Cayley graphs and analysis of quantum cost for reversible circuit synthesis

We propose the theory of Cayley graphs as a framework to analyse gate counts and quantum costs resulting from reversible circuit synthesis. Several methods have been proposed in the reversible logic synthesis literature by considering different libraries whose gates are associated to the generating sets of certain Cayley graphs. In a Cayley graph, the distance between two vertices corresponds to the optimal circuit size. The lower bound for the diameter of Cayley graphs is also a lower bound for the worst case for any algorithm that uses the corresponding gate library. In this paper, we study two Cayley graphs on the Symmetric Group S2n : the first, denoted by In, is defined by a generating set associated to generalized Toffol i gates; and the second, the hypercube Cayley graph Hn, is defined by a generating set associated to multiple-control Toffoli gat es. Those two Cayley graphs have degree n2 n−1 and order 2 n !. Maslov, Dueck and Miller proposed a reversible circuit synthesis that we model by the Cayley graph In. We propose a synthesis algorithm based on the Cayley graph Hn with upper bound of (n 1)2 n +1 multiple-control Toffoli gates. In addition, the diameter of the Cayley graph Hn gives a lower bound of n2 n−1 . Index Terms—circuit synthesis, quantum complexity, Cayley graphs.