  It is proposed to shoot a molecule through a pipe of mesoscopic scale. The
molecule should effectifely carry two spins and antiferromagnetic chains should
be embedded in the wall of the tube. If the momentum is detected before and
behind the tube, a projection of the molecular spins s1 and s2 takes place. If
the molecule did not loose momentum inside the tube, it has a vanishing squared
total spin in the direction of its anisotropy axis L. Whereas [L(s1+s2)]^2=1 if
it did so.
