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Two-level System (two-level + system)

Distribution by Scientific Domains

Physics and Astronomy	50%
Chemistry	33%

Selected Abstracts

Coherent Properties of Quantum Dot Two-Level Systems

ISRAEL JOURNAL OF CHEMISTRY, Issue 4 2006
Artur Zrenner
In a single self-assembled InGaAs quantum dot, the one-exciton ground-state transition defines a two-level system, which appears as an extremely narrow resonance of only a few ,eV width. The resonant interaction of this two-level system with cw laser fields can be studied in detail by photocurrent spectroscopy, revealing the fine structure of the excitonic ground state as well as the effects of nonlinear absorption and power broadening. For the case of pulsed laser fields and in the absence of decoherence, the two-level system represents a qubit. Excitations with ps laser pulses result in qubit rotations, which appear as Rabi oscillations in photocurrent experiments. Double pulse experiments further allow us to infer the decoherence time and to perform coherent control on a two level system. [source]

Dynamic resonances in ultra-short laser pulses

LASER PHYSICS LETTERS, Issue 6 2007
N.V. Bordyug
Abstract Dynamic multiphoton resonances in ultra-short superintense laser pulses consisting of 50,200 field periods are considered numerically for a two-level system. The impact of nonresonant (Bloch-Siegert) and carrier-envelope effects on the dynamics of a two-level system was studied numerically, and conditions for complete population inversion were derived. The widths of multiphoton resonances increase with the increasing of the laser field. Results do not depend practically on the pulse duration. Effective resonance frequencies for multiphoton resonances increases strongly in a super-intense field. Multiphoton resonances are shifted and unified with each other in super-intense laser fields. (© 2007 by Astro Ltd., Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source]

The one-warehouse multiretailer problem with an order-up-to level inventory policy

NAVAL RESEARCH LOGISTICS: AN INTERNATIONAL JOURNAL, Issue 7 2010
uz Solyal
Abstract We consider a two-level system in which a warehouse manages the inventories of multiple retailers. Each retailer employs an order-up-to level inventory policy over T periods and faces an external demand which is dynamic and known. A retailer's inventory should be raised to its maximum limit when replenished. The problem is to jointly decide on replenishment times and quantities of warehouse and retailers so as to minimize the total costs in the system. Unlike the case in the single level lot-sizing problem, we cannot assume that the initial inventory will be zero without loss of generality. We propose a strong mixed integer program formulation for the problem with zero and nonzero initial inventories at the warehouse. The strong formulation for the zero initial inventory case has only T binary variables and represents the convex hull of the feasible region of the problem when there is only one retailer. Computational results with a state-of-the art solver reveal that our formulations are very effective in solving large-size instances to optimality. © 2010 Wiley Periodicals, Inc. Naval Research Logistics, 2010 [source]

Coherent control of ground state excitons in the nonlinear regime within an ensemble of self-assembled InAs quantum dots

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2009
Thomas Moldaschl
Abstract In this work femtosecond spectral hole burning spectroscopy is used to resonantly excite ground state excitons in an ensemble of self-assembled InAs/GaAs quantum dots with a strong pump pulse. Two fundamental coherent nonlinear effects are observed with the aid of the intrinsic time- and frequency resolution of the setup: The low temperature Rabi oscillation of the two-level system associated with the excitonic ground state transition and the observation of two-photon absorption in the surrounding GaAs crystal matrix. The emergence of the latter effect also infers the existence of charged excitons in the nominally undoped QD sample, backed up by the observation of additional spectral holes next to the excitonic transitions. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Anomalous resonant nonlinear absorption of excitons in CdSe/ZnS quantum dots

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2008
V. Dneprovskii
Abstract We have explained the observed anomalous dependence of colloidal CdSe/ZnS quantum dots' (QDs) absorption upon the intensity of input picosecond laser pulses in the case of one-photon resonant excitation of exciton 1S3/2(h) , 1S (e) transition , the linear absorption at low intensity is replaced by nonlinear decreasing and then increasing of absorption (!) at high intensities , by saturation effect (state filling) of two-level system with variable lifetime of the excited state (variable saturation intensity). The lifetime decreases at high excitation due to nonradiative Auger recombination. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Entanglement of spin chains with general boundaries and of dissipative systems

