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At least 91 records · Page 5Linked to original sources
arXiv1995

Quantum state diffusion with a moving basis: computing quantum-optical spectra

Quantum state diffusion (QSD) as a tool to solve quantum-optical master equations by stochastic simulation can be made several orders of magnitude more efficient if states in Hilbert space are represented in a moving basis of excited coherent states. The large savings in computer memory and time are due to the localization property of the QSD equation. We show how the method can be used to compute spectra and give an application to second harmonic generation.

atom-ph↗
arXiv1995

Band Gaps for Atoms in Light based Waveguides

The energy spectrum for a system of atoms in a periodic potential can exhibit a gap in the band structure. We describe a system in which a laser is used to produce a mechanical potential for the atoms, and a standing wave light field is used to shift the atomic levels using the Autler-Townes effect, which produces a periodic potential. The band structure for atoms guided by a hollow optical fiber waveguide is calculated in three dimensions with quantised external motion. The size of the band gap is controlled by the light guided by the fiber. This variable band structure may allow the construction of devices which can cool atoms. The major limitation on this device would be the spontaneous emission losses.

atom-ph↗
arXiv1996

Prospects for Parity Non-conservation Experiments with Highly Charged Heavy Ions

We discuss the prospects for parity non-conservation experiments with highly charged heavy ions. Energy levels and parity mixing for heavy ions with two to five electrons are calculated. We investigate two-photon-transitions and the possibility to observe interference effects between weak-matrix elements and Stark matrix elements for periodic electric field configurations.

atom-ph↗
arXiv1996

Actio causes reactio: Gravito-optical trapping of three-level atoms

We investigate an atomic three-level $Λ$-system which is exposed to two counterpropagating laser fields (inducing Raman transitions) and which is closed by a magnetic hyperfine field tuned to be in resonance with the transition between the two ground states. The influence of a homogeneous gravitational field is included in a full quantum treatment of the internal and external dynamics of the atom. It is shown that the combined influence of the gravitational field and the lasers lead for specific momentum values with a very high probability to a transition of the Landau-Zener type. This is accompanied by a momentum transfer resulting in an upward kick. For appropriate initial conditions a sequence of up and down motions is obtained. No mirror is needed. A gravito-optical trapping of atoms based on this effect seems to be realizable.

atom-ph↗
arXiv1996

Motion Tomography of a single trapped ion

A method for the experimental reconstruction of the quantum state of motion for a single trapped ion is proposed. It is based on the measurement of the ground state population of the trap after a sudden change of the trapping potential. In particular, we show how the Q function and the quadrature distribution can be measured directly. In an example we demonstrate the principle and analyze the sensibility of the reconstruction process to experimental uncertainties as well as to finite grid limitations. Our method is not restricted to the Lamb-Dicke Limit and works in one or more dimensions.

atom-ph↗
arXiv1996

Right-unitary transformation theory and applications

We develop a new transformation theory in quantum physics, where the transformation operators, defined in the infinite dimensional Hilbert space, have right-unitary inverses only. Through several theorems, we discuss the properties of state space of such operators. As one application of the right-unitary transformation (RUT), we show that using the RUT method, we can solve exactly various interactions of many-level atoms with quantized radiation fields, where the energy of atoms can be two levels, three levels in Lambda, V and equiv configurations, and up to higher (>3) levels. These interactions have wide applications in atomic physics, quantum optics and quantum electronics. In this paper, we focus on two typical systems: one is a two-level generalized Jaynes-Cummings model, where the cavity field varies with the external source; the other one is the interaction of three-level atom with quantized radiation fields, where the atoms have Lambda-configuration energy levels, and the radiation fields are one-mode or two-mode cavities.

atom-ph↗
arXiv1996

Density Matrix Functional Calculations for Matter in Strong Magnetic Fields: I. Atomic Properties

