# Homework Help: Eigenvalues, eigenvectors, eigenstates and operators

1. May 11, 2013

### pigletbear

1. The problem statement, all variables and given/known data

Good evening :-)

I have an exam on Wednesday and am working through some past papers. My uni doesn't give the model answers out, and I have come a bit stuck with one question. I have done part one, but not sure where to go from here, would be great if someone could point me in the right direction:

S2) Show that the state vectors |Sx+> = $\frac{1}{\sqrt{}2}$ times a 2x1 matrix (1,1) and |Sy+> = $\frac{1}{\sqrt{}2}$ times a 2x1 matrix (1,-1) are eigenvectors of Sx = h/2 times a 2x2 matrix (0 1, 1 0) with respective eigenvalues plus and minus h/2...

Part two... Of what operator is the state $\frac{1}{\sqrt{}2}$(|Sx+> + |Sy+>) and eigenstate, and with what eigenvalue...

Any help would be great and much appreciated

2. Relevant equations

All in question

3. The attempt at a solution

Part 1: This I can do by using |A - λI| = 0, finding the eigenvalues, then using A.v=λv and setting up simutaneous quations to find the eigenvalues.

Part 2... this is where I need help please :-)
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. May 11, 2013

### TSny

Hello.

For part (1) you don't need to go through solving for the eigenvalues and eigenvectors. You just want to verify that |Sx+>, say, is an eigenvector of the given matrix. So, just multiply the matrix times the 2x1 vector representing |Sx+> and verify that you get a constant factor times |Sx+>. The constant factor is your eigenvalue.

For part (2), what 2x1 vector do you get when you add $\frac{1}{\sqrt{}2}$(|Sx+> + |Sy+>) ? Can you recognize it?

[Aside: The notation|Sy+> for the second given vector is a bit odd. It seems like |Sx-> would be more appropriate.]

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