Qubit polarization
= Qubit polarization
{title2=$\mathbf P=\operatorname{Tr}(\rho\boldsymbol\sigma)$}
The ensemble <Bloch vector> of a two-level <quantum state>. With $\rho=(I+\mathbf P\cdot\boldsymbol\sigma)/2$, full polarization has $|\mathbf P|=1$ and is pure, while zero polarization has $\rho=I/2$ and is maximally mixed. This internal-state ensemble polarization differs from the <polarization> of a light wave used to drive it.