Showing papers on "Quantum published in 1991"
Book•
01 Jan 1991
TL;DR: The distinction between level clustering and level repulsion is one of the quantum analogues of the classical distinction between globally regular and predominantly chaotic motion (see Figs. 1, 2, 3) as mentioned in this paper.
Abstract: The distinction between level clustering and level repulsion is one of the quantum analogues of the classical distinction between globally regular and predominantly chaotic motion (see Figs. 1, 2, 3). In order to reveal level repulsion under conditions of global classical chaos special care may be necessary: (i) subspectra referring to different values of the quantum numbers related to symmetries must be dealt with separately and (ii) for systems with quantum localization only levels whose wavefunctions have overlapping support must be admitted. A “level” may either be an energy eigenvalue E in the case of autonomous systems or, for periodically driven systems, a quasi-energy φ, i.e. an eigenphase of the unitary Floquet operator transporting the wavevector from period to period.
2,495 citations
TL;DR: It is proposed that electrons captured from the barriers in the upper levels of quantum boxes are retained in their cascade to the fundamental states for more than nanoseconds, and no luminescence can be obtained from these stored electrons with reasonable assumptions for the hole population.
Abstract: The poor radiative efficiency in quantum-box luminescence is tentatively explained as an intrinsic effect rather than the usually invoked effect of etch damages From the recently calculated decreased relaxation rate in zero-dimensional (0D) systems under 100\char21{}200-nm lateral quantization, we propose that electrons captured from the barriers in the upper levels of quantum boxes are retained in their cascade to the fundamental states for more than nanoseconds Due to the mutual orthogonality of quantum states in a box, no luminescence, or much less than in 2D or 3D, can be obtained from these stored electrons with reasonable assumptions for the hole population Magnetic-confinement experiments in quantum-well lasers support our conclusion A realistic model at low temperature describes more quantitatively the observed strong decay of the radiative efficiency in quantum boxes and pseudowires with decreased lateral dimensions
744 citations
Book•
12 May 1991
TL;DR: In this article, the authors present a survey of the electronic properties of 1D and 0D Semiconductor Heterostructures and their applications in 1D, 0D, and 1D applications.
Abstract: Introduction. The Advent of Ultrathin, Well-Contained Semiconductor Heterostructures. A Prerequisite. The Mastering of Semiconductor Purity and Interfaces. The Electronic Properties of Thin Semiconductor Heterostructures. Quantum Well Energy Levels. Triangular Quantum Well Energy Levels. Two-Dimensional Density of States. Excitons and Shallow Impurities in Quantum Wells. Tunneling Structures, Coupled Quantum Wells, and Superlattices. Modulation Doping of Heterostructures. n-i-p-i Structures. Optical Properties of Thin Heterostructures. Optical Matrix Element. Selection Rules. Energy Levels, Band Discontinuities, and Layer Fluctuations. Low Temperature Luminescence. Carrier and Exciton Dynamics. Inelastic Light Scattering. Non-Linear and Electro-Optic Effects. Electrical Properties of Thin Heterostructures. Mobility in Parallel Transport. Hot Electron Effects in Parallel Transport. Perpendicular Transport. Quantum Transport. Applications of Quantized Semiconductor Heterostructures. Electronic Devices Based on Parallel Transport. Electronic Devices Based on Perpendicular Transport. Quantum Well Lasers. Towards 1D and 0D Physics and Devices. One- and Zero-Dimensional Systems. 1D and 0D Semiconductor Fabrication Techniques. Electrical Applications of 1D and 0D Structures. 1D and 0D Optical Devices. Selected Bibliography. References. Index.
444 citations
420 citations
TL;DR: It is shown that a natural extension of canonical Heisenberg-picture quantum mechanics is well defined and can be used to describe the "non-Schr\"odinger regime," in which a fundamental time variable is not defined.
