Title:EPR argument and Bell inequalities for Bose-Einstein spin condensates

Abstract: We discuss the properties of two Bose-Einstein condensates in different spin states, represented quantum mechanically by a double Fock state. Individual measurements of the spins of the particles are performed in transverse directions (perpendicular to the spin quantization axis), giving access to the relative phase of the two macroscopically occupied states. Before the first spin measurement, the phase is completely undetermined; after a few measurements, a more and more precise knowledge of its value emerges under the effect of the quantum measurement process. This naturally leads to the usual notion of a quasi-classical phase (Anderson phase) and to an interesting transposition of the EPR (Einstein-Podolsky-Rosen) argument to macroscopic physical quantities. The purpose of this article is to discuss this transposition, as well as situations where the notion of a quasi-classical phase is no longer sufficient to account for the quantum results, and where significant violations of Bell type inequalities are predicted.