How can we find the variance of S_x, S_y, and S_z?

1. Dec 5, 2007

man@SUT

Given |Psi> = a|up>+b|down>, in principle it should not be so difficult but when I calculate

Delta S=Sqrt[<S^2_x,S^2_y, or S^2_z>-<S_x,S_y, or S_z>^2]

the second term gives the problem. Lots of many terms a*, a, b*, b which is not cancelled involve.

Whoever knows the way to get rid of this problem, please let me know.

2. Dec 6, 2007

Galileo

Can you show what you've tried so far?
For example, how did you go about finding <S_x>?