A continuous-time quantum walk on a graph is a matrix-valued function
exp(−iAt) over the reals, where
A is the adjacency matrix of the graph. Such a quantum walk has universal perfect state transfer if for all vertices
u,v, there is a time where the
(v,u) entry of the matrix exponential has unit magnitude. We prove new characterizations of graphs with universal perfect state transfer. This extends results of Cameron et al. (Linear Algebra and Its Applications, 455:115-142, 2014). Also, we construct non-circulant families of graphs with universal perfect state transfer. All prior known constructions were circulants. Moreover, we prove that if a circulant, whose order is prime, prime squared, or a power of two, has universal perfect state transfer then its underlying graph must be complete. This is nearly tight since there are universal perfect state transfer circulants with non-prime-power order where some edges are missing.
The chiral quantum walk is an emerging tool for state transfer as it helps circumvent the barrier of no-go theorems in quantum transport. Yet, it remains largely unexplored. We prove a conjecture that universal monogamy violations for perfect quantum transfer in large graphs require couplings beyond the simple imaginary [Formula: see text]. This motivates our constructions of new monogamy violations on sparse graph products. Then, we show a novel violation of periodicity and the first known examples of one-way perfect quantum transfer via transcendental couplings.This article is part of the theme issue 'Numerical analysis, spectral graph theory, orthogonal polynomials and quantum algorithms'.