The diffusive spectral gap of the qubit eSWAP chain

We prove that the second-moment Hamiltonian of the open qubit eSWAP chain has a diffusive spectral gap, resolving a conjecture of Li, Zheng, Liu, Jiang, and Liu. We also characterise the ground space on every connected interaction graph with at least five vertices and prove a uniform lower bound for the unnormalised complete-graph gap. We apply these results to sequential and brickwork circuits and prove that the periodic chain also has a diffusive spectral gap.