9.5 Entropy of more general states
The pure quantum state, which is the starting point in the entanglement-entropy calculation, should not
necessarily be a vacuum state. According to [55, 53, 52, 54], if one starts with a mixed state of the
vacuum and an excited state, the entanglement entropy receives some power law corrections,
with always less than unity, and the power law correction is due to the excited state.