State Transfer on Paths with Weighted Loops

Abstract: We consider the fidelity of state transfer on an unweighted path on $n$ vertices, where a loop of weight $w$ has been appended at each of the end vertices. It is known that if $w$ is transcendental, then there is pretty good state transfer from one end vertex to the other; we prove a companion result to that fact, namely that there is a dense subset of $[1,\infty)$ such that if $w$ is in that subset, pretty good state transfer between end vertices is impossible. Under mild hypotheses on $w$ and $t$, we derive upper and lower bounds on the fidelity of state transfer between end vertices at readout time $t$. Using those bounds, we localise the readout times for which that fidelity is close to $1$. We also provide expressions for, and bounds on, the sensitivity of the fidelity of state transfer between end vertices, where the sensitivity is with respect to either the readout time or the weight $w$. Throughout, the results rely on detailed knowledge of the eigenvalues and eigenvectors of the associated adjacency matrix.