We report an experimental and theoretical study of momentum transfer from a modulated standing wave of light to a sample of ultracold atoms. This system is a quantum realization of the periodically driven rotor where the underlying classical phase space goes from stable to chaotic as a control parameter is varied. Our experimental results are in good absolute agreement with a quantum Floquet analysis and with a quantum simulation. We relate the quantum evolution to the underlying classical dynamics in this mixed phase space regime.

We show that magnetic quantization of the carrier free motion into discrete Landau levels for quantum wells inserted in a Fabry-Pérot microcavity allows continuous tuning of the electromagnetic interaction from the weak coupling regime (irreversible regime described by Fermi's golden rule) to the strong coupling regime (where coherent time dependence and the vacuum Rabi splitting prevail).

Clay-water suspensions are shear-thinning yield stress fluids whose apparent viscosity increases with solid fraction. It is shown experimentally that, in a wide solid fraction range, their behavior is similar and goes through a transition from non-Newtonian to Newtonian when a nondimensional shear rate increases.

A vertical soap film channel driven by gravity is used to study two-dimensional turbulence. Velocity differences on different scales and the spectrum of velocity fluctuations show behavior very different from their three-dimensional counterparts. Our findings are consistent with the theoretically predicted enstrophy cascade picture for two-dimensional turbulence. However, significant differences were observed.

The interaction of a dipole gas with a strong laser field $E_1\left(t\right)$ and a strong, much lower frequency electric field $E_2\left(t\right)$ is analytically investigated. A theory of stimulated scattering in dipole gases is developed and used in numerical calculations of the amplification of the field $E_2\left(t\right)$ under resonance conditions. This theory may serve as a basis for constructing tunable devices for the generation and/or amplification of infrared or microwave radiation. Such devices may make inroads into a wavelength range longer than that of analogous devices based on the traditional stimulated Raman scattering in nondipole media.

We establish rigorous inequalities for short-distance scaling exponents in 2D incompressible turbulence. Using only the condition of constant ultraviolet enstrophy flux, we show that $〈({\Delta }_{\mathit{l}}\omega {)}^p〉\sim {\ell }^{{\zeta }_p}$ must have ${\zeta }_2\le 2/3$ (Sulem-Frisch bound) and ${\zeta }_p\le 0$, for $p\ge 3$. If the minimum Hölder singularity of the vorticity is negative, $h_{\min }<0\mathrm{}$, then the bounds can be improved to ${\zeta }_p\le {\tau }_{p/3\mathrm{}}$, where ${\tau }_p$ is the scaling exponent of a local enstrophy flux: $〈|Z_{\ell }{|}^p〉\sim {\ell }^{{\tau }_p}$. However, if $h_{\min }=0\mathrm{}$, then ${\zeta }_p=0\mathrm{}$ for $p\ge 2$ and Kraichnan theory is exact.

Experimental data on the rate of advance $\left(s\right)$ of a propagating front in a stirred medium was obtained using reaction-diffusion fronts in aqueous solution and two types of hydrodynamic flows. Two distinct propagation modes are observed depending on a Damköhler number (Da) indicating the ratio of a chemical rate to a hydrodynamic strain rate. For low Da, values of $s$ are consistent with a Huygens propagation model employing renormalization group analysis. For high Da, values of $s$ are consistent with a classical model. These results are compared to traditional experiments using premixed combustible gases.

Observations of unusual diffraction and interference by two-photon correlation measurements are reported. The signal and idler beams produced by spontaneous parametric down-conversion are sent in different directions, and detected by two distant pointlike photon counting detectors. A double slit or a single slit is inserted into the signal beam. Interference-diffraction patterns are observed in coincidences by scanning the detector in the idler beam.

Spontaneous emission lifetime in a one-dimensional AlGaAs distributed Bragg reflector microcavity has been systematically measured as a function of emission wavelength tuned by the quantum-confined Stark effect in order to confirm the cavity-induced alteration of the SE lifetime. The result manifests a substantial cavity-induced fractional change of the exciton decay times, $\sim$30% around the on-axis resonant wavelength, despite the relatively large broadening of the excitonic emission being comparable to the cavity resonance width in the low- $Q$ cavity.