ANNALEN DER PHYSIK, Issue 7-8 2009
T. Stauber
Abstract We analyze the entanglement properties of spins (qubits) close to the boundary of spin chains in the vicinity of a quantum critical point and show that the concurrence at the boundary is significantly different from the one of bulk spins. We also discuss the von Neumann entropy of dissipative environments in the vicinity of a (boundary) critical point, such as two Ising-coupled Kondo-impurities or the dissipative two-level system. Our results indicate that the entanglement (concurrence and/or von Neumann entropy) changes abruptly at the point where coherent quantum oscillations cease to exist. The phase transition modifies significantly less the entanglement if no symmetry breaking field is applied and we argue that this might be a general property of the entanglement of dissipative systems. We finally analyze the entanglement of an harmonic chain between the two ends as function of the system size. [source]

Entanglement of spin chains with general boundaries and of dissipative systems

Self-Assembled Quantum Dot Molecules

ADVANCED MATERIALS, Issue 25-26 2009
Lijuan Wang
Abstract Semiconductor quantum dot molecules (QDMs) are systems composed of two or more closely spaced and interacting QDs. QDMs are receiving much attention both as playground for studying coupling and energy transfer processes between "artificial atoms" and as new systems, which substantially extend the range of possible applications of QDs. QDMs can be conveniently fabricated by self-assembly either through chemical synthesis or epitaxial growth. Although QDMs relying on the random occurrence of nearby QDs can be used for fundamental studies, special fabrication protocols must be used to create QDMs with well-defined properties. In this article, we focus on self-assembled QDMs obtained by epitaxial growth and embedded in a semiconductor matrix, which are appealing for the possible realization of quantum gates based on two-level systems defined in QDs. We provide a comprehensive overview of the development and current stage of the research on QDMs composed of vertically (in the growth direction) or laterally (in the growth plane) aligned QDs. The review highlights some recent milestone works and points out the challenges and future directions in the field. [source]

Theoretical study on quantum dynamics of bose system interacting with photon field

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2001
Hidemi Nagao
Abstract We investigate the quantum dynamics of two-boson and two-level systems interacting with a one-mode photon field. The time evolution of the population for each state is calculated in terms of the Jaynes,Cummings model. We find the collapses and revivals of the order parameter for the Bose,Einstein condensation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 401,408, 2001 [source]

Quasilocal vibrations and the Boson peak in glasses

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004
D. A. ParshinArticle first published online: 16 NOV 200
Abstract The problem of the low-frequency harmonic excitations and of the Boson peak in glasses is reviewed in the scope of recent theoretical developments. It is shown that the Boson peak inevitably appears in the reduced density of states g(,)/,2 of quasilocal vibrations in glasses which are additional to phonons harmonic excitations. We show that the same physical mechanism is fundamental for such seemingly different phenomenon as formation of the two-level systems in glasses. The fundamental reason for the Boson peak and two-level system formation is an instability of the spectrum of quasilocal harmonic modes weakly interacting with the high frequency surrounding and with each other. The instability controlled by the anharmonicity creates a new stable universal spectrum of harmonic vibrations with a Boson peak feature. We show that under pressure the Boson peak in glasses is always shifted to higher frequencies. For high enough pressures P the Boson peak frequency ,b , P1/3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

A Fourier optics approach to the dynamical theory of X-ray diffraction , continuously deformed crystals

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2004
Giovanni Mana
X-ray diffraction in continuously deformed crystals is considered by application of Fourier optics and from the viewpoint of the analogy between X-ray dynamics and the motion of two-level systems in quantum mechanics. Different forms of Takagi's equations are traced back to a common framework and it is shown that they are different ways to represent the same propagation equation. A novel way to solve Takagi's equations in the presence of a constant strain gradient is presented and approximation methods derived from quantum mechanics are considered. Crystal deformation in X-ray interferometry and two-crystal spectrometry are discussed and it is demonstrated that Si lattice-parameter measurements depend on the diffracting plane spacing on the crystal surface. [source]