A mixed graph is a graph in which both directed and undirected edges may occur.
If only directed edges exist, the graph is called a directed graph.
If only undirected edges exist, it is called an undirected graph.
For a mixed graph with no loops or multiple edges, the Hermitian adjacency matrix represents
an undirected graph edge by 1 and an arc by the conjugate
pair and
. This extends spectral questions such as perfect
state transfer to mixed graphs (Song and Lin 2026).