Abstract: A solution to the issue of time in quantum gravity is proposed. The hypothesis that time is not defined at the fundamental level (at the Planck scale) is considered. A natural extension of canonical Heisenberg-picture quantum mechanics is defined. It is shown that this extension is well defined and can be used to describe the "non-Schr\"odinger regime," in which a fundamental time variable is not defined. This conclusion rests on a detailed analysis of which quantities are the physical observables of the theory; a main technical result of the paper is the identification of a class of gauge-invariant observables that can describe the (observable) evolution in the absence of a fundamental definition of time. The choice of the scalar product and the interpretation of the wave function are carefully discussed. The physical interpretation of the extreme "no time" quantum gravitational physics is considered.
379 citations
TL;DR: The most efficient set of operations of that type that the authors were able to design falls short of a single combined measurement, performed on both system together.
Abstract: Two quantum systems are identically prepared in different locations. An observer's task is to determine their state. A simple example shows that a pair of measurements of the von Neumann type is less effective than a sequence of nonorthogonal probability-operator measures, alternating between the two quantum systems. However, the most efficient set of operations of that type that we were able to design falls short of a single combined measurement, performed on both system together.
291 citations
TL;DR: Coupled cluster (CC) techniques have been developed to calculate excited states, energies of open-shell systems, density matrices and hence other properties, sum rules, and sub-sum-rules that follow from imbedding linear response theory within the NCCM.
Abstract: What has since become known as the normal coupled cluster method (NCCM) was invented about thirty years ago to calculate ground-state energies of closed-shell atomic nuclei. Coupled cluster (CC) techniques have since been developed to calculate excited states, energies of open-shell systems, density matrices and hence other properties, sum rules, and the sub-sum-rules that follow from imbedding linear response theory within the NCCM. Further extensions deal both with systems at nonzero temperature and with general dynamical behaviour. More recently, a new version of CC theory, the so-called extended coupled cluster method (ECCM) has been introduced. It has the potential to describe such global phenomena as phase transitions, spontaneous symmetry breaking, states of topological excitation, and nonequilibrium behaviour. CC techniques are now widely recognized as providing one of the most universally applicable, most powerful, and most accurate of all microscopicab initio methods in quantum many-body theory. The number of successful applications within physics is now impressively large. In most such cases the numerical results are either the best or among the best available. A typical case is the electron gas, where the CC results for the correlation energy agree over the entire metallic density range to within less than 1 millihartree (or <1%) with the essentially exact Green's function Monte Carlo results. The role of CC theory within modern quantum many-body theory is first surveyed, by a comparison with other techniques. Its full range of applications in physics is then reviewed. These include problems in nuclear physics, both for finite nuclei and infinite nuclear matter; the electron gas; various integrable and nonintegrable models; various relativistic quantum field theories; and quantum spin chain and lattice models. Particular applications of the ECCM include the quantum hydrodynamics of a zero-temperature, strongly-interacting condensed Bose fluid; a charged impurity in a polarizable medium (e.g., positron annihilation in metals); and various anharmonic oscillator and spin systems.
268 citations
TL;DR: In this article, the authors show that all experimental results to date are consistent with a mechanism based on a concerted direct dissociation breaking a single CH bond on impact.
240 citations
TL;DR: In this article, the authors consider the evolution of an open quantum mechanical system which together with its environment forms a closed system and perform numerical calculations for the nuclear system at low as well as at high level density.
Abstract: The author considers the evolution of an open quantum mechanical system which together with its environment forms a closed system. The numerical calculations are performed for the nuclear system at low as well as at high level density. In each case, the relevant modes are discussed and compared to the results of the standard methods developed for their description. The influence of the respective remaining modes is compared with existing experimental data. The transition from the resonance reaction mechanism at low level density to the direct reaction mechanism at high level density takes place at a stochasticity threshold. The many-body properties are conserved, at high level density, in long-lived traps but the spectroscopic information is lost. The evolution to the thermal equilibrium takes place via the formation of quantum chaos in accordance with the second law of thermodynamics. The evolution is accompanied, in the open system, by the formation of a new order with less degrees of freedom. These modes are far from thermal equilibrium. They screen the long-lived modes which are near to thermal equilibrium.
