# Homework Help: [QM] Expectation value in spin-1/2 state

1. Dec 26, 2012

### Lengalicious

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

Basically I need to produce a state for a spin-1/2 particle such that the expectation value of <Jz> = 0 where <Jz> is for a spin-1 particle.

2. Relevant equations

Jz = (1 0 0, 0 0 0, 0 0 -1) <--[3x3] matrix

3. The attempt at a solution

I don't quite understand how to do this because up till now we have been trying to find expectation values for observables in same spin states.

Anyways heres my attempt:

ψ* Jz ψ = <Jz>
= ψ* (1 0 0, 0 0 0, 0 0 -1) ψ

the reason I get no further than this is cuz I don't see how i'm supposed to find a spin-1/2 state which would be in the form of a [2x1] matrix and then multiply the [3x3] matrix by a [2x1] matrix, whats the deal here? I'm guessing i've completely misinterpreted something but I can't see what. .

Last edited: Dec 26, 2012
2. Dec 27, 2012

### cragar

can you be more precise with your question.
going from spin-1 to spin-1/2 doesn't really make sense, can you give us more context
on what your trying to do.

3. Dec 27, 2012

### Lengalicious

The exact question is as follows:
Produce a state of spin 1/2 particle such that when measuring <Jz> = 0, normalise that state. Can it be that at the same time <Jz> = 0 and <Jx> = 0?

It doesn't explicitly tell me that <Jz> is for a spin-one particle but I've just assumed that the J matrices are since thats what our lecturer has used as notation for spin-one particles and S for spin 1/2 particles. Hope this makes the question clearer?