Q: Why is photon linear polarization, important in EPR experiments, constrained
by angular momentum conservation?
Answers
A: Photons are boson particles (see below) that have an intrinsic spin angular
momentum of one -
unit. If this spin vector points in the direction of motion of
the photon, the photon is in a state of left circular polarization; if the spin vector
points against the direction of motion, it is in a state of right circular polarization.
Because the photon travels at the speed of light, special relativity does not allow
its spin to point in any other directions. States of linear polarization can be formed
by a superposition of the right and left circular polarization states, as discussed in
Sect. 6.6 and quantified by Eqs. 6.4 and 6.5. Since angular momentum conservation
constrains the spins of the photons of a system, it also constrains the states of linear
polarization, and EPR experiments make use of this.