Showing papers in "Physical Review A in 1993"
Journal Article•
TL;DR: In this article, a lattice Boltzmann model is developed which has the ability to simulate flows containing multiple phases and components, each of the components can be immiscible with the others and can have different mass values.
Abstract: A lattice Boltzmann model is developed which has the ability to simulate flows containing multiple phases and components. Each of the components can be immiscible with the others and can have different mass values. The equilibrium state of each component can have a nonideal gas equation of state at a prescribed temperature exhibiting thermodynamic phase transitions. The scheme incorporated in this model is the introduction of an interparticle potential. The dynamical rules in this model are local so it is highly efficient to compute on massively parallel computers. This model has many application in large-scale numerical simulations of various types of fluid flows
2,300 citations
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TL;DR: The concept of quantum random walk is introduced, and it is shown that due to quantum interference effects the average path length can be much larger than the maximum allowed path in the corresponding classical random walk.
Abstract: We introduce the concept of quantum random walk, and show that due to quantum interference effects the average path length can be much larger than the maximum allowed path in the corresponding classical random walk A quantum-optics application is described
1,518 citations
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TL;DR: Two proposed mechanisms, referred to as one-axis twisting and two-axis countertwisting, are shown to reduce the standard quantum noise S/2 of the coherent S-spin state down to 1/2(S/3${)}^{1/3}$ and 1/3, respectively.
Abstract: The basic concept of squeezed spin states is established and the principles for their generation are discussed. Two proposed mechanisms, referred to as one-axis twisting and two-axis countertwisting, are shown to reduce the standard quantum noise S/2 of the coherent S-spin state down to 1/2(S/3${)}^{1/3}$ and 1/2, respectively. Implementations of spin squeezing in interferometers are also discussed.
1,326 citations
TL;DR: In the experiments described here, a single ion or a few identical ions were prepared in well-defined superpositions of two internal energy eigenstates, and the populations of the energy levels were measured.
Abstract: Measurements of internal energy states of atomic ions confined in traps can be used to illustrate fundamental properties of quantum systems, because long relaxation times and observation times are available. In the experiments described here, a single ion or a few identical ions were prepared in well-defined superpositions of two internal energy eigenstates. The populations of the energy levels were then measured. For an individual ion, the outcome of the measurement is uncertain, unless the amplitude for one of the two eigenstates is zero, and is completely uncertain when the magnitudes of the two amplitudes are equal. In one experiment, a single $^{199}\mathrm{Hg}^{+}$ ion, confined in a linear rf trap, was prepared in various superpositions of two hyperfine states. In another experiment, groups of $^{9}\mathrm{Be}^{+}$ ions, ranging in size from about 5 to about 400 ions, were confined in a Penning trap and prepared in various superposition states. The measured population fluctuations were greater when the state amplitudes were equal than when one of the amplitudes was nearly zero, in agreement with the predictions of quantum mechanics. These fluctuations, which we call quantum projection noise, are the fundamental source of noise for population measurements with a fixed number of atoms. These fluctuations are of practical importance, since they contribute to the errors of atomic frequency standards.
525 citations
TL;DR: The correlation energies for 3- to 10-electron atomic ions are recomputed using more recent experimental and theoretical data and the correlation energies obtained from some density-functional models are also compared to these correlation energy data.
Abstract: Recently Davidson [ital et] [ital al]. [Phys. Rev. A 44, 7071 (1991)] have estimated nonrelativistic correlation energies and relativistic corrections to ionization potentials for atomic ions with up to 10 electrons. In this work, this approach is extended to atomic ions with 11 to 18 electrons. The correlation energies for 3- to 10-electron atomic ions are also recomputed using more recent experimental and theoretical data. Unlike other work the method focuses on the correlation contribution to the individual ionization energies which are obtained by comparing experimental data with relativistic complete-valence-space energies. [ital Ab] [ital initio] estimates of correlation contributions to the ionization energies with extensive configuration-interaction calculations of 3- to 10-electron atomic ions with nuclear charge from 4 through 10 and 18, 36, 50, 72, 100, and 144 have been obtained. The correlation energies obtained from some density-functional models are also compared to these correlation energy data.
