Krylov spread complexity of quantum walks

Abstract

Given the recent advances in quantum technology, the complexity of quantum states is an important notion. The idea of the Krylov spread complexity has come into focus recently with the goal of capturing this in a quantitative way. The present paper sheds light on the Krylov complexity measure by exploring it in the context of continuous-time quantum walks on graphs. A close relationship between Krylov spread complexity and the concept of limiting distributions for quantum walks is established. Moreover, using a graph optimization algorithm, quantum-walk graphs are constructed that have vertex states with minimal and maximal (long-time average) Krylov $\overline{\mathcal{C}}$ complexity. This reveals an empirical upper bound for the $\overline{\mathcal{C}}$ complexity as a function of Hilbert-space dimension and an exact lower bound.