  The competition between electron localization and de-localization in Mott
insulators underpins the physics of strongly-correlated electron systems.
Photo-excitation, which re-distributes charge between sites, can control this
many-body process on the ultrafast timescale. To date, time-resolved studies
have been performed in solids in which other degrees of freedom, such as
lattice, spin, or orbital excitations come into play. However, the underlying
quantum dynamics of bare electronic excitations has remained out of reach.
Quantum many-body dynamics have only been detected in the controlled
environment of optical lattices where the dynamics are slower and lattice
excitations are absent. By using nearly-single-cycle near-IR pulses, we have
measured coherent electronic excitations in the organic salt ET-F2TCNQ, a
prototypical one-dimensional Mott Insulator. After photo-excitation, a new
resonance appears on the low-energy side of the Mott gap, 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.
