  We demonstrate systematic resonance fluorescence recovery with near-unity
emission efficiency in single quantum dots embedded in a charge-tunable device
in a wave-guiding geometry. The quantum dot charge state is controlled by a
gate voltage, through carrier tunneling from a close-lying Fermi sea,
stabilizing the resonantly photocreated electron-hole pair. The electric field
cancels out the charging/discharging mechanisms from nearby traps toward the
quantum dots, responsible for the usually observed inhibition of the resonant
fluorescence. Fourier transform spectroscopy as a function of the applied
voltage shows a strong increase of the coherence time though not reaching the
radiative limit. These charge controlled quantum dots act as quasi-perfect
deterministic single-photon emitters, with one laser pulse converted into one
emitted single photon.
