Subcycle 2-Electron Quantum Interference in Nonsequential Double Ionization

Strong-field nonsequential double ionization (NSDI) is a key process for probing electron–electron correlations. Yet, while its classical aspects are well understood, the quantum nature of these correlations has remained largely hidden. Here, we report the direct observation of subcycle 2-electron quantum interference in the NSDI of xenon. This observation was achieved using triple-coincidence momentum spectroscopy with 400-nm laser pulses at relatively low intensities. The interference stems from the coherent superposition of time-delayed recollision–excitation with subsequent ionization pathways with electron exchange. Strong-field approximation simulations qualitatively reproduce the interference patterns and reveal that they encode the information about the time delay between recollision of the first electron and ionization of the second electron. This work demonstrates a new route for time-resolving quantum-correlated electron dynamics with attosecond precision.