  We present a new treatment for the spin of a massive relativistic particle in
the context of quantum information based on a physical interpretation of the
Wigner rotations, obtaining different results in relation to the previous
works. We are lead to the conclusions that it is not possible to define a
reduced density matrix for the particle spin and that the Pauli-Lubanski (or
similar) spin operators are not suitable to describe measurements where spin
couples to an electromagnetic field in the measuring apparatus. These
conclusions contradict the assumptions made by most of the previous papers on
the subject. We also propose an experimental test of our formulation.
