High-Rayleigh-number turbulent convection has been studied using the technique of photon-correlation homodyne spectroscopy to measure velocity differences at various length scales. The measured intensity correlation function G(q,t,L) is found to be of the scaling form G(qu(L)t), with u(L)∼${\mathit{L}}^{0.6}$ being the characteristic turbulent velocity at the legth scale L and q the scattering vector. It is also found that the lower cutoff length ${\mathit{l}}_{\mathit{c}}$ of the scaling range obeys a power law ${\mathit{l}}_{\mathit{c}}$∼${\mathrm{Ra}}^{\mathrm{-}0.3}$. The measured power-law exponents are in excellent agreement with theoretical predictions.

We consider the quantum electrodynamics of an atom uniformly moving through a single spatially periodic field mode. The shape and periodicity of the field modulation can be designed by an appropriate choice of a defect in a periodic structure that possesses a forbidden spectral band (a ‘‘photonic band gap’’). This design of the periodic modulation can improve our control over the evolution and properties (e.g., photon statistics) of nonclassical ‘‘Schrödinger-cat’’ states of the field, generated by resonant interaction with the atom.

The concepts of absolute and convective instability are extended to nonlinear systems with broken Galilean invariance. As an illustrative model we describe the behavior of a flow, homogeneous in a semi-infinite domain, which undergoes a subcritical pitchfork bifurcation. The classical bifurcation phenomenology is shown to be nontrivially affected by the presence of a nonremovable advection term. In particular the existence of a hysteresis loop is shown to be restricted to the nonlinear absolute instability range. A qualitative description of the possible scenarios likely to arise in subcritically bifurcating open flows is outlined and a practical test is suggested to determine the nature of the bifurcation.

The autoresonance in nonlinear three-wave interactions is discussed. The phenomenon is characteristic of inhomogenous (or time-dependent) media in the presence of nonlinear wave-vector (frequency) shifts and leads to a persistent and efficient redistribution of the wave action fluxes (action densities) via a continuing spatial (temporal) self-adjustment of the nonlinear resonance relation. Conditions for the autoresonance are found and the phenomenon is illustrated by numerical examples.

Polarized, stored beams are becoming a more and more important tool in nuclear and high-energy physics. In order to measure the beam polarization in a storage ring the polarization vector of the stored beam has to aim, revolution for revolution, over a period of seconds to minutes, into the same, so-called ‘‘stable’’ direction. In this paper measurements at the Indiana University Cooler Ring are described in which for the first time in a storage ring, oscillations around this stable direction have been measured. The existence and the dynamics of such oscillations are, for instance, important for a new proposed technique for polarizing stored hadron beams.

Electrically coupled light-emitting diodes have been configured to demonstrate multiple light beams with correlated sub-shot-noise intensity fluctuations. Photocurrent covariance measurements of the two light beams from a pair of series-connected diodes show a positive correlation, similar to that observed with nonlinear crystal down-converters. Shunt-connected diodes are shown to generate negatively correlated light beams.

The classical-quantum correspondence is studied for the ‘‘wedge billard,’’ a system known to exhibit hard chaos. The Gutzwiller trace formula is recast as a damped sine or cosine transform, yielding peaks and zero crossings at the values of the actions of the periodic orbits. The lowest 190 energy eigenvalues of the quantum system are shown to contain a great deal of information about the shortest periodic orbits of the classical system.

A direct numerical study of a model of superflow, the nonlinear Schrödinger equation, and simple analytical arguments shows a very striking phenomenon: The flow around a disk creates a drag force beyond a well-defined threshold velocity, linked to the emission of vortices from the perimeter of the disk.

The Navier-Stokes equations are utilized to derive upper bounds on the turbulent energy dissipation rate for an incompressible Newtonian fluid confined between parallel comoving plates. These estimates provide a rigorous foundation for one of the basic scaling ideas of turbulence theory, namely, the independence of the dissipation rate and the viscosity at high Reynolds number. The bounds are compared to experiments on turbulence in the Couette-Taylor system.

The correlation between fluorescence photons emitted by an optically pumped single molecule of pentacene in a p-terphenyl host has been investigated at short times. The correlation function shows photon antibunching, a unique feature of a nonclassical radiation field, which decreases if two molecules rather than one are pumped at the same time. The peculiarities of the correlation function for a three-level molecule are discussed and the theoretical description outlined.

