Interaction-Free Measurements of the Second Type: Concepts and Proposal for a Feasible Experiment

Abstract

Wave aspects inherent in quantum physics lead to observable consequences, which are far removed from classical intuition. The possibility of interaction-free measurement is one consequence of quantum wave nature at the single particle level. To date, all experiments, which have studied interaction-free measurements, have dealt with modification of the amplitude of the wave function. In this paper, we discuss interaction-free measurements of a second type, where only the phase of the wave function is modified. We show that an example discussed by Penrose is in this class and clarify the physical reasons for loss of interference in such measurements. We discuss the experimental feasibility of interaction-free measurements of the second type, employing micromaser cavities and atom interferometers. One scheme leads to the possibility of interactionfree measurement of the finesse of the cavity by observing atoms that never interacted with the cavity. The loss of interference, in this case, is due to a random spinor phase and there is no Heisenberg momentum back-action. This is the first experimental proposal for interaction-free measurements of pure phase changes.