Abstract
Let A be the adjacency matrix of a graph X. The quantum walk based on X is determined by the family of unitary matrices U(t) = exp(itA). The walk allows perfect state transfer from vertex a to vertex b if there is a time t such that |U(t)_{a,b}|=1, and physicists are interested in knowing when this is possible. I will discuss how tools from algebraic graph theory and number theory can be used to study this question, and questions related to it.