Femtosecond probing of few-fermion dynamics and deterministic single-photon gain in a single semiconductor quantum dot

We report on femtosecond readout of the optical properties of a single CdSe/ZnSe quantum dot. Owing to the uncertainty principle, this timescale represents the ultimate limit for coherent quantum manipulation of such an artificial atom. After resonant excitation of a hot electron-hole pair the absorption of the fundamental exciton resonance is switched off via instantaneous Coulomb renormalization. Subsequently, optical gain builds up after ultrafast intraband relaxation. The speed of thermalization is dominated by the electron spin, since our system is charged permanently with one excess electron. When operating in the nonlinear regime, the number of quanta in a femtosecond light pulse may be changed by exactly ±1. We demonstrate that this deterministic single photon amplifier is characterized by a flat gain spectrum.