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

Abstract

The ability to coherently manipulate single electron and photon states is vital for quantum information processing. However, typical quantization and correlation energies restrict processing rates in real implementations owing to the time energy uncertainty. Here we report optical initialization, manipulation and probing of a single CdSe/ZnSe semiconductor quantum dot on femtosecond timescales, the ultimate limit for clean quantum operations in such 'artificial atoms'. Resonant pump probe measurements on a donor-charged quantum dot reveal that the fundamental exciton absorption is switched off through instantaneous Coulomb renormalization. Optical gain builds up following ultrafast intraband relaxation, with a thermalization rate determined by the electron spin. Operating the system in a nonlinear regime, we demonstrate the ability to change the number of quanta in a femtosecond light pulse by exactly plusminus1. This deterministic single-photon amplifier is characterized by a flat gain spectrum.