We can then send the beam through another apparatus like this one, although in this second apparatus we may choose to filter away the top beam instead of the bottom one. If we filter away the bottom beam in the first apparatus and then we insert a filter in the top beam trajectory in the second apparatus, we will filter away all light in the beam and end up with nothing. Suppose that light in the top beam is polarized vertically and light in the bottom beam is polarized horizontally. We will use symbols v and h to denote polarization states of the beams. Now we are going to repeat our procedure with turning down light intensity until we can see individual photons, and what we used to call light inensity in classical physics now become probability.
Photons carry polarization like classical beams.
The probability amplitude that vertically polarized photon can be found in the horizontally polarized state is zero. Then we can block the bottom beam in both apparatuses, and of course, the top beam will emerge at the end undisturbed. The probability that a photon that is in vertical polarization to begin with is found again in the same state is 1. Now suppose the second apparatus has been rotated about the beam axis by some angle. Now the situation becomes really interesting.