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 mode. 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 non-resonant luminescence.The spin orientation of the resident electron is then reflected by the photoluminescence yield of doubly (X 2− ) 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 non perturbative 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 ≫ 10 6 s −1 W −1 cm 2 ) 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 quasi-continuous, non perturbative readout of the charge state of the dot allows to increase the dark time available for undisturbed spin manipulation and storage into the millisecond range, whilst 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 is possible.
We optically probe the spectrum of ground and excited state transitions of an individual, electrically tunable self-assembled quantum dot molecule. Photocurrent absorption measurements show that the spatially direct neutral exciton transitions in the upper and lower dots are energetically separated by only ∼ 2 meV. Excited state transitions ∼ 8 − 16 meV to higher energy exhibit pronounced anticrossings as the electric field is tuned due to the formation of hybridized electron states. We show that the observed excited state transitions occur between these hybridized electronic states and different hole states in the upper dot. By simultaneously pumping two different excited states with two laser fields we demonstrate a strong (88% on-off contrast) laser induced switching of the optical response. The results represent an electrically tunable, discrete coupled quantum system with a conditional optical response.Quantum dot (QD) nanostructures formed by strain driven self-assembly are ideal for solid state quantum optics experiments due to their discrete optical spectrum, strong interaction with light and robust quantum coherence for both interband polarization 1,2 and spin 3 . The ease with which such nanostructures can be embedded into electrically active devices allows for tuning of the transition frequency and control of charge occupancy 4 . Self-assembly provides a natural way to realize few dot systems via vertical stacking to produce more sophisticated nanostructures with coherent inter-dot coupling due to carrier tunneling 5-12 . When combined with the potential to coherently manipulate excitons over ultrafast timescales using precisely timed laser and electrical control pulses 13-15 such systems raise exciting prospects for the operation of small scale few qubit systems in a solid-state device. Very recently, conditional quantum dynamics for a single resonantly driven QD-molecule (QDM) 16 and spin dependent quantum jumps have been observed 8,17 .In this paper we employ photocurrent (PC) absorption, photoluminescence (PL) emission and PLexcitation (PLE) spectroscopy to trace the spectrum of ground and excited state transitions of an individual selfassembled QD-molecule as their character is electrically tuned from spatially direct to indirect. PC absorption allows us to identify the spatially direct neutral exciton transitions in both the upper (X ud ) and lower (X ld ) dots in the molecule. A number of excited state transitions are identified in PLE ∼ 8 − 16 meV above X ud . These excited states exhibit pronounced anticrossings (energy splitting ∆E ∼ 3.2 − 3.5 meV) as the electric field F is tuned. Excited state transitions are identified from voltage dependent PLE measurements to correspond to transitions between these hybridized electronic states and different hole orbitals in the upper dot. By performing a multi-color experiment where the QDM is simultaneously excited with different frequency lasers, we demonstrate how the resonant excitation of indirect excitons or exci-X ld X ud X ind FIG. 1. (Color onli...
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