We report the direct observation of quantum coupling in individual quantum dot molecules and its manipulation using static electric fields. A pronounced anticrossing of different excitonic transitions is observed as the electric field is tuned. A comparison of our experimental results with theory shows that the observed anticrossing occurs between excitons with predominant spatially direct and indirect character and reveals a field driven transition of the nature of the molecular ground state exciton wave function. Finally, the interdot quantum coupling strength is deduced optically and its dependence on the interdot separation is calculated.

Recent experimental developments in the field of semiconductor quantum dot spectroscopy will be discussed. First we report about single quantum dot exciton twolevel systems and their coherent properties in terms of single qubit manipulations. In the second part we report on coherent quantum coupling in a prototype "two-qubit" system consisting of a vertically stacked pair of quantum dots. The interaction can be tuned in such quantum dot molecule devices using an applied voltage as external parameter.

We probe acoustic phonon mediated relaxation between tunnel coupled exciton states in an individual quantum dot molecule in which the inter-dot quantum coupling and energy separation between exciton states is continuously tuned using static electric field. Time resolved and temperature dependent optical spectroscopy are used to probe inter-level relaxation around the point of maximum coupling. The radiative lifetimes of the coupled excitonic states can be tuned from

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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...

Strongly confined excitons in self-assembled InGaAs quantum dot clusters produced by a hybrid growth method

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We report on single InGaAs quantum dots embedded in a lateral electric field device. By applying a voltage we tune the neutral exciton transition into resonance with the biexciton using the quantum confined Stark effect. The results are compared to theoretical calculations of the relative energies of exciton and biexciton. Cascaded decay from the manifold of single exciton-biexciton states has been predicted to be a new concept to generate entangled photon pairs on demand without the need to suppress the fine structures splitting of the neutral exciton.The controlled next generation of entanglement is an important concept in both quantum information science [1] and quantum cryptography [2]. Benson et al. [3] proposed the use of biexciton-exciton cascade in semiconductor quantum dots (QDs) to generate such nonclassical states of light. Subsequently, many groups [4][5][6] worldwide have demonstrated the generation of entangled photon pairs via this process. Creating polarization entangled, rather than classically correlated [7], photon pairs has been achieved by tuning the exciton fine structure splitting to zero [8]. Recently, this has been achieved by several groups [9][10][11] by applying an electric field in a lateral geometry in the base plane of the QD. An alternative approach for realizing an entangled photon source, proposed by Avron et al. [12], requires the exciton to be tuned into resonance with the biexciton in order to exploit time reordering of the emitted photon pairs.In this Letter we demonstrate tuning of exciton and biexciton states of a single InGaAs QD into resonance by applying an electric field parallel to the QD layer. Comparison with theory shows that the sign of the energy shift arising from the quantum confined Stark effect [13] is opposing for the exciton and biexciton states. This allows us to bring both transitions into resonance at electric fields of F∼17 kV/cm. Such devices are promising candidates for realizing an electrically tunable source of entangled photon pairs. The samples investigated were grown by molecular beam epitaxy and consist of the following layer sequence: starting with a semi-insulating (100) GaAs wafer we deposited a 300 nm thick GaAs buffer layer, followed by 25 periods of a AlAs (2.5 nm)/GaAs (2.5 nm) superlattice. We grew a 200 nm thick GaAs waveguide, into the center of which a single layer of nominally In 0.5 Ga 0.5 As QDs was incorporated. The areal density of the QDs was varied during the growth by stopping the rotation of the wafer and samples with a density of ∼10 µm −2 , as revealed by atomic force microscope measurements (not shown here), were chosen for further processing. Using a combination of optical lithography and electron beam metalization we established back-to-back Ti/Au Schottky gates on the surface of the sample. In a first evaporation step we deposit a 20 nm titanium undercoating, followed by a 60 nm gold layer. In a second lithography step we produced larger Ti/Au bonding pads with a layer thickness of 20 nm and 200 nm, respectively. In t...