403 research outputs found
Spectroscopy of nanoscopic semiconductor rings
Making use of self-assembly techniques, we demonstrate the realization of
nanoscopic semiconductor quantum rings in which the electronic states are in
the true quantum limit. We employ two complementary spectroscopic techniques to
investigate both the ground states and the excitations of these rings. Applying
a magnetic field perpendicular to the plane of the rings, we find that when
approximately one flux quantum threads the interior of each ring, a change in
the ground state from angular momentum to takes place.
This ground state transition is revealed both by a drastic modification of the
excitation spectrum and by a change in the magnetic field dispersion of the
single-electron charging energy
Tentamen historiae lichenum in genere cui accedunt primae lineae distributionis novae
Inaug.-Diss.--Academia Georgia Augusta.Mode of access: Internet
Self-consistent Coulomb effects and charge distribution of quantum dot arrays
This paper considers the self-consistent Coulomb interaction within arrays of
self-assembled InAs quantum dots (QDs) which are embedded in a pn structure.
Strong emphasis is being put on the statistical occupation of the electronic QD
states which has to be solved self-consistently with the actual
three-dimensional potential distribution. A model which is based on a Green's
function formalism including screening effects is used to calculate the
interaction of QD carriers within an array of QDs, where screening due to the
inhomogeneous bulk charge distribution is taken into acount. We apply our model
to simulate capacitance-voltage (CV) characteristics of a pn structure with
embedded QDs. Different size distributions of QDs and ensembles of spatially
perodic and randomly distributed arrays of QDs are investigated.Comment: submitted to pr
Interaction-induced chaos in a two-electron quantum-dot system
A quasi-one-dimensional quantum dot containing two interacting electrons is
analyzed in search of signatures of chaos. The two-electron energy spectrum is
obtained by diagonalization of the Hamiltonian including the exact Coulomb
interaction. We find that the level-spacing fluctuations follow closely a
Wigner-Dyson distribution, which indicates the emergence of quantum signatures
of chaos due to the Coulomb interaction in an otherwise non-chaotic system. In
general, the Poincar\'e maps of a classical analog of this quantum mechanical
problem can exhibit a mixed classical dynamics. However, for the range of
energies involved in the present system, the dynamics is strongly chaotic,
aside from small regular regions. The system we study models a realistic
semiconductor nanostructure, with electronic parameters typical of gallium
arsenide.Comment: 4 pages, 3ps figure
Giant Coulomb broadening and Raman lasing on ionic transitions
CW generation of anti-Stokes Raman laser on a number of blue-green argon-ion
lines (4p-4s, 4p-3d) has been demonstrated with optical pumping from metastable
levels 3d'^2G, 3d^4F. It is found, that the population transfer rate is
increased by a factor of 3-5 (and hence, the output power of such Raman laser)
owing to Coulomb diffusion in the velocity space. Measured are the excitation
and relaxation rates for the metastable level. The Bennett hole on the
metastable level has been recorded using the probe field technique. It has been
shown that the Coulomb diffusion changes shape of the contour to exponential
cusp profile while its width becomes 100 times the Lorentzian one and reaches
values close to the Doppler width. Such a giant broadening is also confirmed by
the shape of the absorption saturation curve.Comment: RevTex 18 pages, 5 figure
Autonomous Driving and Cybersecurity by Design
So far, real-time requirements for the overall autonomous driving (AD) have been addressed only in a few cases. Cybersecurity and real-time capability are usually addressed separately. However, with regard to a justifiable mobility quality, these requirements are in direct interaction with each other. Therefore, as suggested here, it makes sense to consider the provision of a suitable IT infrastructure with cybersecurity, QoS (Quality of Service) and simultaneous real-time IoT capabilities. The early integration of security and real-time by design, as well as the architecture concepts mentioned, are measures that limit development costs, make the solution modular, scalable and thus sustainable. We introduce the adaptive-real-time-manager (ARM), an innovative concept for continuous assessment and optimization of the real-time capability of autonomous driving systems. The paper also proposes a cloud-broker-concept and simulation as essential building blocks to accelerate the integration of the ARM into an autonomous driving system (ADS). Furthermore, we discuss aspects of multisensory data acquisition and processing, addressing the integration of various data sources and their qualities. Finally, we highlight the importance of driveability for autonomous vehicles, emphasizing its role in comfort, safety, and user acceptance
Collective charge-density excitations of non-circular quantum dots in a magnetic field
Recent photoabsorption measurements have revealed a rich fine structure in
the collective charge-density excitation spectrum of few-electron quantum dots
in the presence of magnetic fields. We have performed systematic computational
studies of the far-infrared density response of quantum dots, using
time-dependent density-functional theory in the linear regime and treating the
dots as two-dimensional disks. It turns out that the main characteristics
observed in the experiment can be understood in terms of the electronic shell
structure of the quantum dots. However, new features arise if a breaking of the
circular symmetry of the dots is allowed, leading to an improved description of
the experimental results.Comment: 18 pages, 5 figures, submitted to Phys. Rev.
Quantum Computers and Quantum Coherence
If the states of spins in solids can be created, manipulated, and measured at
the single-quantum level, an entirely new form of information processing,
quantum computing, will be possible. We first give an overview of quantum
information processing, showing that the famous Shor speedup of integer
factoring is just one of a host of important applications for qubits, including
cryptography, counterfeit protection, channel capacity enhancement, distributed
computing, and others. We review our proposed spin-quantum dot architecture for
a quantum computer, and we indicate a variety of first generation materials,
optical, and electrical measurements which should be considered. We analyze the
efficiency of a two-dot device as a transmitter of quantum information via the
ballistic propagation of carriers in a Fermi sea.Comment: 13 pages, latex, one eps figure. Prepared for special issue of J.
Mag. Magn. Matl., "Magnetism beyond 2000". Version 2: small revisions and
correction
Quantum Computation with Quantum Dots and Terahertz Cavity Quantum Electrodynamics
A quantum computer is proposed in which information is stored in the two
lowest electronic states of doped quantum dots (QDs). Many QDs are located in a
microcavity. A pair of gates controls the energy levels in each QD. A
Controlled Not (CNOT) operation involving any pair of QDs can be effected by a
sequence of gate-voltage pulses which tune the QD energy levels into resonance
with frequencies of the cavity or a laser. The duration of a CNOT operation is
estimated to be much shorter than the time for an electron to decohere by
emitting an acoustic phonon.Comment: Revtex 6 pages, 3 postscript figures, minor typos correcte
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