Pretty good fractional revival on abelian Cayley graphs

Abstract: Let $\Gamma$ be a graph with the adjacency matrix $A$. The transition matrix of $\Gamma$, denoted $H(t)$, is defined as $H(t) := \exp(-\textbf{i}tA)$, where $t$ is a real variable and $\textbf{i} := \sqrt{-1}$. The graph $\Gamma$ is said to exhibit pretty good fractional revival (PGFR in short) between the vertices $a$ and $b$ if there exists a sequence of real numbers ${t_k}$ such that $\lim_{k\to\infty} H(t_k){\textbf{e}{a}} = \alpha{\textbf{e}{a}} + \beta{\textbf{e}_{b}}$, where $\alpha, \beta \in \mathbb{C}$ with $\beta \neq 0$ and $|\alpha|^2 + |\beta|^2 = 1$. In this paper, we present a necessary and sufficient condition for the existence of PGFR on abelian Cayley graphs. Using that necessary and sufficient condition, we obtain some infinite classes of circulant graphs exhibiting PGFR. We also obtain some classes of circulant graphs not exhibiting PGFR. Some of our results generalize the results of Chan et al. [Pretty good quantum fractional revival in paths and cycles. \textit {Algebr. Comb.} 4(6) (2021), 989-1004.] for cycles. We also prove that an unitary Cayley graph on $n$ vertices ($n \geq 4$) exhibits PGFR if and only if $n = 2p$, where $p$ is a prime.