# Spin observables

1. Nov 1, 2008

### intervoxel

What is the criterion to define the reference axes, since $S_x$, $S_y$ and $S_z$ correspond to different Pauli matrices?

2. Nov 1, 2008

### intervoxel

I found the answer: In the theory the ladder operators are defined as

S_{\pm} \equiv S_x \pm iS_y.

That is, they act on the z direction.