Spin State Rotation: Understanding Negative Probability?

[eoghan]

Hi there!
If I have an electron in the spin state |+> and I rotate it by an angle of 360°, then the state becomes -|+>. Now, if I want to find the probability of measuring the spin up, I have:
P(|+>)=<+|(-|+>)=-1
So after a rotation of 360° I have a negative probability??

 

[eoghan]

Ok.. I found the error! I have to consider the square of -1 which is 1... really a stupid error