487 citations
TL;DR: The role of the centrifugal barrier in the final collision channel in reducing the loss of atoms from the trap due to transitions induced by the magnetic dipole-dipole interaction is discussed.
Abstract: We study the magnetic-field dependence of the cross sections for elastic and inelastic collisions of pairs of ultracold cesium atoms in a magnetic trap, calculated with the coupled-channels method. We pay special attention to atoms in the f=3, mf=-3 weak-field seeking state of the lower hyperfine manifold. The cross sections show a pronounced resonance structure. We discuss its origin, starting from the pure bound singlet and triplet rovibrational Cs2 states and introducing perturbations due to the hyperfine and Zeeman interactions. We also discuss the role of the centrifugal barrier in the final collision channel in reducing the loss of atoms from the trap due to transitions induced by the magnetic dipole-dipole interaction
484 citations
TL;DR: An analysis is made of the background level and counter efficiencies actually necessary to perform a loophole-free Einstein-Podolsky-Rosen experiment and both requirements are correlated.
Abstract: An analysis is made of the background level and counter efficiencies actually necessary to perform a loophole-free Einstein-Podolsky-Rosen experiment. Both requirements are correlated. Photon counters do not absolutely have to have more than 82.8% efficiency if the signal-over-noise ratio is very high.
477 citations
TL;DR: The quantum effects on a cavity mode of the electromagnetic field caused by measuring one of its quadrature components is analyzed and it is shown that homodyne measurement cannot produce a squeezed state from a classical initial state.
Abstract: The quantum effects on a cavity mode of the electromagnetic field caused by measuring one of its quadrature components is analyzed. We consider three measurement schemes: an intracavity quantum-nondemolition coupling to another mode, simple homodyne detection, and balanced homodyne detection. It is shown that, for suitable initial conditions, the first scheme has an effect which approaches that of a projective collapse of the state vector for long measurement times. However, the two homodyne schemes (which are shown to be equivalent for large local-oscillator amplitudes) do not approximate a projective measurement in any limit. In particular, it is shown that homodyne measurement cannot produce a squeezed state from a classical initial state. All three schemes are analyzed in terms of ‘‘quantum trajectories’’ which link measurement theory with stochastic quantum-jump processes.
416 citations
TL;DR: The problem of the Schr\"odinger equation for two electrons (interacting with Coulomb potentials) in an external harmonic-oscillator potential is revisited and shown to be solvable analytically for a particular, denumerably infinite set of oscillator frequencies.
Abstract: The problem of the Schr\"odinger equation for two electrons (interacting with Coulomb potentials) in an external harmonic-oscillator potential is revisited and shown to be solvable analytically for a particular, denumerably infinite set of oscillator frequencies. Solutions are given for ground and excited states in the singlet and triplet spin configurations.
382 citations
TL;DR: The comparison of the calculated cross-section energymore » dependence with the experimental data gives evidence for a positive value for the Cs-Cs scattering length, which makes cesium Bose gas stable.
Abstract: A simple analytical formula, [ital a]=[ital [bar a]][1[minus]tan([pi]/[ital n][minus]2)]tan[l brace][Phi][minus][[pi]/2([ital n][minus]2)][r brace], is obtained for the scattering length in atomic collisions. Here [ital [bar a]]=cos[[pi]/([ital n][minus]2)][l brace] [radical]2[ital M][alpha] /[[h bar]([ital n][minus]2)][r brace][sup 2]/([ital n][minus]2)[[Gamma]([ital n][minus]3)/([ital n][minus]2)]/[[ital T]([ital n][minus]1)/([ital n][minus]2)] is the mean scattering length determined by the asymptotic behavior of the potential [ital U]([ital r])[similar to][minus][alpha]/[ital R][sup [ital n]], ([ital n]=6 for atom-atom scattering or [ital n]=4 for ion-atom scattering), [ital M] is the reduced mass of the atoms, and [Phi] is the semiclassical phase calculated at zero energy from the classical turning point to infinity. The value of [ital [bar a]], the average scattering length, also determines the slope of the [ital s]-wave phase shifts beyond the near-threshold region. The formula is applicable to the collisions of atoms cooled down in traps, where the scattering length determines the character of the atom-atom interaction. Our calculation shows that repulsion between atoms ([ital a][gt]0) is more likely than attraction with a probability'' of 75%. For the Cs-Cs scattering in the [sup 3][Sigma][sub [ital u]] state, [ital [bar a]]=95.5[ital a][sub [ital B]] has been obtained, where [ital a][sub [ital B]] is the Bohr radius. The comparison of the calculated cross-section energymore » dependence with the experimental data gives evidence for a positive value for the Cs-Cs scattering length, which makes cesium Bose gas stable.« less
355 citations
TL;DR: A two-photon interference experiment proposed by Franson is reported, in which sinusoidal fringes with visibilities greater than 70.7%, such as those predicted by quantum mechanics, violate a Bell inequality.
