Lorentz covariant reduced-density-operator theory for relativistic   quantum information processing
  In this paper, we derived Lorentz covariant quantum Liouville equation for the density operator which describes the relativistic quantum information processing from Tomonaga-Schwinger equation and an exact formal solution for the reduced-density-operator is obtained using the projector operator technique and the functional calculus. When all the members of the family of the hypersurfaces become flat hyperplanes, it is shown that our results agree with those of non-relativistic case which is valid only in some specified reference frame. To show that our new formulation can be applied to practical problems, we derived the polarization of the vacuum in quantum electrodynamics up to the second order. The formulation presented in this work is general and might be applied to related fields such as quantum electrodynamics and relativistic statistical mechanics. 
Doyeol Ahn, Hyuk-jae Lee, and Sung Woo Hwang