  We consider entanglement extraction into two two-level Unruh-DeWitt detectors
from a vacuum of a neutral massless quantum scalar field in a four-dimensional
spacetime, where the general monopole coupling to the scalar field is assumed.
Based on the reduced density matrix of the two detectors derived within the
perturbation theory, we show that the single copy of the entangled pair of the
detectors can be utilized in quantum teleportation even when the detectors are
separated acausally, while we observe no violation of the Bell-CHSH inequality.
In the case of the Minkowski vacuum, in particular, we find that entanglement
usable in quantum teleportation is extracted due to the special relativistic
effect when the detectors are in a relative inertial motion, while it is not
when they are comoving inertially and the switching of the detectors is
executed adiabatically at infinite past and future.
