diff --git a/ueb1.tex b/ueb1.tex index 46ae8c7..ce227b8 100644 --- a/ueb1.tex +++ b/ueb1.tex @@ -6,7 +6,8 @@ \section{Aufgabe 2: Pauli Matrizen} \subsection*{a)} - +\subsection*{b)} +\subsection*{c)} \begin{math} \textbf{\sigma} = (\sigma_x, \sigma_y, \sigma_z) \\ \textbf{\a},\textbf{\b} \in \setR @@ -20,11 +21,79 @@ \sum a_\alpha b_\beta \krondelta{\alpha \beta} \cdot \one + \i a_\alpha b_\beta \levicivita{\alpha,\beta,\gamma} \sigma_\gamma &= \\ \one (\textbf{a \cdot b} + \i \sigma \cdot (a \times b) \end{align} -\subsection*{b)} -\subsection*{c)} -\subsection*{d)} +\subsection*{d)} + $e^{\i \frac{\i}{2} n \sigma} = \one cos(\frac{\alpha}{2}) - \i (\textbf{n \cdot \sigma}) sin(\frac{\alpha}{2})$ \\ + Mit der Reihenentwicklung von $e^x$ ergibt sich: + \begin{align} + e^{- \i \frac{\alpha}{2} \textbf{n \cdot \sigma} &= \sum_k \frac{(- \i \frac{\alpha}}{2} \textbf{n \cdot \sigma})^k}{k!} \\ + &= \sum_k (- \i \frac{\alpha}{2})^k \cdot \frac{(\textbf{n \¢dot \sigma)^k}}{k!} + \end{align} + Desweiteren gilt nach Aufgabe 2 c): + \begin{align} + (\textbf{n \cdot \sigma} &= \sigma \\ \\ + (\textbf{n \cdot \sigma}^2 &= \one (\textbf{n \cdot n} + \underbrace{\i \textbf{\sigma} \cdot (\textbf{\n} \times \textbf{n})}_{=0} \\ + &= \one (\textbf{n \cdot n} + \end{align} + + $\Rightarrow$ + + \begin{align} + \sum_k (- \i \frac{\alpha}{2})^k \cdot \frac{(\textbf{n \cdot \sigma)^k}}{k!} &= \\ + + \sum_k ( (-\i)^{2k} (\frac{\alpha}{2})^{2k+1} \frac{(\textbf{n \cdot \sigma})^{2k}}{(2k)!} + + \sum_k ( (-\i)^{2k+1} (\frac{\alpha}{2})^{2k+1} \frac{(\textbf{n \cdot \sigma})^{2k+1}}{(2k+1)!} &= \\ + + \sum_k ( (-1)^k (\frac{\alpha}{2})^{2k+1} \frac{(\textbf{n \cdot \sigma})^{2k}}{(2k)!} + + \sum_k ( \i \cdot (-1)^k (\frac{\alpha}{2})^{2k+1} \frac{(\textbf{n \cdot \sigma})^{2k} \cdot (\textbf{n \cdot \sigma})}{(2k+1)!} &= \\ + + \sum_k ( (-1)^k (\frac{\alpha}{2})^{2k+1} \one \frac{1}{(2k)!} + + \i \cdot \sum_k (-1)^k (\frac{\alpha}{2})^{2k+1} \one \cdot (\textbf{n \cdot \sigma}) \cdot \frac{1}{(2k+1)!} &= \\ + + \one \cos(\frac{\alpha}{2}) + \i (\textbf{n \cdot \sigma}) \sin(\frac{\alpha}{2}) + + \end{align} + + + + \section{Aufgabe 3: Operator-Identitäten} \subsection*{a)} + $f(t) = e^{tA} \cdot B e^{-tA}$ + \begin{align} + \diffTm{}{f(t)}{t} &= [A,f(t)] \\ + \diffTm{2}{f(t)}{t} &= [A,[A,f(t)]] \\ \\ + \diffTm{}{f(t)}{t} &= A e^{tA} B e^{-tA} t - e^{tA} B e^{-tA} A &= [A,f(t)] = A \cdot f(t) - f(t) \cdot A \\ + \diffT{t} [A,f(t)] &= \diffT{t} A \cdot f(t) - \diffT{t} f(t) \cdot A \\ + &= A \cdot [A,f(t)] - [A,f(t)] \cdot A \\ + &= [A,[A,f(t)]] + \end{align} + Die Taylorreihenentwicklung lautet dann: + $f(t) = e^{tA} \cdot B e^{-tA} = B + \frac{t}{1!}[A,B]+\frac{t^2}{2!}[A,[A,B]]+\ldots$ + \subsection*{b)} -\subsection*{c)} \ No newline at end of file + $\forall A,B,C : [A,B] = \i C \and [B,C] = \i A$ \\ + \begin{align} + e^{\i B t} \cdot B e^{-\i B t} &= A \cos(t) + C \sin(t) \\ + &= A + [\i B,A] + \frac{t^2}{2}[\i B,[\i B, A]] \\ + [\i B, A] &= \i [B,A] &= -\i [A,B] \\ + [\i B, [\i B, A]] &= [\i B, C] &= \i \i A = -A }} \\ + e^{\i B t} A e^{-\i B t} &= A + C - \frac{t^2}{2} A \frac{t^3}{6} C + \frac{t^4}{24} A \\ \\ + &= A \cos(t) + C \sin(t) + \end{align} + +\subsection*{c)} +Es gelte: $[A,[A,B]] = [B,[A,B]] = 0$ \\ +\begin{align} + e^{A+B} &= e^A \cdot e^B \cdot e^{-\frac{1}{2}[A,B]} \\ + g(t) &= e^{tA} \cdot e^{tB} \cdot e^{-\frac{t^2}{2}[A,B]} \\ + \diffTfrac{g(t)}{t} &= A \cdot g(t) + e^{tA} \cdot B \cdot e^{-tA} \cdot g(t) - t \cdot g(t) \cdot [A,B] \\ + e^{x} \cdot e^{-x} &= \one \\ + \diffTfrac{g}{t} &= \right( A - t [A,B] + B + \frac{t}{1!} \cdot [A,B] \left) \cdot g(t) \\ + \diffTfrac{g}{t} &= [A+B] \cdot g(t) \\ + \frac{\diffTfrac{g}{t}}{g} &= [A+B] \\ + g(t) &= e^{[A+B] \cdot t + c} \\ + e^A \cdot e^B &= e^B \cdot e^A \cdot e^[A,B] \\ + e^(A+B) &= e^A \cdot e^B \cdot e^{-\frac{1}{2}[A,B]} \\ + e^(B+A) &= e^B \cdit e^A \cdot e^{-\frac{1}{2}[A,B]} +\end{align}