TL;DR: It is shown that the zero-field quantization and quantization in high magnetic fields are two limiting cases of a more general quantization phenomenon, and the combination of a QPC and a bulk Ohmic contact can act as a controllable edge-channel mixer.
Abstract: We present an experimental and theoretical study of quantum ballistic transport in single quantum point contacts (QPC's), defined in the two-dimensional electron gas (2DEG) of a high-mobility GaAs/${\mathrm{Al}}_{0.33}$${\mathrm{Ga}}_{0.67}$As heterojunction. In zero magnetic field the conductance of quantum point contacts shows the formation of quantized plateaus at multiples of 2${\mathit{e}}^{2}$/h. The experimental results are explained with a simple model. Deviations from ideal quantization are discussed. The experimental results are compared with model calculations. Energy averaging of the conductance has been studied, both as a function of temperature and voltage across the device. The application of a magnetic field leads to the magnetic depopulation of the one-dimensional subbands in the QPC. It is shown that the zero-field quantization and quantization in high magnetic fields are two limiting cases of a more general quantization phenomenon. We use quantum point contacts to study the high-magnetic-field transport in a 2DEG. Quantum point contacts are used to selectively populate and detect edge channels. The experiments show that scattering between adjacent edge channels can be very weak, under certain circumstances even on length scales longer than 200 \ensuremath{\mu}m. This adiabatic transport has resulted in the observation of an anomalous integer quantum Hall effect, in which the quantization of the Hall conductance is not determined by the number of Landau levels in the bulk 2DEG, but by the number of Landau levels in the QPC's instead. Related effects are the anomalous quantization of the longitudinal resistance and the adiabatic transport through QPC's in series. A theoretical description for transport in the presence of Shubnikov--de Haas (SdH) backscattering is given. This model explains the experimentally observed suppression of the SdH oscillations due to the selective population or detection of edge channels. Finally, we demonstrate that the combination of a QPC and a bulk Ohmic contact can act as a controllable edge-channel mixer.
TL;DR: In this paper, the quantum Fokker-Planck equation for a Gaussian-Markovian bath is deduced by applying a method proposed by Tanimura and Kubo.
Abstract: The quantum Fokker-Planck equation for a Gaussian-Markovian bath is deduced by applying a method proposed by Tanimura and Kubo [J. Phys. Soc. Jpn. 58, 101 (1989)]. The results are expressed in the form of simultaneous differential equations in terms of density operators and can treat strong system-bath interactions where the correlated effects of the noise play an important role. The classical Fokker-Planck equation for a Gaussian-Markovian noise is obtained by performing the Wigner transformation, and its equilibrium state is shown to be the Maxwell-Boltzmann distribution. The method is convenient for numerical studies. Calculations for quantum-system harmonic oscillators and the double-well potential problems are demonstrated for cases of Gaussian-white noise and Gaussian-Markovian noise.
TL;DR: In this article, a waveguide with a doubly periodic grating coupler was used to achieve a quantum efficiency of 92% with respect to unpolarized radiation, with a response linewidth of 0.8 μm.
Abstract: Quantum well infrared detectors based on a waveguide with a doubly periodic grating coupler are shown to provide quantum efficiencies of nearly unity with respect to unpolarized polarization. This is a factor of six larger than for a conventional 45° polished edge detector with the same quantum well characteristics. As a further advantage the detector response becomes nearly insensitive to the polarization direction of the incident radiation. Detectors have been fabricated and tested. Measurements lead to a maximum quantum efficiency of 92% with respect to unpolarized radiation, with a response linewidth =0.8 μm, thus confirming theory.
TL;DR: Techniques to make quantum wires, and quantum wells of controlled size and shape, from compound semiconductor materials, are discussed and some of the properties of these structures are described.
