  Narrow line-widths and the possibility of enhanced spontaneous emission via
coupling to microcavities make semiconductor quantum dots ideal for harnessing
coherent quantum phenomena at the single photon level. So far, however, all
approaches have relied on incoherent pumping, which limits the desirable
properties of the emission. In contrast, coherent excitation was recognized to
be necessary for providing both improved photon indistinguishability and high
efficiency, and offers the quantum control capabilities required for basic
qubit manipulations. Here we achieve, for the first time, resonant and coherent
excitation of a quantum dot with simultaneous collection of the non-classical
photon emission. Second-order correlation measurements show the unique
signature of a coherently-driven two-level quantum emitter: the photon
statistics become oscillatory at high driving fields, reflecting the coherent
evolution of the excitonic ground state of the quantum dot.
