Unraveling interchannel quantum interference in below-threshold nonsequential double ionization with statistical measures

We present a systematic study of interchannel quantum interference in laser-induced nonsequential double ionization within the strong-field approximation. Focusing on the below-threshold intensity regime where the recollision excitation with subsequent ionization (RESI) pathway dominates, we derive analytical phase conditions governing interference between distinct excitation channels for arbitrary driving fields. To quantify the interplay between channels resulting from a vast number of interfering processes, we introduce statistical metrics based on the earth mover's distance, allowing us to assess the relative weight of each channel's contribution to the two-electron photoelectron momentum distributions. We identify key factors that determine whether interchannel interference is appreciable, such as comparable channel intensities, strong spatial overlap between the excited-state wave functions, and the energy difference between contributing channels. We demonstrate that for linearly polarized few-cycle pulses, the typical intrachannel interference features associated with exchange, temporal shifts, and combined exchange-temporal interference are retained with interchannel interference. Our findings establish a hierarchy of interference mechanisms in RESI and may provide practical guidelines for enhancing or suppressing interference in different regions of the momentum plane. The tool kit presented in this work is transferable to a wide range of interferometric schemes involving different excitation channels.