Abstract: Structures in which electrons are confined to move in two dimensions (quantum wells) have led to new physical discoveries and technological applications. Modification of these structures to confine the electrons to one dimension (quantum wires) or release them in the third dimension, are predicted to lead to new electrical and optical properties. This article discusses techniques to make quantum wires, and quantum wells of controlled size and shape, from compound semiconductor materials, and describes some of the properties of these structures.
TL;DR: In this paper, a class of supersymmetric quantum mechanics whose eigenvalue problem is (in part) exactly solvable is presented. In concrete realizations, the class includes supersymetric quantum mechanical models associated with one-dimensional or radial Schrodinger operators with potentials of a special type, called shape-invariant potentials.
TL;DR: In this paper, the authors investigated the spectra of affine Toda field theories with imaginary coupling constant and found soliton solutions that have real classical masses and are stable to small perturbations.
Abstract: The spectra of $A_r$ affine Toda field theories with imaginary coupling constant, are investigated. Soliton solutions are found, which, despite the non-unitary form of the Lagrangian, have real classical masses and are stable to small perturbations. The quantum corrections to the soliton masses are determined, to lowest order in $\hbar$. The solitons have the same spectrum as the fundamental Toda particles; a feature that is preserved in the quantum theory.
TL;DR: In this paper, the classical dynamics of near-collinear electron configurations in helium were analyzed and it was shown that the resulting motion is fully chaotic in the corresponding symmetry plane whereas the linearized motion of the plane is stable.
Abstract: We analyse the classical dynamics of near-collinear electron configurations in helium. The dynamics turns out to be fully chaotic. An application of periodic orbit quantization techniques yields the energy of doubly excited states with high accuracy. The analysis shows that near-collinear intra-shell resonances are associated with an asymmetric stretch like molion of the electron pair rather than the symmetric stretch motion along the Wannier ridge. The purpose of this letter is threefold. We first analyse the classical dynamics for near-collinear arrangements of the two electrons in the helium atom. We find strong evidence that the resulting motion is fully chaotic in the corresponding symmetry plane whereas the linearized motion of the plane is stable. We then apply modern semi- classical techniques to quantize the chaotic dynamics and obtain the energies of certain doubly excited states. Finally, our results (together with numerically highly accurate quantum mechanical calculations) unambiguously show that the widely accepted viewpoint of electron pair propagation along the Wannier ridge for doubly excited 11 Prrmanent address: Max-Planck lnrlitut Tir Kcmphyrik. Postfach 103980. 6900 Heidelberg. Federal Republic of Gemany.
TL;DR: Application of information theory to the quantized electromagnetic field reveals a special role for the two-mode squeezed states and concludes that for any correlated pure state a given pair of single-system observables contains at most only half the information about the correlations.
Abstract: Information theory allows us to make quantitative statements about the strength and nature of the correlations between systems. Application of this theory to the quantized electromagnetic field reveals a special role for the two-mode squeezed states. The nonclassical properties of these states arise from the intermode correlations, and we apply information-theoretic methods to determine the strength of the correlation between specific pairs of observables. This analysis leads to the important general result that for any correlated pure state a given pair of single-system observables contains at most only half the information about the correlations. We discuss the implications of this result for the distinction between classical and quantum systems.
TL;DR: In this article, the results of accurate quantum mechanical calculations including geometric phase effects were reported for the simple H + H 2 system over the energy range 0.7 to 1.2 eV.
TL;DR: In this article, the authors simulate the Liouville-Poisson system using the splitting scheme and the double Fourier transformation in x and p space and study the quantum effects in the one-dimensional electrostatic plasma, i.e., nonlinear Landau damping and two stream instability problems.
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01 Jan 1991
TL;DR: The quantum groups gl ∞ and A ∞ are constructed in this paper, and the representation theory of these algebras is developed and the universal R-matrix is presented.
