4 A conclusive experiment to throw more light on ” light ”

We describe a new realization of Ghose, Home, Agarwal experiment on wave particle duality of light where some limitations of the former experiment, realized by Mizobuchi and Ohtaké, are overcome. Our results clearly indicate that wave-particle complementarity must be understood between interference and " whelcher weg " knowledge and not in a more general sense. PACS 03.65.Ta Wave particle duality is one of the fundamental aspects of Quantum Mechanics , central element of the knowledge of every physics student. Nevertheless, the original Bohr statement about the term complementarity (in particular complementarity between wave and particle behaviours) " to denote the relation of mutual exclusion characteristic of the quantum theory with regard to an application of the various classical concepts and ideas " [1] has been recently subject of a wide debate and a paradigm where this " mutual exclusion " must be interpreted in a weaker sense is emerging. Incidentally, it must be noticed that the mutual exclusion was thought by Bohr as a necessary condition for internal consistency of his interpretation attributing a classical behaviour to detection apparatuses [1,2,3] (we do not enter in this letter in the extensively debated and still unclarified issue of measurement in quantum Mechanics, see Ref.s [3,4] and Ref.s therein). In particular a series of experiments with single photons and atoms in in-terferometers have shown how a gradual transition takes place between the 1 [email protected] 1 two aspects (wave and particle) [3], described by the Greenberger-Gisen inequality P 2 + V 2 ≤ 1, where P denotes the predictability of path (particle behaviour) and V the visibility of interference (wave behaviour). The knowledge of " welcher weg " (which path) is therefore alternative to coherence (and thus to the possibility of having interference) with a smooth transition between a perfect " welcher weg " knowledge and a 100% interference visibility. The extension of complementarity to classical concepts of waves and particle in every situation (including, also, for example tunnel effect or birefringence) is nevertheless not contained in the mathematical formalism of Quantum Mechanics and can be questioned [4]. Furthermore, in interference experiments [5,3] the use of beam splitters (or similar devices) can be somehow modellized with classical particles transmission and reflection [2]. In this sense a large interest arouses an experiment [7] based on the theoretical proposal of Ref. [6] where the coincidences between photodetectors after a tunnel effect in a double prism of …