302 research outputs found
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 are fabricated by site-selective ion beam
implantation in a 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
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