1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I have totally no idea how to solve this question. But I find it somehow similar to the Larmor precession problem. Therefore I try to solve my problem by referring to that. Are there any mistakes if I do it like this? p.s. since the matrix elements are quite complicated, in order to save time, I use "c" to stand for cosine, "e" to stand for exponential that repeat over and over again.