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,
A- A- −β S ∼ 𝜖2(1 + c(𝜖2) ) , (336 )
with β always less than unity, and the power law correction is due to the excited state.
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