The most essential performance parameters of the upgraded neutron interfero
meter instrument S18 at the ILL including the novel concert of beam polariz
ation by birefringent permanent magnetic field prisms are presented. A spin
superposition experiment is proposed to test the complementarity of which-
way knowledge and fringe visibility as well as for the concept of quantum e
rasure. (C) 2000 Elsevier Science B.V. All rights reserved.