7.9. SU(2) → SO(3) homomorphism#
The Cornwell theorem from the group theory (see for example Section 3, Chapter 5 of [3]) gives the relation between the rotation of the transition amplitude and the physical vector of polarization sensitivity:
\[
\begin{align}
R_{ij}(\phi,\theta,\chi) &= \frac{1}{2}\mathrm{tr}\left(
D^{1/2*}(\phi,\theta,\chi) \sigma^P_i D^{1/2*\dagger}(\phi,\theta,\chi) \sigma^P_j
\right)\,,
\end{align}
\]
where \(\mathrm{tr}\) represents the trace operation applied to the product of the two-dimensional matrices, \(D\) and \(\sigma^P\), and \(R_{ij}(\phi,\theta,\chi)\) is a three-dimensional rotation matrix implementing the Euler transformation to a physical vector.
\[\begin{split}\displaystyle \left[\begin{matrix}- \sin{\left(\chi \right)} \sin{\left(\phi \right)} + \cos{\left(\chi \right)} \cos{\left(\phi \right)} \cos{\left(\theta \right)} & - \sin{\left(\chi \right)} \cos{\left(\phi \right)} \cos{\left(\theta \right)} - \sin{\left(\phi \right)} \cos{\left(\chi \right)} & \sin{\left(\theta \right)} \cos{\left(\phi \right)}\\\sin{\left(\chi \right)} \cos{\left(\phi \right)} + \sin{\left(\phi \right)} \cos{\left(\chi \right)} \cos{\left(\theta \right)} & - \sin{\left(\chi \right)} \sin{\left(\phi \right)} \cos{\left(\theta \right)} + \cos{\left(\chi \right)} \cos{\left(\phi \right)} & \sin{\left(\phi \right)} \sin{\left(\theta \right)}\\- \sin{\left(\theta \right)} \cos{\left(\chi \right)} & \sin{\left(\chi \right)} \sin{\left(\theta \right)} & \cos{\left(\theta \right)}\end{matrix}\right]\end{split}\]
\[\begin{split}\displaystyle \left[\begin{matrix}- \sin{\left(\chi \right)} \sin{\left(\phi \right)} + \cos{\left(\chi \right)} \cos{\left(\phi \right)} \cos{\left(\theta \right)} & - \sin{\left(\chi \right)} \cos{\left(\phi \right)} \cos{\left(\theta \right)} - \sin{\left(\phi \right)} \cos{\left(\chi \right)} & \sin{\left(\theta \right)} \cos{\left(\phi \right)}\\\sin{\left(\chi \right)} \cos{\left(\phi \right)} + \sin{\left(\phi \right)} \cos{\left(\chi \right)} \cos{\left(\theta \right)} & - \sin{\left(\chi \right)} \sin{\left(\phi \right)} \cos{\left(\theta \right)} + \cos{\left(\chi \right)} \cos{\left(\phi \right)} & \sin{\left(\phi \right)} \sin{\left(\theta \right)}\\- \sin{\left(\theta \right)} \cos{\left(\chi \right)} & \sin{\left(\chi \right)} \sin{\left(\theta \right)} & \cos{\left(\theta \right)}\end{matrix}\right]\end{split}\]
assert R_SO3 == R_SU2