Charge and spin readout scheme for single self-assembled quantum dots

We propose an all optical spin initialization and readout concept for single self-assembled quantum dots, and demonstrate its feasibility. Our approach is based on a gateable single dot photodiode structure that can be switched between charge and readout modes. After optical electron generation and storage, we propose to employ a spin-conditional absorption of a circularly polarized light pulse tuned to the single negatively charged exciton transition to convert the spin information of the resident electron to charge occupancy. Switching the device to the charge readout mode then allows us to probe the charge state of the quantum dot (1e,2e) using nonresonant luminescence. The spin orientation of the resident electron is then reflected by the photoluminescence (PL) yield of doubly (X2-) and singly (X-1) charged transitions in the quantum dot. To verify the feasibility of this spin readout concept, we have applied time gated photoluminescence to confirm that selective optical charging and efficient nonperturbative measurement of the charge state can be performed on the same dot. The results show that, by switching the electric field in the vicinity of the quantum dot, the charging rate can be switched between a regime of efficient electron generation (Γ 106 s-1 W-1 cm2) and a readout regime, where the charge occupancy and, therefore, the spin state of the dot can be tested via PL over millisecond timescales, without altering it. Our results show that such a quasicontinuous, nonperturbative readout of the charge state of the dot allows increasing the dark time available for undisturbed spin manipulation and storage into the millisecond range, while still providing sufficient signal for high fidelity readout. Consequently, our readout scheme would allow the investigation of spin relaxation and decoherence mechanisms over the long timescales, predicted by theory.