We show analytically and numerically that the linearly stable flat interface in the Saffman-Taylor problem becomes unstable against finite amplitude perturbations when the surface tension is modified in proportion to the local curvature. This nonlinear instability is identified with unstable saddle points in a flow diagram in a weakly nonlinear approximation. Numerical simulations confirm this prediction and reveal that this instability leads to spiky celluar patterns.

We report a real-time, two-dimensional light scattering study of the evolution of structure in a concentrated electrorheological fluid during the ‘‘liquid-solid’’ phase transition. We find that after particle chaining along the electric field lines, strong light scattering lobes appear at a finite scattering wave vector q orthogonal to the field lines, and then brighten as they move to q=0. This indicates the existence of an unstable concentration fluctuation that signifies the segregation of chains into columns. We suggest a theoretical explanation for our results.

Nonequilibrium steady-state flows governed by time-reversible equations of motion are described in terms of unstable periodic orbits. Important properties, such as transport coefficients and the multifractal spectrum, are related to the difference in probability of observing periodic orbits and their time reverses. We apply the theory to the periodic Lorentz gas driven by a constant external field and numerically calculate the conductivity.

The large dispersion of a phase-coherent medium, at a point of vanishing absorption, is applied to interferometric measurements of detuning between atomic and radiation frequencies. It is shown that, under certain conditions, the interferometer quantum-limited operation is determined by vacuum-fluctuation shot noise while the noise introduced by the interaction of the probe field with the phase-coherent atoms can be made negligible. As a possible application, an optical magnetometer is analyzed whose sensitivity is shown to be potentially superior to the present state-of-the-art devices.

Recently, in connection with the problem of Anderson localization of light, a new physical mechanism was considered for the decreasing of the diffusion coefficient, in which it is supposed that the velocity of energy transport, which appears in the diffusion constant D=${\mathit{v}}_{\mathit{E}}$${\mathit{l}}_{\mathrm{tr}}$/3, may be much smaller than the phase velocity. Using an exact definition of the diffusion coefficient and generalized Ward identity, we, however, show that in the low-density approximation ${\mathit{v}}_{\mathit{E}}$ coincides with the phase velocity.

Convection and fluidization in a vibrated bed of powder are reproduced in a numerical simulation. In the simulation, each particle of the powder, during a collision, has a viscoelastic interaction with the other colliding particle. Because of the discreteness of the particles, this elasticity causes convection. The critical values of fluidization and convection agree with experiments.

We present molecular dynamics simulations of granular material submitted to vibrations in a two-dimensional system. We find various types of convection cells, due either to the existence of walls or to spatial modulations in the amplitude of the vibration. The direction of the motion relative to the walls depends on shear friction. We measure the strength of the convection velocity and find a characteristic resonance frequency as in experiments. We propose an explanation for the mechanisms that are at the origin of the different motions.

We report the observation of a key feature of intermittency in the tokamak edge turbulence. The probability distribution functions of broadband plasma density and potential fluctuations in an Ohmically heated tokamak plasma are measured and shown to be non-Gaussian. The intermittency is observed on scales large compared to the dissipation scale and it is found to evolve during the flattop current phase of the discharge.

Using velocity data obtained in the atmospheric surface layer, we examine Kolmogorov’s refined hypotheses. In particular, we focus on the properties of the stochastic variable V=Δu(r)/(r${\mathrm{\varepsilon }}_{\mathit{r}}$${)}^{1/3}$, where Δu(r) is the velocity increment over a distance r, and ${\mathrm{\varepsilon }}_{\mathit{r}}$ is the dissipation rate averaged over linear intervals of size r. We show that V has an approximately universal probability density function for r in the inertial range and discuss its properties; we also examine the properties of V for r outside the inertial range.

The clocklike emission of picosecond pulses of light with a peak power over 30 mW has been observed to originate from a bubble trapped at the velocity node of a resonant sound field in water. The spectrum of this bright sonoluminescence is broadband and our measurements show that if a spectral peak exists, it lies at photon energies above 6 eV.