Spin-selective AC Stark shifts in a charged quantum dot*

The energy levels of an optically active quantum system can be shifted via the AC Stark effect by applying a strong, far-detuned laser. We achieve an AC Stark shift of up to 20 GHz in a single negatively charged InGaAs quantum dot. In addition to the AC Stark shift we observe a small Overhauser shift of <1 GHz which we attribute to dynamic nuclear polarization via electron spin pumping induced by the high power, although far-detuned AC Stark laser. Both shifts are polarization selective, meaning polarization control of the applied laser provides control over the energy level structure of the system. Applying a circularly polarized laser therefore allows for a spin-selective modification to the energies, shifting one spin manifold and not the other. The reconfiguration of the energy levels is reversible and can be applied or removed rapidly on the timescale of a few nanoseconds. In principal this ability to rapidly and coherently reconfigure the energy structure and polarization selection rules may enable single-shot fluorescence readout in a small transverse magnetic field.