- #1
Telemachus
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The thing is that often in the problems on quantum mechanics I've found that an operator is given, but not the base on which it is represented. I'll give an especific example in a moment. So then the problem asks me to find the eigenvalues and eigenvectors for a given operator and to express it in the basis on which the operator is given, but the base is not given. I know that the information of the base is implicit in the operator it self, but I was wondering on which method I should use to find that base (I thought of using the typical matrix reduction of linear algebra, but I don't think that's the better way to do this). I'll take a problem from Cohen Tannoudji to picture what I mean.
Page 203, Cohen Tannoudji, problem 2, complement HII (I'll do incise c):
Consider an operator whose matrix, in an orthornormal basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ## is:
##L_y=
\begin{bmatrix}
0 & \sqrt{2} & 0\\
-\sqrt{2} & 0 & \sqrt{2}\\
0 & -\sqrt{2} & 0\\
\end{bmatrix}##
So, the exercise asks me to find the eigenvalues and the eigenvectors, and give their normalized expansion in terms of the basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ##.So, the question is, what should I do to know what the vectors of the basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ## are? so then I can express the eigenvectors in terms of the vectors of this basis.
From the secular equation I get the eigenvalues ##\lambda_1=0, \lambda_2=2i, \lambda_3=-2i##
From the first eigenvalue I get the eigenvector: ##\left| \lambda_1 \right\rangle=\frac{1}{2} \begin{bmatrix}
1\\
0\\
1\\
\end{bmatrix}##
So, what I want is to express this vector in the basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ##, but what that kets are? how do I obtain them?
I thought of using the fact that the matrix elements are given by ##L_{y;m,n}= \langle m | \hat L_y \left| n \right\rangle## but I've tried with a random matrix, only for one element, and get to an identity 0=0, so then I desisted, but I should try with this specific example now.
Page 203, Cohen Tannoudji, problem 2, complement HII (I'll do incise c):
Consider an operator whose matrix, in an orthornormal basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ## is:
##L_y=
\begin{bmatrix}
0 & \sqrt{2} & 0\\
-\sqrt{2} & 0 & \sqrt{2}\\
0 & -\sqrt{2} & 0\\
\end{bmatrix}##
So, the exercise asks me to find the eigenvalues and the eigenvectors, and give their normalized expansion in terms of the basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ##.So, the question is, what should I do to know what the vectors of the basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ## are? so then I can express the eigenvectors in terms of the vectors of this basis.
From the secular equation I get the eigenvalues ##\lambda_1=0, \lambda_2=2i, \lambda_3=-2i##
From the first eigenvalue I get the eigenvector: ##\left| \lambda_1 \right\rangle=\frac{1}{2} \begin{bmatrix}
1\\
0\\
1\\
\end{bmatrix}##
So, what I want is to express this vector in the basis ##\{ \left| 1 \right\rangle , \left| 2 \right\rangle, \left| 3 \right\rangle \} ##, but what that kets are? how do I obtain them?
I thought of using the fact that the matrix elements are given by ##L_{y;m,n}= \langle m | \hat L_y \left| n \right\rangle## but I've tried with a random matrix, only for one element, and get to an identity 0=0, so then I desisted, but I should try with this specific example now.
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