# Finding normalized eigenfunctions of a linear operator in Matrix QM

1. Oct 18, 2013

1. The problem statement, all variables and given/known data
Hey everyone!
The question is this:
Consider a two-state system with normalized energy eigenstates $\psi_{1}(x)$ and $\psi_{2}(x)$, and corresponding energy eigenvalues $E_{1}$ and $E_{2} = E_{1}+\Delta E; \Delta E>0$

(a) There is another linear operator $\hat{S}$ that acts by swapping the two energy eigenstates: $\hat{S}\psi_{1}=\psi_{2}$ and $\hat{S}\psi_{2}=\psi_{1}$. Show that the corresponding normalized eigenfunctions of $\hat{S}$ are $\phi_{1}(x)=\frac{1}{\sqrt{2}}(\psi_{1}(x)+\psi_{2}(x))$ and $\phi_{2}(x)=\frac{1}{\sqrt{2}}(\psi_{1}(x)-\psi_{2}(x))$, with eigenvalues $\lambda_{1}=+1$ and $\lambda_{2}=-1$

2. Relevant equations
- The regular eigenvalue/eigenvector stuff.
- In matrix mechanics, the wavefunction $\psi_{n}(x)=C_{1}(1,0...n)^{T}+C_{2}(0,1,0...n)^{T}+...C_{n}(0....n)^{T}$

3. The attempt at a solution
So I've found the matrix associated with the linear operator $\hat{S}$ in the natural basis $U_{1}=(1,0)^{T}, U_{2}=(0,1)^{T}$; which is:
http://imageshack.com/a/img266/5713/bfo6.jpg [Broken]
This gives the correct eigenvalues of +1 and -1. But I dont know how to find the eigenfunctions that the question is talking about. I'm not even sure if the matrix is right...

Last edited by a moderator: May 6, 2017
2. Oct 19, 2013

### dextercioby

I think the hint I can give is to use that S is linear and consider a linear combination of the 2 vectors psi1 and psi2.

3. Oct 19, 2013

But is my matrix correct for S..?

4. Oct 20, 2013

### dextercioby

Who cares, you needn't use any matrix to solve the problem.

5. Oct 20, 2013