Solving the Kadanoff-Baym equations for inhomogeneous systems: application to atoms and molecules

We implement time propagation of the nonequilibrium Green function for atoms and molecules by solving the Kadanoff-Baym equations within a conserving self-energy approximation. We here demonstrate the usefulness of time propagation for calculating spectral functions and for describing the correlated electron dynamics in a nonperturbative electric field. We also demonstrate the use of time propagation as a method for calculating charge-neutral excitation energies, equivalent to highly advanced solutions of the Bethe-Salpeter equation.     

Control of light pulse propagation with only a few cold atoms in a high-finesse microcavity

Propagation of a light pulse through a high-Q optical microcavity containing a few cold atoms (N<10) in its cavity mode is investigated experimentally. With less than ten cold rubidium atoms launched into an optical microcavity, up to 170 ns propagation lead time ("superluminal"), and 440 ns propagation delay time (subluminal) are observed. Comparison of the experimental data with numerical simulations as well as future experiments are discussed.     

Transverse heating due to nonadiabatic propagation of cold atoms in an atomic guide

We study propagation of cold atoms along a curved atomic guide following an arbitrary trajectory in space. Transverse energy of the atomic beam increases as the beam propagates along the guide. Our model explains results of recent experiments on optical and magnetic guiding of cold atoms.     

Slow light with cavity electromagnetically induced transparency

We report an experimental observation of slow light propagation in cold Rb atoms exhibiting cavity electromagnetically induced transparency (EIT). The steep slope of the atomic dispersion manifested by EIT reduces the light group velocity. The cavity filtering and feedback further contribute to the slowdown and delay of the light pulse propagation. A combination of the cavity and the EIT atomic system significantly improves the performance of the slow light propagation. A propagation time delay of approximately 200 ns was observed in the cavity and Rb EIT system, which is approximately 70 times greater than the time delay calculated for the light pulse propagation through the same Rb EIT system without the cavity.     

Opacity of electromagnetically induced transparency for quantum fluctuations

We analyze the propagation of a pair of quantized fields inside a medium of three-level atoms in a Lambda configuration. We calculate the stationary quadrature noise spectrum of the field, in the case where the probe field is in a squeezed state and the atoms show electromagnetically induced transparency. We find an oscillatory transfer of the initial quantum properties between the probe and pump fields which is most strongly pronounced when both fields have comparable intensities. This implies that the quantum state measured after propagation can be completely different from the initial state, even though the mean values of the field are unaltered.     

The propagation of sound waves and neutron waves in bounded plane geometry

Integral equations are formulated for neutron wave propagation parallel to the faces of a slab and sound wave propagation in a gas confined between parallel plates. The main physical differences arise from the different conditions satisfied by the particle distribution functions at the boundaries of the systems, i.e. neutrons can pass through a boundary unhindered whilst gas atoms are reflected from it.     

Numerical solutions to 2D Maxwell–Bloch equations

Rare-earth-doped crystals contain inhomogeneously broadened two-level atoms. Optical propagation and nonlinear interaction in the crystals can be described by the Maxwell–Bloch equations. We show a consistent numerical approach that solves Maxwell’s equations by using the FFT-finite difference beam propagation method and the Bloch equations by using the finite difference method. Numerical simulation results are given for an off-axis 3-pulse photon echo.     

Propagation of cold atoms along a miniature magnetic guide

A cloud of laser-cooled 85Rb atoms is coupled through a magnetic funnel into a miniature waveguide formed by four current-carrying wires embedded in a silica fiber. The atom cloud has a approximately 100 &mgr;m radius within the fiber and propagates over cm distances. We study the coupling, propagation, and transverse distribution of atoms in the fiber, and find good agreement with theory. This prototype demonstrates the feasibility of miniature guides as a tool in the new field of integrated atom optics, leading to single-mode propagation of de Broglie waves and the possible preparation of 1D atom clouds.     

Nonlinear polariton waves in an optical double lattice with photo-induced transport of atoms

The process of photo-induced transport of cold atoms in optical lattices is discussed for the case in which the direct tunneling of atoms is suppressed but transport of an atom is possible as a result of simultaneous absorption and reemission of two photons with different energies. Proceeding from the Bose-Hubbard model for two kinds of traps of a one-dimensional optical lattice, a set of equations describing the propagation of an ultrashort pulse of biharmonic radiation accompanied by a transfer of populations of traps of such a double lattice is derived. It was found that, depending on the choice of boundary conditions, there exist two types of stationary solitary waves corresponding to the propagation of a coupled pair of soliton pulses: a pair of bright solitons and a coupled pair of dark and bright solitons.     

The effect of initial inversion of resonant atoms on the propagation dynamics of laser pulses in a continuous resonant photonic crystal

The nonlinear interaction of high-intensity coherent optical radiation and a continuous resonant photonic crystal is analytically and numerically studied in the framework of the semiclassical approximation using the two-wave Maxwell-Bloch equations at various initial excitations of resonant atoms. It is demonstrated that the initially unexcited resonant photonic crystal with an arbitrary concentration function of resonant atoms allows the propagation of the Bragg soliton of the self-induced transparency. The Bragg reflection is suppressed at zero initial inversion when the number of atoms in the excited state is equal to the number of atoms in the stationary state.     

Dynamic control of light propagation and optical switching through an RF-driven cascade-type atomic medium

We study nonlinear optical behaviors in pulse propagation through a medium consisting of four-level cascade-type cold atoms, where a radio-frequency (RF) field couples upper two-folded levels and double-dark resonances (DDRs) can arise. By numerically solving the coupled Bloch-Maxwell equations for atom and field simultaneously in space and time, we demonstrate dynamic control of light propagation and optical switching in such a four-level atomic medium. The proposed scheme may have potential applications in the design of optical switching and optical storage devices.     

