Transform-limited single photons from a single quantum dot

A semiconductor quantum dot mimics a two-level atom. Performance as a single photon source is limited by decoherence and dephasing of the optical transition. Even with high quality material at low temperature, the optical linewidths are a factor of two larger than the transform-limit. A major contributor to the inhomogeneous linewdith is the nuclear spin noise. We show here that the nuclear spin noise depends on optical excitation, increasing (decreasing) with increasing resonant laser power for the neutral (charged) exciton. Based on this observation, we discover regimes where we demonstrate transform-limited linewidths on both neutral and charged excitons even when the measurement is performed very slowly.