Abstract: We report on a two-photon interference experiment proposed by Franson [Phys. Rev. Lett. 62, 2205 (1989)], in which sinusoidal fringes with visibilities greater than 70.7%, such as those predicted by quantum mechanics, violate a Bell inequality. We observe visibility of 80.4\ifmmode\pm\else\textpm\fi{}0.6%, implying a violation of the inequality by 16 standard deviations. Here the elements of reality under consideration are energy and time rather than spin components. Any classical field models describing separate beams in a Franson interferometer are limited to visibilities less than 50%, and hence ruled out as well, without the need for any supplementary assumptions.
TL;DR: It is shown that the evolution of an open quantum system whose density operator obeys a Markovian master equation can in some cases be meaningfully described in terms of stochastic Schrodinger equations (SSE’s) for its state vector.
Abstract: It is shown that the evolution of an open quantum system whose density operator obeys a Markovian master equation can in some cases be meaningfully described in terms of stochastic Schrodinger equations (SSE’s) for its state vector. A necessary condition for this is that the information carried away from the system by the bath (source of the irreversibility) be recoverable. The primary field of application is quantum optics, where the bath consists of the continuum of electromagnetic modes. The information lost from the system can be recovered using a perfect photodetector. The state of the system conditioned on the photodetections undergoes stochastic quantum jumps. Alternative measurement schemes on the outgoing light (homodyne and heterodyne detection) are shown to give rise to SSE’s with diffusive terms. These three detection schemes are illustrated on a simple quantum system, the two-level atom, giving new perspectives on the interpretation of measurement results. The reality of these and other stochastic processes for state vectors is discussed.
Journal Article•
TL;DR: This paper showed that local icosahedral order increases as random hard-sphere packings (one and two-component) generated on the computer are densified from the (recently established) "loose random-packing" limit to the "dense random packing" limit.
Abstract: Local icosahedral order is found to increase as random hard-sphere packings (one- and two-component) generated on the computer are densified from the (recently established) «loose random-packing» limit to the «dense random-packing» limit. While icosahedral ordering in «atomic» systems is frequently ascribed to the energetic stability or icosahedral clusters, the present results show that icosahedral ordering can arise from packing constraints alone. However, the icosahedra are often distorted, partly due to the lack of preferred distance between hard spheres. At high density one-third to one-half of the pairs in the first peak of the radial distribution function (RDF) are icosahedral fragments
Journal Article•
TL;DR: A phase-field model for isothermal solidification of a binary alloy is developed that includes gradient energy contributions for the phase field and for the composition field to predict an alloy solidification with no redistribution of composition.
Abstract: A phase-field model for isothermal solidification of a binary alloy is developed that includes gradient energy contributions for the phase field and for the composition field. When the gradient energy coefficient for the phase field is smaller than that for the solute field, planar steady-state solutions exhibit a reduction in the segregation predicted in the liquid phase ahead of an advancing front (solute trapping), and, in the limit of high solidification speeds, predict alloy solidification with no redistribution of composition. Such situations are commonly observed experimentally
Journal Article•
TL;DR: The rigidity of the filaments was measured by extracting the correlation function given by the mean dot product between unit tangent vectors of an isolated filament, which is consistent with current rigidity values found in the literature.
