On weak odd domination and graph-based quantum secret sharing

A weak odd dominated (WOD) set in a graph is a subset B of vertices for which there exists a distinct set of vertices C such that every vertex in B has an odd number of neighbors in C. We point out the connections of weak odd domination with odd domination, ? , ? -domination, and perfect codes. We introduce bounds on ? ( G ) , the maximum size of WOD sets of a graph G, and on ? ' ( G ) , the minimum size of non-WOD sets of G. Moreover, we prove that the corresponding decision problems are NP-complete.The study of weak odd domination is mainly motivated by the design of graph-based quantum secret sharing protocols: a graph G of order n corresponds to a secret sharing protocol whose threshold is ? Q ( G ) = max ? ( ? ( G ) , n - ? ' ( G ) ) . These graph-based protocols are very promising in terms of physical implementation, however all such graph-based protocols studied in the literature have quasi-unanimity thresholds (i.e. ? Q ( G ) = n - o ( n ) where n is the order of the graph G underlying the protocol). In this paper, we show using probabilistic methods the existence of graphs with smaller ? Q (i.e. ? Q ( G ) ? 0.811 n where n is the order of G). We also prove that deciding for a given graph G whether ? Q ( G ) ? k is NP-complete, which means that one cannot efficiently double check that a graph randomly generated has actually a ? Q smaller than 0.811n.