  Violation of the null energy condition plays an important role both in the
general theory of relativity and quantum field theory in curved spacetimes.
Over the years, it has been shown that the violation of the null energy
condition leads to instability and violation of causality. In quantum field
theory, violation of the energy condition also depends on the quantum states
apart from the geometry of curved spacetime. Hence, the quantum effects play an
important role in the violation of the null energy condition. We show that the
set of all the coherent states does not violate the null energy condition.
Further, we also show that under certain conditions, the null energy condition
is violated either by a pure state or a mixed state. Furthermore, the dynamical
violation of the null energy condition by the quantum states is also discussed
here.
