- #1
Silicon-Based
- 51
- 1
- TL;DR Summary
- Rotation of spin-1/2 state
Suppose I have a positive spin-##1/2## eigenstate pointing in the ##z##-direction. If I apply a rotation operator by an angle ##\theta## around the ##z##-axis the state should of course not change. However, if I write it out explicitly, I find something different:
$$R_z(\theta)|\uparrow\rangle =
\begin{pmatrix}
e^{-i\theta/2} & 0 \\
0 & e^{i\theta/2}
\end{pmatrix}
\begin{pmatrix}
1 \\
0
\end{pmatrix} =
e^{-i\theta/2}|\uparrow\rangle.
$$Is this factor of ##e^{-i\theta/2}## like a phase that can be added and removed at will because it's not an observable? I don't see a different way to rationalize this with the physical expectation above.
$$R_z(\theta)|\uparrow\rangle =
\begin{pmatrix}
e^{-i\theta/2} & 0 \\
0 & e^{i\theta/2}
\end{pmatrix}
\begin{pmatrix}
1 \\
0
\end{pmatrix} =
e^{-i\theta/2}|\uparrow\rangle.
$$Is this factor of ##e^{-i\theta/2}## like a phase that can be added and removed at will because it's not an observable? I don't see a different way to rationalize this with the physical expectation above.