  We show the decrease of spin-spin entanglement between two s=1/2 fermions or
two photons due to local transfer of correlations from the spin to the momentum
degree of freedom of one of the two particles. We explicitly show how this
phenomenon operates in the case where one of the two fermions (photons) passes
through a local homogeneous magnetic field (optically-active medium), losing
its spin correlations with the other particle.

Quantum Physics