  We study perfect state transfer of quantum walks on signed graphs. Our aim is
to show that negative edges are useful for perfect state transfer. Specific
results we prove include: (1) The signed join of a negative 2-clique with any
positive (n,3)-regular graph has perfect state transfer even if the unsigned
join does not. Curiously, the perfect state transfer time improves as n
increases. (2) A signed complete graph has perfect state transfer if its
positive subgraph is a regular graph with perfect state transfer and its
negative subgraph is periodic. This shows that signing is useful for creating
perfect state transfer since no complete graph (except for the 2-clique) has
perfect state transfer. (3) The double-cover of a signed graph has perfect
state transfer if the positive subgraph has perfect state transfer and the
negative subgraph is periodic. Here, signing is useful for constructing
unsigned graphs with perfect state transfer. Furthermore, we study perfect
state transfer on a family of signed graphs called the exterior powers which is
derived from a many-fermion quantum walk on graphs.
