Theorem: Loschmidt FGR Gaussian log-slope log M(t)/t^2 = -sigma^2*delta^2/hbar^2 (exact)

Theorem (Loschmidt FGR Gaussian-slope canonical): in the Fermi-golden-rule regime of Peres (1984), the Loschmidt echo M(t) = exp(-sigma^2 delta^2 t^2/hbar^2) has logarithmic decay log M(t) = -sigma^2 delta^2 t^2/hbar^2. Dividing by t^2…