unentangled and Bell states. In the ensuing subsection, we present a relation between generalized zeta functions and von Neumann entropy which may be useful for the regularization of the entropy of infinite-dimensional density matrices. Further comments and conclusions are presented in the final section.

As far as it is convenient to do so, we shall follow the conventions and normalizations in Weinberg’s tome[@weibqft], with $\eta^{\mu\nu}=diag(-1,+1,+1,+1)$. Space-time Lorentz indices are denoted by Greek letters, spatial indices by Latin letters; and summation over repeated discrete index is assumed. Our computations shall concentrate, for convenience, only on the homogeneous Lorentz group since translations can be incorporated rather readily[@weibqft] once the behaviour of fields under the homogeneous group has been worked out.

Wigner rotations
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In quantum field theory, one-particle states ${\ensuremath{|p,s\rangle}}$ are classified by eigenvalues of the Casimir invariants of the Poincaré group. Any value of the momentum, $p^\mu$ can be reached by Lorentz transformation $L(p)$ on a standard $k^\mu$ for which $p^\mu = L^\mu_{\,\nu}(p) k^\nu$; and the states can be defined as $$\label{e:sp}
{\ensuremath{|p,s\rangle}} \equiv \sqrt{\frac{k^0}{p^0}}U(L(p)){\ensuremath{|{k},s\rangle}},$$ with $P^\mu{\ensuremath{|p,s\rangle}} = p^\mu{\ensuremath{|p,s\rangle}}$. It follows(see, for instance, Ref.[@weibqft]) that the effect of an arbitrary Lorentz transformation $\Lambda$ unitarily implemented as $U(\Lambda)$ on one-particle states is 