Entanglement creation in a quantum dot-nanocavity system

We explore the possibility to entangle an excitonic two-level system in a semiconductor quantum dot (QD) with a cavity defined on a photonic crystal by sweeping the cavity frequency across its resonance with the exciton transition. The dynamic cavity detuning is established by a radio frequency surface acoustic wave (SAW). It induces Landau-Zener (LZ) transitions between the excitonic and the photonic degrees of freedom and thereby creates a superposition state. We optimize this scheme by using tailored Fourier-synthesized SAW pulses with up to five harmonics. The theoretical study is performed with a master equation approach for present state-of-the-art setups. Assuming experimentally demonstrated system parameters, we demonstrate that the composed pulses increase both the maximum entanglement and its persistence. The latter is only limited by the dominant dephasing mechanism; i.e., the photon loss from the cavity.Comment: 7 pages, 4 figure

Ion beam synthesis of nanothermochromic diffraction gratings with giant switching contrast at telecom wavelengths

Nanothermochromic diffraction gratings based on the metal-insulator transition of $\mathrm{VO_2}$ are fabricated by site-selective ion beam implantation in a $\mathrm{SiO_2}$ matrix. Gratings were defined either (i) directly by spatially selective ion beam synthesis or (ii) by site-selective deactivation of the phase transition by ion beam induced defects. The strongest increase of the diffracted light intensities was observed at a wavelength of 1550\,nm exceeding a factor of 20 for the selectively deactivated gratings. The observed pronounced thermal hysteresis extending down close to room temperature makes this system ideally suited for optical memory applications.Comment: 4 pages, 3 figures, 1 table - corrected Figure

Electrical control of the exciton-biexciton splitting in a single self-assembled InGaAs quantum dots

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

Vertical quantum wire realized with double cleaved-edge overgrowth

A quantum wire is fabricated on (001)-GaAs at the intersection of two overgrown cleaves. The wire is contacted at each end to n+ GaAs layers via two-dimensional (2D) leads. A sidegate controls the density of the wire revealing conductance quantization. The step height is strongly reduced from 2e^2/h due to the 2D-lead series resistance. We characterize the 2D density and mobility for both cleave facets with four-point measurements. The density on the first facet is modulated by the substrate potential, depleting a 2um wide strip that defines the wire length. Micro-photoluminescence shows an extra peak consistent with 1D electron states at the corner.Comment: 4 pages, 4 figure

Forster signatures and qubits in optically driven quantum dot molecules

An interesting approach to achieve quantum gate operations in a solid state device is to implement an optically driven quantum gate using two vertically coupled self-assembled quantum dots, a quantum dot molecule (QDM). We present a realistic model for exciton dynamics in InGaAs/GaAs QDMs under intense laser excitation and applied electric fields. The dynamics is obtained by solutions of the Lindblad master equation. A map of the dressed ground state as function of laser energy and applied electric field exhibits rich structure that includes excitonic anticrossings that permit the identification of the relevant couplings. The optical signatures of the dipole-dipole Forster energy transfer mechanism show as splittings of several (spatially) indirect excitonic lines. Moreover, we construct a model for exciton qubit rotations by adiabatic electric field cyclic sweeps into a Forster-tunneling regime which induces level anticrossings. The proposed qubit exhibits Rabi oscillations among two well defined exciton pairs as function of the residence time at the anticrossing.Comment: Paper presented in the International Conference on Electronic Properties of Two-dimensional Systems and Modulated Semiconductor Structures Genova Magazzini del Cotone, July 15-20 200

Optically Probing Spin and Charge Interactions in an Tunable Artificial Molecule

We optically probe and electrically control a single artificial molecule containing a well defined number of electrons. Charge and spin dependent inter-dot quantum couplings are probed optically by adding a single electron-hole pair and detecting the emission from negatively charged exciton states. Coulomb and Pauli blockade effects are directly observed and hybridization and electrostatic charging energies are independently measured. The inter-dot quantum coupling is confirmed to be mediated predominantly by electron tunneling. Our results are in excellent accord with calculations that provide a complete picture of negative excitons and few electron states in quantum dot molecules.Comment: shortened version: 6 pages, 3 figures, 1 table, to appear in Phys. Rev. Let

Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves

Individual self-assembled Quantum Dots and Quantum Posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For Quantum Posts, switching occurs continuously from predominantely charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent on whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is non-monotonic and shows a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of positively charged and neutral excitons is observed at high surface acoustic wave amplitudes. These findings are explained by carrier trapping and localization in the thin and disordered two-dimensional wetting layer on top of which Quantum Dots nucleate. This limitation can be overcome for Quantum Posts where acoustically induced charge transport is highly efficient in a wide lateral Matrix-Quantum Well.Comment: 11 pages, 5 figure

