Increasing Quantum Dot Electron Spin Coherence with Persistent Spin Narrowing

We demonstrate reproducible initialization of the Overhauser field in a single InAs self-assembled quantum dot using the hole assisted nuclear feedback mechanism. This fixes the mean the Overhauser field to a value determined by two pump lasers and dramatically reduces the statistical broadening of the electron spin resonance arising from averaging over the nuclear spin ensemble, (1/T2*). By initializing for tens of milliseconds, the prepared Overhauser field distribution lasts for well over a second even in the presence of a fluctuating electron spin. Furthermore, we find a mechanism which will initialize the nuclear spins using only a single laser, and that this mechanism involves the evolution of the nuclear spins ``in the dark'', that is, absent any optical field. This new method is directly compatible with the CW readout technique used in recent time-domain spin manipulation experiments.