Nonlocal stabilization of nonlinear beams in a self-focusing atomic vapor

We show that ballistic transport of optically excited atoms in an atomic vapor provides a nonlocal nonlinearity which stabilizes the propagation of vortex beams and higher order modes in the presence of a self-focusing nonlinearity. Numerical experiments demonstrate stable propagation of higher order nonlinear states (dipole, vortices, and rotating azimuthons) over a hundred diffraction lengths, before dissipation leads to decay of these structures.     

Longitudinal atomic beam spin echo experiments: a possible way to study parity violation in hydrogen

We discuss the propagation of hydrogen atoms in static electric and magnetic fields in a longitudinal atomic beam spin echo (lABSE) apparatus. Depending on the choice of the external fields the atoms may acquire both dynamical and geometrical quantum mechanical phases. As an example of the former, we show first in-beam spin rotation measurements on atomic hydrogen, which are in excellent agreement with theory. Additional calculations of the behaviour of the metastable 2S states of hydrogen reveal that the geometrical phases may exhibit the signature of parity-(P-)violation. This invites for possible future lABSE experiments, focusing on P-violating geometrical phases in the lightest of all atoms.     

Parity violation in hydrogen and longitudinal atomic beam spin echo I

We discuss the propagation of hydrogen atoms in static electric and magnetic fields in a longitudinal atomic beam spin echo (lABSE) apparatus. There the atoms acquire geometric (Berry) phases that exhibit a new manifestation of parity-(P-)violation in atomic physics. We provide analytical as well as numerical calculations of the behaviour of the metastable 2S states of hydrogen. The conditions for electromagnetic field configurations that allow for adiabatic evolution of the relevant atomic states are investigated. Our results provide the theoretical basis for the discussion of possible measurements of P-violating geometric phases in lABSE experiments.     

Quasi-coarse-grained dynamics: modelling of metallic materials at mesoscales

A computationally efficient modelling method called quasi-coarse-grained dynamics (QCGD) is developed to expand the capabilities of molecular dynamics (MD) simulations to model behaviour of metallic materials at the mesoscales. This mesoscale method is based on solving the equations of motion for a chosen set of representative atoms from an atomistic microstructure and using scaling relationships for the atomic-scale interatomic potentials in MD simulations to define the interactions between representative atoms. The scaling relationships retain the atomic-scale degrees of freedom and therefore energetics of the representative atoms as would be predicted in MD simulations. The total energetics of the system is retained by scaling the energetics and the atomic-scale degrees of freedom of these representative atoms to account for the missing atoms in the microstructure. This scaling of the energetics renders improved time steps for the QCGD simulations. The success of the QCGD method is demonstrated by the prediction of the structural energetics, high-temperature thermodynamics, deformation behaviour of interfaces, phase transformation behaviour, plastic deformation behaviour, heat generation during plastic deformation, as well as the wave propagation behaviour, as would be predicted using MD simulations for a reduced number of representative atoms. The reduced number of atoms and the improved time steps enables the modelling of metallic materials at the mesoscale in extreme environments.     

Atomic recoil effects in slow light propagation

We theoretically investigate the effect of atomic recoil on the propagation of ultraslow light pulses through a coherently driven Bose-Einstein condensed gas. For a sample at rest, the group velocity of the light pulse is the sum of the group velocity that one would observe in the absence of mechanical effects (infinite mass limit) and the velocity of the recoiling atoms (light-dragging effect). We predict that atomic recoil may give rise to a lower bound for the observable group velocities, as well as to pulse propagation at negative group velocities without appreciable absorption.     

Coherent control of laser pulse temporal duration: An experimental proposal

We present a study of temporal compression resulting from the coherent control peculiarities of electromagnetically induced transparency propagation dynamics. We discuss the crucial conditions required to accomplish temporal compression in an experiment with a sample of hot atoms.     

Symmetry of carrier-envelope phase difference effects in strong-field, few-cycle ionization of atoms and molecules

In few-cycle pulses, the exact value of the carrier-envelope phase difference (CEPD) has a pronounced influence on the ionization dynamics of atoms and molecules. We show that, for atoms in circularly polarized light, a change in the CEPD is mapped uniquely to an overall rotation of the system, and results for arbitrary CEPD are obtained by rotation of the results from a single calculation with fixed CEPD. For molecules, this is true only for linear molecules aligned parallel with the propagation direction of the field. The effects of CEPD are classified as geometric or nongeometric. The observations are exemplified by strong-field calculations on hydrogen.     

Directional anisotropy in the cleavage fracture of silicon

Total-energy pseudopotential calculations are used to study the cleavage anisotropy in silicon. It is shown that cracks propagate easily on 111 and 110 planes provided crack propagation proceeds in the <1;10> direction. In contrast, if the crack is driven in a <001> direction on a 110 plane the bond breaking process is discontinuous and associated with pronounced relaxations of the surrounding atoms, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the experimentally observed cleavage anisotropy in silicon single crystals.     

Frequency-filtered parametric fluorescence interacting with an atomic ensemble

The frequency spectrum of the fluorescence must be reduced when studying interactions between atoms and parametric fluorescence using the photon counting method since photon counting does not distinguish the light frequency. An interference filter and etalons successfully reduced the frequency spectrum of the parametric fluorescence from 6.6 THz to 1.7 GHz. The parametric fluorescence after frequency filtering showed the non-classical feature violating a Cauchy-Schwartz inequality for the intensity correlation function. We used slow light propagation with Rb gas to demonstrate that the obtained light source interacts with the atoms.