Ankerlos

Die Arbeitswelt hat sich grundlegend gewandelt. Das alte Paradigma der fordistischen industriellen Massenproduktion wurde vom neuen flexiblen Kapitalismus (Sennett 1998: 10) abgelöst. Veränderte Märkte erfordern die Anpassung der Betriebe und der ArbeitnehmerInnen, vor allem in Form der Flexibilisierung von Arbeitszeit, -ort und Tätigkeit. Die Anforderungen an die neue flexible Arbeitskraft verlangen nach neuen Kompetenzen, die aus lebensweltlichen Ressourcen gezogen werden müssen. Die Grenzen zwischen Arbeit und Privat hinsichtlich Zeit, Ort und Fähigkeiten verschwimmen, sodass es zur Entgrenzung von Arbeitswelt und Lebenswelt kommt. Das ökonomisch-kapitalistische Prinzip greift auf alle Lebensbereiche über. Die Verdinglichung zur austauschbaren Ware Arbeitskraft als Humanressource wird zunehmend verstärkt. Die Freisetzung aus regulierten Strukturen führt zu Ungewissheit. Mangelnde Selbstbestimmungsmöglichkeit über das eigene Leben, eine ungewisse Zukunft, Planungslosigkeit ziehen den Kontrollverlust über die eigene Biographie nach sich. Ankerlosigkeit breitet sich auf alle Lebensbereiche aus. Grundlegende, als selbstverständlich empfundene Errungenschaften im Wohlfahrtsstaat erodieren und werden von zunehmend ungewissen sowie riskanten Erwerbs- und damit Lebensbedingungen abgelöst. Arbeitslosigkeit hat sich mittlerweile vom Einzelschicksal zum Teil einer neuen ‚Normalbiographie‘ entwickelt. Das Normalarbeitsverhältnis wird immer mehr von a-typischen und prekären Beschäftigungsformen abgelöst. Als Beispiel für die Entwicklungen der Arbeitswelt und sich ausbreitende a-typische Beschäftigungsformen wird in der vorliegenden Arbeit die Arbeitskräfteüberlassung herangezogen. Angefangen von ihrer Definition und historischen Entwicklung, wird ein kurzer jedoch nötiger Überblick über die rechtliche Situation gegeben. Es folgt die ausführliche Darstellung der Situation in Österreich und eine anschließende ‚Typologie überlassener Arbeitskräfte‘ anhand statistischen Materials und der Literatur. Und schließlich werden die spezifischen Chancen und Risiken dieser Beschäftigungsform aus Sicht der Unternehmen sowie der Arbeitskräfte beleuchtet. Überlassene Arbeitskräfte sind trotz gesetzlich (relativ klar) geregelter Bestimmungen mit riskanten Lebensbedingungen und der (betrieblich strategischen) Zuweisung als zweite Klasse der Belegschaft konfrontiert

Direct observation of acoustic phonon mediated relaxation between coupled exciton states in a single quantum dot molecule

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 ~2 ns to ~10 ns as the spatially direct and indirect character of the wavefunction is varied by detuning from resonance. Acoustic phonon mediated inter-level relaxation is shown to proceed over timescales comparable to the direct exciton radiative lifetime, indicative of a relaxation bottleneck for level spacings in the range $\Delta E\$ ~3-6 meV.Comment: 6 pages, 4 figures, submitted for publicatio

Dynamic modulation of photonic crystal nanocavities using gigahertz acoustic phonons

Photonic crystal membranes (PCM) provide a versatile planar platform for on-chip implementations of photonic quantum circuits. One prominent quantum element is a coupled system consisting of a nanocavity and a single quantum dot (QD) which forms a fundamental building block for elaborate quantum information networks and a cavity quantum electrodynamic (cQED) system controlled by single photons. So far no fast tuning mechanism is available to achieve control within the system coherence time. Here we demonstrate dynamic tuning by monochromatic coherent acoustic phonons formed by a surface acoustic wave (SAW) with frequencies exceeding 1.7 gigahertz, one order of magnitude faster than alternative approaches. We resolve a periodic modulation of the optical mode exceeding eight times its linewidth, preserving both the spatial mode profile and a high quality factor. Since PCMs confine photonic and phononic excitations, coupling optical to acoustic frequencies, our technique opens ways towards coherent acoustic control of optomechanical crystals.Comment: 11 pages 4 figure