  The entanglement between spins of a pair of particles may change because the
spin and momentum become mixed when viewed by a moving observer [R.M. Gingrich
and C. Adami, Phys. Rev. Lett. 89, 270402 (2002)]. In this paper, it is shown
that, if the momenta are appropriately entangled, the entanglement between the
spins of the Bell states can remain maximal when viewed by any moving observer.
Further, we suggest a relativistic-invariant protocol for quantum
communication, with which the non-relativistic quantum information theory could
be invariantly applied to relativistic situations.
