On the Aharonov–anandan Experiment

An experiment that would measure non–commuting quantum mechanical observables without collapsing the wave function has been recently proposed by Y Aharonov and J Anandan. These authors argue that this " protected measurement " may give indication on " the reality of the wave function ". We argue that, depending of the precise version of the experiment considered, either the author's prediction is incorrect and the wave function does collapse, or the measurement is not a measurement on a quantum system. In either case, the experiment does not provide a way for measuring non–commuting observables without collapse, and it does not bear on the issue of the " reality of the wave function ". Yakir Aharonov and Jeeva Anandan have recently discussed the problem of the interpretation of quantum mechanics, and have proposed an experiment which could provide indications concerning the " reality " of the wave function [1]. This proposal has received a certain attention [2]. The experiment consists in a standard Stern–Gerlach experiment, to which an additional homogeneous strong magnetic field B has been added. According to these authors the presence of this additional magnetic field prevents the beam's 1