Qubit polarization ($\mathbf{P}=\mathrm{Tr}(\rho \mathbf{\sigma })$)

The ensemble Bloch vector of a two-level quantum state. With $\rho =(I+\mathbf{P}\cdot \mathbf{\sigma })/2$, 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.