时间演化算符
时间演化
\begin{align} U(t_0 + \mathrm{d}t, t_0) = 1 - \frac{i}{\hbar}Ht \end{align}那么
\begin{align} U(t + \mathrm{d}t, t_0) = \left(1 - \frac{i}{\hbar}Ht \right)U(t, t_0) \end{align}所以
\begin{align} U(t + \mathrm{d}t, t_0) - U(t + , t_0) =& - \frac{i}{\hbar}\mathrm{d}t U(t, t_0) \\ \Downarrow &\\ \mathrm{i}\hbar \frac{\partial}{\partial t} U(t, t_0) =& H(t) U(t, t_0) \end{align}关键在于如果 Hamiltonian 中含时, 它和演化算符中是同一个 \(t\) . 由此得到最 general 的演化算符的形式
\begin{align} U(t,t_0) = T \left\{ e^{-\frac{\mathrm{i}}{\hbar}\int _{t_0}^t \mathrm{d}t' H(t')} \right\} \end{align}也就是说最 general 的情况下, \([U(t, t_0), H(t)] \neq 0\) .
Heisenberg 运动方程为
\begin{align}
\frac{\mathrm{d}A^H}{\mathrm{d}t} = -\frac{i}{\hbar}
U^{\dagger}(t)[A, H(t)] U(t) + U^{\dagger}(t)\frac{\partial A}{\partial t} U(t)
\end{align}
上式中的 \(H(t), U(t)\) 也是同一个 \(t\) .
Liouville–von Neumann equation
密度矩阵的演化满足 Liouville–von Neumann equation
\begin{align} \frac{\mathrm{d}}{\mathrm{d}t} \rho(t) = \frac{\mathrm{i}}{\hbar} [\rho(t), H] \end{align}它的 Heisenberg 运动方程类似, 但要注意差了一个负号, 并且它是在 Schrodinger picture 中的.
密度算符
\begin{align} \rho(t) = \sum_n p_n U(t)|n\rangle\langle n| U^{\dagger}(t) \end{align}从上式可以看出负号的来源. Heisenberg picture 中的算符是 \(U^{\dagger}AU\) , 而密度算符是 \(U|n\rangle\langle n|U^{\dagger}\) , \(U\) 和 \(U^{\dagger}\) 的位置刚好相反.
Reference
- J. J. Sakurai, Jim Napolitano, Modern Quantum Mechanics 2nd
- https://en.wikipedia.org/wiki/Heisenberg_picture
- https://en.wikipedia.org/wiki/Density_matrix
- https://physics.stackexchange.com/questions/112984/liouville-von-neumann-equation-can-be-directly-derived-from-heisenberg-picture