  The evolution of the entanglement measure during Compton scattering is
studied. Our analytical results show that the corresponding measure coincides
with the concurrence of the two-qubit state arising after scattering. The state
never collapses to a separable one, contrary to what was previously assumed.
The behavior of quantum entanglement during scattering is identical to the
behavior of initially classically correlated photons up to a constant factor
equal to two. This is consistent with local quantum field theory, and "spooky
action at a distance" is not required to explain the change in state of
nonlocally entangled qubits during the measurement of one of them. Our
dedicated experiment with annihilation photons confirms these results and
explains the "Puzzle of Decoherence" observed recently.
