When I am studying photon physics, one question come to my mind about mixed states. Let a photon be in the state |h> = (a;b) where (a;b) is a column vector with complex entries. How can we calculate the probability of its passing through an x-polaroid? My approach: p(h->x) = |<h|x>||<h|x>|* = |a|^2. But I'm not sure about it.. Can anyone check this idea whether it is true or not.