Quantum chaos and complexity from string scattering amplitudes

Abstract: We introduce Krylov spread complexity in the context of black hole scattering by studying highly excited string states (HESS). Krylov complexity characterizes chaos by quantifying the spread of a state or operator under a known Hamiltonian. In contrast, quantum field theory often relies on S-matrices, where the Hamiltonian density becomes non-trivially time-dependent rendering the computations of complexity in Krylov basis exponentially hard. We define Krylov spread complexity for scattering amplitudes by analyzing the distribution of extrema, treating these as eigenvalues of a fictional Hamiltonian that evolves a thermo-field double state non-trivially. Our analysis of black hole scattering, through highly excited string states scattering into two or three tachyons, reveals that the Krylov complexity of these amplitudes mirrors the behavior of chaotic Hamiltonian evolution, with a pre-saturation peak indicating chaos. This formalism bridges the concepts of chaos in scattering and state evolution, offering a framework to distinguish different scattering processes.