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Aug 22, 2014 at 14:21 comment added Carlo Beenakker $\rho(t)=U(t)\rho(0)U^{\dagger}(t)$ where $U(t)=e^{-iHt}$ has matrix elements $e^{-iE_n t}\delta_{nm}$ in the eigenbasis of $H$.
Aug 22, 2014 at 13:30 comment added Xingdong Zuo Thank you for the answer. When it comes to the form with the energy eigenvalues $\exp(-i(E_m - E_n))t$, is it still possible to consider it as an operator such that $U'(t, 0) = \exp(-i(E_m - E_n))t \neq I$, where $I$ denotes the identity operator. Or we can only simply say $\exp(-i(E_m - E_n))t \neq 1$ ?
Aug 22, 2014 at 12:07 vote accept Xingdong Zuo
Aug 22, 2014 at 11:16 history answered Carlo Beenakker CC BY-SA 3.0