Title: Strong-Field Nonsequential Double Photoionization Using Virtual Detector Theory with Path Summation

Abstract: We present an ab initio study of the nonsequential strong-field ionization dynamics of a model two-electron atom with helium character. Single- and double-ionization events are characterized and displayed using detector signals extracted at different points in the two-electron two-dimensional space. The double photoelectron momentum distribution is calculated via coherent path-summation over virtual particle trajectories. Insights into different ionization and electron recollision pathways are gained from detailed virtual-particle tracking and energy-time readouts. This study demonstrates the extension of virtual detector theory to strong-field multi-electron quantum dynamics and highlights the importance of the evolving quantum phase in quasi-classical electron propagation.