Abstract: Single actin filaments were confined between bovine-serum-albumine-coated glass plates, with a separation of about 1 μm, and their flickering Brownian movement was observed by fluorescence microscopy. The rigidity of the filaments was measured by extracting the correlation function given by the mean dot product between unit tangent vectors of an isolated filament. The result is consistent with current rigidity values found in the literature
TL;DR: This work examines the mutual coherence and phase dynamics of two solid-state lasers generated adjacent to each other in a single Nd:YAG rod, and theoretical predictions are obtained that are in excellent agreement with the measurements.
Abstract: We examine the mutual coherence and phase dynamics of two solid-state lasers, generated adjacent to each other in a single Nd:YAG rod. The coupling of the lasers is varied by changing the separation of the pump beams. A model is formulated to interpret the experimental results, and theoretical predictions are obtained that are in excellent agreement with the measurements.
Journal Article•
TL;DR: It is shown that spiral breakup, leading to turbulence, can occur in a two-dimensional reaction-diffusion system with delayed-inhibitor production.
Abstract: Excitable media are extended spatial systems, which support the propagation of waves including pulses and rotating spirals. They are well described by sets of partial differential equations involving a fast activator and a slow inhibitor variable. Here we show that spiral breakup, leading to turbulence, can occur in a two-dimensional reaction-diffusion system with delayed-inhibitor production. Upon a decrease of excitability, spirals become unstable because their wavelengths and periods are too short to be sustained in the system
Journal Article•
TL;DR: In this article, the authors formulate the general theory of random walks in continuum, essential for treating a collision rate which depends smoothly upon direction of motion, and also consider a smooth probability distribution of correlations between the directions of motion before and after collisions.
Abstract: We formulate the general theory of random walks in continuum, essential for treating a collision rate which depends smoothly upon direction of motion. We also consider a smooth probability distribution of correlations between the directions of motion before and after collisions, as well as orientational Brownian motion between collisions. These features lead to an effective Smoluchowski equation. Such random walks involving an infinite number of distinct directions of motion cannot be treated on a lattice, which permits only a finite number of directions of motion, nor by Langevin methods, which make no reference to individual collisions. The effective Smoluchowski equation enables a description of the biased random walk of the bacterium Escherichia coli during chemotaxis, its search for food
TL;DR: The far-detuned trap confines atoms with a strong, nearly conservative optical force and negligible atomic excitation confined in an optical dipole force trap with a very large detuning from resonance of up to 65 nm.
Abstract: We confine $^{85}\mathrm{Rb}$ atoms in an optical dipole force trap with a very large detuning from resonance of up to 65 nm. Confinement times of 200 ms, limited only by background-gas collisions, are obtained without additional cooling. A typical trap contains 1300 atoms at a temperature of 0.4 mK and a peak density of 8\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. We measure spontaneous photon scatter rates of the trapped atoms to be less than 1.6\ifmmode\times\else\texttimes\fi{}${10}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, corresponding to recoil heating rates below 0.6 mK/s. The far-detuned trap confines atoms with a strong, nearly conservative optical force and negligible atomic excitation.
TL;DR: Photodepletion spectroscopy is used to measure the optical activity of sputtered Ag ionic clusters with up to 70 atoms and shows a blue shift, qualitatively explained in terms of a reduced s-d screening interaction in the surface region of the particles.
Abstract: Photodepletion spectroscopy is used to measure the optical activity of sputtered Ag ionic clusters with up to 70 atoms. With decreasing cluster size the giant resonance caused by collective excitation of the valence electrons shifts to higher frequencies. This blue shift is qualitatively explained in terms of a reduced s-d screening interaction in the surface region of the particles.
TL;DR: The dependence of the high-order harmonic cutoff with the laser intensity in neon and argon is found to be lower than that predicted in single-atom theories.
Abstract: We present experimental studies of high-order harmonic generation in the rare gases performed with a short-pulse titanium sapphire laser operating at 794 nm in the 10(14)-10(15) W/cm2 range. The harmonic yields generated in neon and in argon are studied for all orders as a function of the laser intensity. They vary first rather steeply, in the cutoff region, then much more slowly in the plateau region, and, finally, they saturate when the medium gets ionized. The dependence of the high-order harmonic cutoff with the laser intensity in neon and argon is found to be lower than that predicted in single-atom theories. We observe high-order harmonics in argon and xenon (up to the 65th and 45th, respectively) at 10(15) W/cm2, which we attribute to harmonic generation from ions. We also show how the harmonic and fundamental spectra get blueshifted when the medium becomes ionized. (Less)
TL;DR: A linear stability analysis demonstrates the existence of self-pulsing for a finite range of pumping rates and a theoretical model which describes the active medium as a mixture of isolated ions and ion pairs is presented.
