The polarization of a single photon is a quantum property, so it's essentially a probability distribution. Passing a photon through a polarizer modifies the pro

The polarization of a single photon is a quantum property, so it's essentially a probability distribution. Passing a photon through a polarizer modifies the probability distribution such that "more perpendicular" polarizations are now less likely and "more parallel" ones more likely. (Polarization is a superposition (ie. a linear combination) of two orthogonal basis vectors, and a polarizer projects a polarization vector onto one of the basis vectors.)