Abstract: The quantum groups gl ∞ and A ∞ are constructed The representation theory of these algebras is developed and the universal R-matrix is presented
TL;DR: In this paper, a semi-classical treatment of laser cooling of neutral atoms is presented, where the cooling mechanism studied is the Sisyphus process for 1-dimensional optical molasses.
Abstract: We present a quantum treatment of laser cooling of neutral atoms. The cooling mechanism studied is the "Sisyphus" process for 1-dimensional optical molasses. We first derive the energy eigenstates for the atom moving in the potential associated with the light-shifts due to the laser. Then, taking into account optical pumping, we calculate the steady-state populations of these quantum levels in the secular approximation. This approach allows us to determine the atomic eigenfrequencies in the optical wells, as well as momentum and position distributions. In particular, the minimum r.m.s. atomic momentum is 6 single-photon momenta; this result was not accessible to previous semi-classical treatments.
TL;DR: This work focuses on the trace maps of generalized Thue-Morse lattices, a detailed analysis of the attractor of the associated dynamical system, the electronic spectra through the trace-map approach, and spin excitations in a quantum Ising model in a transverse magnetic field.
Abstract: We study the physical properties of the Thue-Morse chain and its generalizations. After a preliminary discussion of its basic features (e.g., structure factor, location, and relative magnitude of spectral gaps), we focus on (1) the trace maps of generalized Thue-Morse lattices, (2) a detailed analysis of the attractor of the associated dynamical system, (3) the electronic spectra through the trace-map approach, (4) spin excitations in a quantum Ising model in a transverse magnetic field, (5) light transmission through a multilayer, and (6) the diamagnetic properties of Thue-Morse superconducting wire networks and Josephson-junction arrays.
TL;DR: A formalism within the linear-response theory is developed to investigate the transport through a quantum point contact between two electron-gas reservoirs and finds that the simple phase relation results in resonance structures superimposed on the plateaus between two steps of quantized conductance.
Abstract: We developed a formalism within the linear-response theory to investigate the transport through a quantum point contact between two electron-gas reservoirs. It is valid for two-terminal conductance through a constriction of a two-dimensional (2D) or 3D potential and has a wide range of applicability covering ballistic as well as tunneling regimes. We studied the quantization of conductance and examined several effects influencing the quantum transmission. Among these effects we found that the simple phase relation results in resonance structures superimposed on the plateaus between two steps of quantized conductance. These resonances are destroyed by the smooth entrance, finite temperature and bias, and variation of the potential. The simulation of adiabatic transmission in constrictions having smoothly varying widths resulted in the conductance with sharp quantum steps without the resonance structure. The quality of quantization is strongly affected by the length of constriction, Fermi-level smearing, the obstacle at the entrance, impurity scattering, nonuniformities of geometry and potential, and in particular by the variation of the longitudinal potential resulting in a sharp saddle-point structure.
TL;DR: In this article, the coherent states for the simplest quantum groups (q-Heisenberg-Weyl, SU petertodd qcffff (2) and SUcffff q�� (1, 1)) are introduced and their properties investigated.
Abstract: The coherent states for the simplest quantum groups (q-Heisenberg-Weyl, SU
q
(2) and the discrete series of representations of SU
q
(1, 1)) are introduced and their properties investigated. The corresponding analytic representations, path integrals, and q-deformation of Berezin's quantization on ℂ, a sphere, and the Lobatchevsky plane are discussed.
TL;DR: Using a simple mean field model of the electron-electron interaction, study has been done of the effect of space charge in a planar diode as discussed by the authors, showing that the classical value for the limiting current in such a diode can be exceeded by a large factor due to tunneling.
Abstract: 1407_69Using a simple mean field model of the electron-electron interaction, study has been done of the effect of space charge in a planar diode. Results show, in particular, that the classical value for the limiting current in such a diode can be exceeded by a large factor due to the effect of tunneling. The smooth transition of the solutions from the quantum to the classical (non-quantum) regime is demonstrated.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.