Journal of Combinatorial Theory. Series A (2013) 120(7) 1649-1662

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If X is a graph with adjacency matrix A, then we define H(t) to be the operator exp(i t A). The Schur (or entrywise) product H(t) {ring operator} H( - t) is a doubly stochastic matrix and because of work related to quantum computing, we are concerned with the average mixing matrix M̂X, defined by. M̂X=limT→∞1T∫0TH(t){ring operator}H(-t)dt. In this paper we establish some of the basic properties of this matrix, showing that it is positive semidefinite and that its entries are always rational. We see that in a number of cases its form is surprisingly simple. Thus for the path on n vertices it is equal to. 12n+2(2J+I+T) where T is the permutation matrix that swaps j and n + 1 - j for each j. If X is an odd cycle or, more generally, if X is one of the graphs in a pseudocyclic association scheme on n vertices with d classes, each of valency m, then its average mixing matrix is. n-m+1n2J+m-1nI. (One reason this is interesting is that a graph in a pseudocyclic scheme may have trivial automorphism group, and then the mixing matrix is more symmetric than the graph itself.). © 2013 Elsevier Inc.

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Godsil, C. (2013). Average mixing of continuous quantum walks. *Journal of Combinatorial Theory. Series A*, *120*(7), 1649–1662. https://doi.org/10.1016/j.jcta.2013.05.006

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