We give a simple theory for recent experiments of Bar-Ziv and Moses [Phys. Rev. Lett. 73, 1392 (1994)] in which tubular vesicles are excited using laser tweezers to a “peristaltic” state. Considering the hydrodynamics of a bilayer membrane under tension, we reproduce some of the qualitative behavior seen and find a value for the wavelength of the instability in terms of independently measured material parameters, in rough agreement with the experimental values.

A Comment on the Letter by Francesco Benvenuto, Guilio Casata and D. L. Shepelyansky, Phys. Rev. Lett. 72, 1818 (1994).

A Reply to the Comment by David Farrelly, Paolo Bellomo and T. Uzer.

We consider the evolution of a magnetic field in a spherical shell of highly conducting fluid surrounded by an insulator. We impose an axisymmetric, time-dependent flow, having large regions of chaotic particle paths. This flow appears to yield fast dynamo action, in which the field grows on the fast advective, rather than on the slow diffusive time scale. We demonstrate that the field adjusts to the Bondi-Gold theorem, according to which the field in the insulators inside and outside the shell cannot grow on the fast time scale, by becoming increasingly self-contained within the shell.

It is shown that a paradigm of classical statistical mechanics—the thermalization of a Brownian particle—has a low-dimensional, deterministic analog: When a heavy, slow system is coupled to fast, deterministic chaos, the resultant forces drive the slow degrees of freedom toward a state of statistical equilibrium with the fast degrees. This illustrates how concepts useful in statistical mechanics may apply in situations where low-dimensional chaos exists.

We propose a coupled system of the Hirota equation and the Maxwell-Bloch equations to describe the wave propagation in an erbium doped nonlinear fiber with higher order dispersion. The Painlevé property of the same is analyzed and the coupled system is found to be integrable. The Lax pair is also constructed and the single-soliton solution is explicitly shown. The coupled system is found to allow soliton-type propagation.

It has been predicted that in the semiclassical regime the level statistics of a classically chaotic system correspond to that of the Gaussian unitary ensemble (GUE) of random matrices when time reversal symmetry is broken. This Letter presents the first experimental test of this prediction. The system employed is a microwave cavity containing a thin ferrite strip adjacent to one of the walls. When a sufficiently large magnetic field is applied to the ferrite (thus breaking the time reversal symmetry) good agreement with GUE statistics is obtained. The transition from Gaussian orthogonal ensemble (GOE) (which applies in the absence of the applied field) to GUE is also investigated.

First results on microwave billiards with broken time reversal symmetry are presented. The billiards are quasi two dimensional with an attached microwave isolator acting as a unidirectional transmission line. Spectral level dynamics was studied by changing the billiard length. For all spectral properties observed, i.e., nearest-neighbor distance, asymptotic curvature, and closest-approach distance at avoided crossings, we have found behavior characteristic of the Gaussian unitary ensemble.

Chaos in vibrational predissociation of the van der Waals complex He-${\mathrm{I}}_2$ is investigated including the bending motion. A four-dimensional Poincaré section is analyzed in terms of homoclinic intersection and Lagrangian singularity. Particular attention is paid to the homoclinic tangency which was first noted by Gillian and Ezra. We find that it leads to a crisis in which a transition occurs from the horseshoe dynamics into topologically new chaos. This is manifested by successive generation of the Whitney cusps. The implications of this result for chemical reactions are briefly discussed.

The distributions of the angular transmission coefficient and of the total transmission are calculated for multiple scattered waves. The calculation is based on a mapping to the known distribution of eigenvalues of the transmission matrix. The distributions depend on the profile of the incoming beam. The distribution function of the angular transmission has a stretched exponential decay. The total-transmission distribution grows log normally whereas it decays exponentially.