Spatial localization and relativistic transformation of quantum spins

Abstract

The Lorentz-boost-induced decrease in the purity of the reduced density matrix for spins was investigated. Such a boost-induced decrease in spin purity observed in a moving reference frame was related to spatial localization properties of the wave packages observed in the rest frames. It was also proved that for any localized pure state with separable spin and momentum in the rest frame, its reduced density matrix for spins appeared to be mixed whenever viewed from a moving reference frame. This observation could be important for relativistic quantum information processing mainly for relativistic quantum communication.