Abstract: Experiments with erbium-doped fiber lasers demonstrate cw, sinusoidal, and self-pulsing operation. The obtained regimes depend on three control parameters: ion-pair concentration, photon lifetime, and pumping rate. We present a theoretical model which describes the active medium as a mixture of isolated ions and ion pairs. Starting with the adapted laser rate equations we show that the description of the dynamical behavior of this system can be reduced to only four first-order coupled equations. A linear stability analysis demonstrates the existence of self-pulsing for a finite range of pumping rates. At both ends of this range Hopf bifurcations occur: one located near the first laser threshold and the other at a higher pumping ratio, whose position is closely related to the pair concentration
TL;DR: The complete analytical solution of the cavity QED of a two-level atom and a field mode at zero temperature is presented, which includes both dissipation of the field due to a finite Q value ofThe cavity and incoherent decay mechanisms for the atom.
Abstract: We present the complete analytical solution of the cavity QED of a two-level atom and a field mode at zero temperature It includes both dissipation of the field due to a finite Q value of the cavity and incoherent decay mechanisms for the atom This analytical solution is provided by a powerful method for treating general master equations that appear in quantum optics As distinct from the usual approaches we first deal with that part of the master equation which describes the dissipative coupling of the field and the atom to their reservoirs Rather than using number-state or dressed-state bases we expand the density operator into the eigenstates of the nonunitary parts of the master equation which model the dissipative part of the dynamics The set of these eigenstates is the damping basis With the aid of this expansion we find the eigenvalues and eigenstates of the total Liouville operator The evolution of an arbitrary initial state is then known We employ these results to give an exact solution of the dynamics of the photon field in realistic experiments with one-atom masers at very low temperatures It includes detuning, cavity leakage effects, spontaneous decay mechanisms for the atoms, a Fizeau-type velocity distribution for the atomic beam, and a statistical parameter for the probability of the excitation of incoming atoms, covering the limits of Poissonian pumping and of regular pumping On the same grounds one can treat the one-atom laser, consisting of a single atom which stays in permanent interaction with the field mode and which is continuously pumped by external heat baths
TL;DR: This work gives an alternative interpretation of the wave function for a single system by means of a measurement which lasts a long time, which determines the expectation values of observables while the wavefunction is protected from collapsing because it undergoes another suitably chosen interaction.
Abstract: So far, the wave function has been interpreted as a probability amplitude, which is given physical meaning by ensemble averages of a large number of identical systems at a given time. We give an alternative interpretation of the wave function for a single system by means of a measurement which lasts a long time. This is a measurement on a single quantum system which determines the expectation values of (not necessarily commuting) observables while the wave function is protected from collapsing because it undergoes another suitably chosen interaction. This type of measurement enables the distinction between states which are not orthogonal, but are protected by a suitable interaction with the states of their environment, even for a single system. It therefore gives a different ontological meaning to the wave function. Several experiments in which such a measurement is realized, which can in principle be performed using electrons, neutrons, or atoms, are studied.
TL;DR: Formulas for the first-order corrections in [alpha] to the dipole form for the angular distribution for atomic photoionization are derived within a nonrelativistic central-potential model.
Abstract: Formulas for the first-order corrections in \ensuremath{\alpha} to the dipole form for the angular distribution for atomic photoionization are derived within a nonrelativistic central-potential model. The results are expressed in terms of two parameters \ensuremath{\gamma} and \ensuremath{\delta} and simple expressions for the angular distribution for unpolarized, polarized, and partially polarized light are given. Calculations for a number of rare-gas subshells are presented and compared with previous calculations.
Journal Article•
TL;DR: Methods for determining the integer-valued dimension of the state space of a system from observed scalar data using the idea of local false nearest neighbors are discussed.
