Progress in Optics

3.1.2: Unpolarized states

Classically, unpolarized states are those that are unchanged by polarization rotations and such states have p = 0. Quantum mechanically, there is a marked difference between states unchanged by polarization rotations and states with p = 0. This strongly underscores the differences between classical and quantum intuition in the realm of polarization.

Quantum states with p = 0 must have $\langle \hat{\mathbf{S}}\rangle =0$. This imposes three constraints onto a generic state that has many more than three degrees of freedom, so it is not surprising that many different states may underly classically unpolarized light.

We begin with unpolarized single-photon ...