# Expectation value of X and Y component of angular momentum

1. ### xinoki

6
1. The problem statement, all variables and given/known data
Show:
<Jx>=<Jy>=0

2. Relevant equations

Jx=1/2(J++J-)
Jy=1/2(J+-J-)

3. The attempt at a solution

<jm l Jx l jm> = < jm l 1/2 J+ l jm> + < jm l j- l jm >
= < jm l h/2 sqrt [(j-m)(j+m+1)] + h/2sqrt[(j+m)(j+m+1) l jm >

i am not sure how to apply the next step

2. ### Matterwave

3,859
Note that |j,m> is NOT an eigenstate of either J+ or J-. In fact, J+|j,m>=k|j,m+1>, etc.

Additionally note that $$<j,m|j,n>=\delta_{m,n}$$

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