Quantum interference between charge excitation paths in a solid-state Mott insulator

Competition between electron localization and delocalization in Mott insulators underpins the physics of strongly correlated electron systems. Photoexcitation, which redistributes charge, can control this many-body process on the ultrafast timescale(1,2). So far, time-resolved studies have been carried out in solids in which other degrees of freedom, such as lattice, spin or orbital excitations(3-5), dominate. However, the underlying quantum dynamics of 'bare' electronic excitations has remained out of reach. Quantum many-body dynamics are observed only in the controlled environment of optical lattices(6,7) where the dynamics are slower and lattice excitations are absent. By using nearly single-cycle near-infrared pulses, we have measured coherent electronic excitations in the organic salt ET-F(2)TCNQ, a prototypical one-dimensional Mott insulator. After photoexcitation, a new resonance appears, which oscillates at 25 THz. Time-dependent simulations of the Mott-Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holon-doublon pairs.