  Efficient excitation of a single two-level system usually requires that the
driving field is at the same frequency as the atomic transition. However, the
scattered laser light in solid-state implementations can dominate over the
single photons, imposing an outstanding challenge to perfect single-photon
sources. Here, we propose a background-free method using a phase-locked
dichromatic electromagnetic field with no spectral overlap with the optical
transition for a coherent control of a two-level system, and we demonstrate
this method experimentally with a single quantum dot embedded in a micropillar.
Single photons generated by pi excitation show a purity of 0.988(1) and
indistinguishability of 0.962(6). Further, the phase-coherent nature of the
two-color excitation is captured by the resonance-fluorescence intensity
dependence on the relative phase between the two pulses. Our two-color
excitation method adds a useful toolbox to the study of atom-photon
interaction, and the generation of spectrally isolated indistinguishable single
photons.
