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We study the time evolution from a pure state with extensive energy
above the ground state in a CFT. We show that the reduced density matrix
of an interval of length ℓ in an infinite system is exponentially
close to a thermal one after times t>ℓ/2. For a finite system of
length L we study the overlap of the state at time t with the
initial state and show that this can expressed in terms of Virasoro
characters. For a rational CFT there are finite revivals when t is a
multiple of L/2, and interesting structure near all rational values of
t/L. We also study the effect of an irrelevant operator which breaks
the integrability and show that the revivals then become increasingly
broadened.
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