Abstract: The time delay reconstruction of the state space of a system from observed scalar data requires a time lag and an integer embedding dimension. The minimum necessary global embedding dimension d E may still be larger than the actual dimension of the underlying dynamics d L . The embedding theorem only guarantees that the attractor of the system is fully unfolded using d E greater than 2d A , with d A the fractal attractor dimension. Using the idea of local false nearest neighbors, we discuss methods for determining the integer-valued d L
TL;DR: Results of experiments testing the influence of elliptical polarization on the production of high-order harmonics are presented and some of these data are compared with the predictions of lowest-order perturbation theory.
Abstract: We present results of experiments testing the influence of elliptical polarization on the production of high-order harmonics. Experiments were conducted both with a 600-nm, 1-psec dye laser and with an 825-nm, 140-fsec Cr:${\mathrm{LiSAF}}_{6}$ (Cr:LiSAF) laser system, over a wide range of intensities and target gases (xenon, argon, and neon), using a detection system with a dynamical range of more than three orders of magnitude. The decrease of the harmonic strength with the ellipticity of the pump beam is rather slow for the low-order harmonics, and becomes much steeper for the high-order harmonics. We compare some of these data with the predictions of lowest-order perturbation theory.
TL;DR: It is shown that there is a dual Aharonov-Casher topological effect on a neutral particle with electric dipole moment interacting with a magnetic field produced by magnetic monopoles.
Abstract: We show that there is a dual Aharonov-Casher topological effect [Phys. Rev. Lett. 53, 319 (1984)] on a neutral particle with electric dipole moment interacting with a magnetic field produced by magnetic monopoles.
TL;DR: The recently developed explicit formalism of orthogonally-spin-adapted, Hilbert-space (or state-universal), multireference (MR), coupled-cluster (CO theory), exploiting the model space spanned by two closed-shell-type reference configurations, is applied to a simple four-electron model system consisting of two interacting hydrogen molecules.
Abstract: In this series, the recently developed explicit formalism of orthogonally spin-adapted Hibert space (or state universal), multireference (MR) coupled-cluster (CC) theory, exploiting the model space spanned by two closed-shell-type reference configurations, is applied to a simple minimum-basis-set four-electron model system consisting of two interacting hydrogen molecules in various geometrical arrangements. In this paper, we examine the nonplanar geometries of this system, generally referred to as the T4 models, and their special cases designated as P4 and V4 models. They correspond to different cross sections of the H[sub 4] potential-energy hypersurface, involving the dissociation or simultaneous stretching of two H---H bonds. They involve various quasidegeneracy types, including the orbital and configurational degeneracies, the twofold degeneracy of the ground electronic state and interesting cases of broken-symmetry solutions. We employ the CC with singles and doubles (SD) approximation, so that the cluster operators are approximated by their one- and two-body components. Comparing the resulting CC energies with exact values, which are easily obtained for these models by using the full configuration-interaction method, and performing a cluster analysis of the exact solutions, we assess the performance of various MRCC Hilbert-space approaches at both linear and nonlinear levels of approximation, while a continuous transition ismore » being made between the degenerate and nondegenerate or strongly correlated regimes. We elucidate the sources and the type of singular behavior in both linear and nonlinear versions of MRCC theory, examine the role played by various intruder states, and discuss the potential usefulness of broken-symmetry MRCCSD solutions.« less
TL;DR: The photoabsorption spectrum of the C molecule and those of Xe and Ba atoms inside the molecule are calculated using a jelliumlike model for the confining cage using time-dependent density-functional theory.
Abstract: The photoabsorption spectrum of the ${\mathrm{C}}_{60}$ molecule and those of Xe and Ba atoms inside the ${\mathrm{C}}_{60}$ molecule are calculated using a jelliumlike model for the confining cage. The dynamic electronic response to an external electric field is obtained through time-dependent density-functional theory. The photoabsorption cross section for ${\mathrm{C}}_{60}$ shows strong collective resonances corresponding to plasmonlike excitations. The resonant 4d photoemission of the free atom is suppressed by the carbon cage, resulting in a weakly oscillating spectrum.