On the problem of non-locality in quantum mechanics

Many experimental results and theoretical reasonings on ''gedanken'' experiments suggest that quantum entities behave in a ''non-local'' way. In order to make the discussion more precise, we propose an operational definition of ''non-locality''. We discuss two examples of spin superposition experiments which illustrate the use of this definition. They show that quantum entities like a neutron or an atom are non-local. We make an analysis of the difficulty to imagine such non-local entities and to conciliate this non-locality with the locality of classical entities. (orig.)