude of the wave function. In this paper, we discuss interaction-free measurements of a sec ond type, where only the phase of the wave function is modified. We show th at an example discussed by Penrose is in this class and clarify the physica l reasons for loss of interference in such measurements. We discuss the exp erimental feasibility of interaction-free measurements of the second type, employing micromaser cavities and atom interferometers. One scheme leads to the possibility of interaction-free measurement of the finesse of the cavi ty by observing atoms that never interacted with the cavity. The loss of in terference, in this case. is due to a random spinor phase and there is no H eisenberg momentum back-action. This is the first experimental proposal for interaction-free measurements of pure phase changes. (C) 2001 MAIK "Nauka/ Interperiodica.