We report on a numerical study of the density matrix functional introduced by Lieb, Solovej and Yngvason for the investigation of heavy atoms in high magnetic fields. This functional describes {\em exactly} the quantum mechanical ground state of atoms and ions in the limit when the nuclear charge $Z$ and the electron number $N$ tend to infinity with $N/Z$ fixed, and the magnetic field $B$ tends to infinity in such a way that $B/Z^{4/3}\to\infty$. We have calculated electronic density profiles and ground state energies for values of the parameters that prevail on neutron star surfaces and compared them with results obtained by other methods. For iron at $B=10^{12}$ G the ground state energy differs by less than 2 \% from the Hartree-Fock value. We have also studied the maximal negative ionization of heavy atoms in this model at various field strengths. In contrast to Thomas-Fermi type theories atoms can bind excess negative charge in the density matrix model. For iron at $B=10^{12}$ G the maximal excess charge in this model corresponds to about one electron.

atom-ph↗
arXiv1996

Interference of Bose condensates

We investigate the prospects of atomic interference using samples of Bose condensed atoms. First we show the ability of two independent Bose condensates to create an interference pattern, even if both condensates are described by Fock states. Thus, the existence of an experimental signature for a broken gauge symmetry, seen in a single run of the experiment, is not necessarily reflected by a broken symmetry on the level of the quantum mechanical state vector. Based on these results, we simulate numerically a recent experiment with two independent Bose condensates, performed by the group of W.Ketterle (MIT). The calculated expansion of the condensates is in good agreement with the experimental data. In addition the existence of interference fringes is predicted based on the nonlinear Schroedinger equation. Finally we study theoretically the influence of finite temperatures on the visibility of the interference in a double pinhole experiment.

atom-ph↗
arXiv1996

Variational calculations of dispersion coefficients for interactions between H, He, and Li atoms

The dispersion coefficients $C_6$, $C_8$, and $C_{10}$ for the interactions between H, He, and Li are calculated using variational wave functions in Hylleraas basis sets with multiple exponential scale factors. With these highly correlated wave functions, significant improvements are made upon previous calculations and our results provide definitive values for these coefficients.

atom-ph↗
arXiv1996

Stimulated Enhancement of Cross-Section by a Bose-Einstein Condensate

This paper examines the feasibility of constructing an experiment which detects the atomic stimulation of a photon emission process. A beam of atoms (bosons) in an excited state is passed over an atomic trap which traps the atoms when they are in their internal ground state. When the trap contains a Bose-Einstein condensate, the cross-section for absorption of the atomic beam is increased. We examine a particular model in which this atom-stimulation is observable, and is also characterised by the emission of photons in a narrow cone in the direction of the atomic beam.

atom-ph↗
arXiv1996

Some exact analytic results for the linear and non-linear response of atoms in a trap with a model interaction

We present an exact expression for the evolution of the wavefunction of $N$ interacting atoms in an arbitrarily time-dependent, $d$-dimensional parabolic trap potential $ω(t)$. The interaction potential between atoms is taken to be of the form $ξ/r^2$ with $ξ>0$. For a constant trap potential $ω(t)=ω_0$, we find an exact, infinite set of relative mode excitations. These excitations are relevant to the linear response of the system; they are universal in that their frequencies are independent of the initial state of the system (e.g. Bose-Einstein condensate), the strength $ξ$ of the atom-atom interaction, the dimensionality $d$ of the trap and the number of atoms $N$. The time evolution of the system for general $ω(t)$ derives entirely from the solution to the corresponding classical 1D single-particle problem. An analytic expression for the frequency response of the $N$-atom cluster is given in terms of $ω(t)$. We consider the important example of a sinusoidally-varying trap perturbation. Our treatment, being exact, spans the `linear' and `non-linear' regimes. Certain features of the response spectrum are found to be insensitive to interaction strength and atom number.

