Does Quantum Mechanics imply influences acting backward in time in   impact series experiments?
  A real two-particle experiment is proposed in which one of the particles undergoes two successive impacts on beam-splitters. It is shown that the standard quantum mechanical superposition principle implies the possibility of influences acting backward in time ("retrocausation"), in striking contrast with the principle of causality. It is argued that nonlocality and retrocausation are not necessarily entangled. 
Antoine Suarez (Center for Quantum Philosophy, Zurich)