Question

I'm aware that unpolarized light can be represented by a mixed state12(|x⟩⟨x|+|y⟩⟨y|). It bothers me that in this framework, unpolarized light is a symptom of our classical aleatoric uncertainty. I do remember my professor in my undergraduate quantum physics class telling me once that "unpolarized light is in the superposition of all polarizations". But this doesn't seem to be true? Is there a way to sensibly model unpolarized light as a purely quantum phenomenon or is unpolarization simply a matter of classical probability? I was thinking that one could just brute force model unpolarized light as a normalized vector onL2(PGL(2,R))for all the possible orientations, but that just opens up other questions - why don't we see other continuous superpositions of polarizations - why is unpolarized light special? Maybe it is silly to want unpolarized light to make sense in the world of purely quantum statistics - but it bothers me that unpolarized light involves an inherently different sort of uncertainty than say, a superposition of horizontal and vertically polarized light. Unpolarized light in quantum mechanics is described by a mixed state. That is a probability for a photon to be in either of 2 quantum states. This can arise, for example, if the photon is correlated, i.e. entangled, with other degrees of freedom that you don't measure. In that case the cross terms in a superposition which would normally cause interference have the other degrees of freedom in different states so they give zero. If you know you aren't going to measure those other degrees of freedom, the photon properties can be described by the reduced density matrix, which describes this mixed state.

Answer 1

Unpolarized light is composed of many rays having random polarization directions. It can be polarized by passing it through a polarizing filter or other polarizing material. The intensity of polarized light after passing through a polarizing filter is given by I = Io cos² θ.

Step-by-step explanation:

Unpolarized light is composed of many rays having random polarization directions.

It can be polarized by passing it through a polarizing filter or other polarizing material.

The intensity of polarized light after passing through a polarizing filter is given by I = Io cos² θ,

where Io is the incident intensity and

θ is the angle between the direction of polarization and the axis of the filter.