cond-mat↗
arXiv1996

Influence of radiative interatomic collisions on an atom laser

We discuss the role of light absorption by pairs of atoms (radiative collisions) in the context of a model for an atom laser. The model is applied to the case of VSCPT cooling of metastable triplet helium. We show that, because of radiative collisions, for positive detuning of the driving light fields from an atomic resonance the operating conditions for the atom laser can only be marginally met. It is shown that the system only behaves as an atom laser if a very efficient sub-Doppler precooling mechanism is operative. In the case of negative frequency detuning the requirements on this sub-Doppler mechanism are less restricting, provided one avoids molecular resonances.

atom-ph↗
arXiv1996

Separation of the Exchange-Correlation Potential into Exchange plus Correlation: an Optimized Effective Potential Approach

Most approximate exchange-correlation functionals used within density functional theory are constructed as the sum of two distinct contributions for exchange and correlation. Separating the exchange component from the entire functional is useful since, for exchange, exact relations exist under uniform density scaling and spin scaling. In the past, accurate exchange-correlation potentials have been generated from essentially exact densities constructed using information from either quantum chemistry or quantum Monte Carlo calculations but they have not been correctly decomposed into their separate exchange and correlation components, except for two-electron systems. exchange and correlation components (except for two-electron systems). Using a recently proposed method, equivalent to the solution of an optimized effective potential problem with the corresponding orbitals replaced by the exact Kohn-Sham orbitals, we obtain the separation according to the density functional theory definition. We compare the results for the Ne and Be atoms with those obtained by the previously used approximate separation scheme.

cond-mat↗
arXiv1996

Phase ordering kinetics of the Bose gas

We study the approach to equilibrium of a Bose gas to a superfluid state. We point out that dynamic scaling, characteristic of far from equilibrium phase-ordering systems, should hold. We stress the importance of a non-dissipative Josephson precession term in driving the system to a new universality class. A model of coarsening in dimension $d=2$, involving a quench between two temperatures below the equilibrium superfluid transition temperature ($T_c$), is exactly solved and demonstrates the relevance of the Josephson term. Numerical results on quenches from above $T_c$ in $d=2,3$ provide evidence for the scaling picture postulated.

cond-mat↗
arXiv1996

Stability of Bose condensed atomic Li-7

We study the stability of a Bose condensate of atomic $^7$Li in a (harmonic oscillator) magnetic trap at non-zero temperatures. In analogy to the stability criterion for a neutron star, we conjecture that the gas becomes unstable if the free energy as a function of the central density of the cloud has a local extremum which conserves the number of particles. Moreover, we show that the number of condensate particles at the point of instability decreases with increasing temperature, and that for the temperature interval considered, the normal part of the gas is stable against density fluctuations at this point.

cond-mat↗
arXiv1996

Renormalization Group Theory of the Three-Dimensional Dilute Bose Gas

We study the three-dimensional atomic Bose gas using renormalization group techniques. Using our knowledge of the microscopic details of the interatomic interaction, we determine the correct initial values of our renormalization group equations and thus obtain also information on nonuniversal properties. As a result, we can predict for instance the critical temperature of the gas and the superfluid and condensate density of the Bose-Einstein condensed phase in the regime $naΛ_{th}^2\ll 1$.

cond-mat↗
arXiv1996

Dynamics, correlations and phases of the micromaser

The micromaser possesses a variety of dynamical phase transitions parametrized by the flux of atoms and the time-of-flight of the atom within the cavity. We discuss how these phases may be revealed to an observer outside the cavity using the long-time correlation length in the atomic beam. Some of the phase transitions are not reflected in the average excitation level of the outgoing atom, which is the commonly used observable. The correlation length is directly related to the leading eigenvalue of the time evolution operator, which we study in order to elucidate the phase structure. We find that as a function of the time-of-flight the transition from the thermal to the maser phase is characterized by a sharp peak in the correlation length. For longer times-of-flight there is a transition to a phase where the correlation length grows exponentially with the flux. We present a detailed numerical and analytical treatment of the different phases and discuss the physics behind them.

atom-ph↗