Photon rescattering in anisotropic clouds of laser cooled atoms is investigated by studying the amplification of a weak probe beam propagating through the atoms. In the case where the atomic ensemble is optically thick in all directions, the amplification of the probe is rapidly saturated due to multiple scattering of spontaneous photons. As the aspect ratio of the cloud is increased, the amplification converges to the values predicted by Mollow’s theory indicating a suppression of light reabsorption.

We study the dynamics of a beam of fermions diffracted off a density grating formed by fermionic atoms in the limit of a large grating. An exact description of the system in terms of particle-hole operators is developed. We use a combination of analytical and numerical methods to quantitatively explore the Raman-Nath and the Bragg regimes of diffraction. We discuss the limits in diffraction efficiency resulting from the dephasing of the grating due to the distribution of energy states occupied by the fermions. We propose several methods to overcome these limits, including the technique of “atom echoes.”

We investigate strong-coupling superfluidity in a uniform two-component gas of ultracold Fermi atoms attractively interacting via quasimolecular bosons associated with a Feshbach resonance. This interaction is tunable by the threshold energy $2\nu$ of the Feshbach resonance, becoming large as $2\nu$ is decreased (relative to $2{\varepsilon }_{\mathrm{F}},$ where ${\varepsilon }_{\mathrm{F}}$ is the Fermi energy of one component). In recent work, we showed that the enhancement of this tunable pairing interaction naturally leads to the BCS-BEC (Bose-Einstein condensation) crossover, where the character of the superfluid phase transition changes from the BCS type to a BEC of composite bosons consisting of preformed Cooper-pairs and Feshbach-induced molecules. In this paper, we extend our previous work and study both the quasiparticles and the collective dynamics of the superfluid phase below the phase-transition temperature $T_{\mathrm{c}},$ limiting ourselves to a uniform gas. We show how the superfluid order parameter changes from the Cooper-pair amplitude $\Delta$ to the square root of the number of condensed molecules $\left({\phi }_{\mathrm{m}}\right)$ associated with the Feshbach resonance, as the threshold energy $2\nu$ is lowered. In the intermediate coupling regime, the superfluidity is shown to be characterized by an order parameter consisting of a superposition of $\Delta$ and ${\phi }_{\mathrm{m}}.$ We also discuss the Goldstone mode associated with superfluidity, and show how its character smoothly changes from the Anderson-Bogoliubov phonon in the BCS regime to the Bogoliubov phonon in the BEC regime in the BCS-BEC crossover. The velocity of this Goldstone phonon mode is shown to strongly depend on the value of $2\nu .$ We also show that this Goldstone mode appears as a resonance in the spectrum of the density-density correlation function, which is experimentally accessible.

We study the surface modes of some of the vortex liquids recently found by means of exact diagonalizations in systems of rapidly rotating bosons. In contrast to the surface modes of Bose condensates, we find that the surface waves have a frequency linear in the excitation angular momentum, $ħl>\mathrm{}0.\mathrm{}$ Furthermore, in analogy with the edge waves of electronic quantum Hall states, these excitations are chiral, that is, they can be excited only for values of l that increase the total angular momentum of the vortex liquid. However, differently from the quantum Hall phenomena for electrons, we also find other excitations that are approximately degenerate in the laboratory frame with the surface modes, and which decrease the total angular momentum by l quanta. The surface modes of the Laughlin as well as other scalar and vector boson states are analyzed and their observable properties characterized. We argue that measurement of the response of a vortex liquid to a weak time-dependent potential that imparts angular momentum to the system should provide valuable information for characterizing the vortex liquid. In particular, the intensity of the signal of the surface waves in the dynamic structure factor has been studied and found to depend on the type of vortex liquid. We point out that the existence of surface modes has observable consequences on the density profile of the Laughlin state. These features are due to the strongly correlated behavior of atoms in the vortex liquids. We point out that these correlations should be responsible for a remarkable stability of some vortex liquids with respect to three-body losses.

We study the expansion of an ultracold boson-fermion mixture released from an elongated magnetic trap, by using a scaling approach. We discuss in detail the role of the boson-fermion interaction on the evolution of the radial-to-axial aspect ratio of the condensate, and show that the latter depends crucially on the relative dynamics of the condensate and degenerate Fermi gas in the radial direction, which is characterized by the ratio between the trapping frequencies for fermions and bosons. The numerical solution of the scaling equations provides a reasonable agreement with the recent experiment [G. Roati et al., Phys. Rev. Lett. 89, 150403 (2002)].

The energy and structure of dilute hard- and soft-sphere Bose gases are systematically studied in the framework of several many-body approaches, such as the variational correlated theory, the Bogoliubov model, and the uniform limit approximation, valid in the weak-interaction regime. When possible, the results are compared with the exact diffusion Monte Carlo ones. Jastrow-type correlation provides a good description of the systems, both hard- and soft-spheres, if the hypernetted chain energy functional is freely minimized and the resulting Euler equation is solved. The study of the soft-sphere potentials confirms the appearance of a dependence of the energy on the shape of the potential at gas paremeter values of $x\sim \mathrm{0.001.}$ For quantities other than the energy, such as the radial distribution functions and the momentum distributions, the dependence appears at any value of x. The occurrence of a maximum in the radial distribution function, in the momentum distribution, and in the excitation spectrum is a natural effect of the correlations when x increases. The asymptotic behaviors of the functions characterizing the structure of the systems are also investigated. The uniform limit approach is very easy to implement and provides a good description of the soft-sphere gas. Its reliability improves when the interaction weakens.

Generation of high-order harmonics in Xe was studied in a static cell at high pump energies, 5 m focal length, and up to 14 cm interaction lengths. Self-guided propagation of the pulse was observed experimentally and confirmed by a three-dimensional model. Phase-matched generation was demonstrated in the self-guided beam, and the high energy and low divergence of the harmonic radiation were explained. Harmonic field calculations, in good agreement with experimental results, allow for the explanation of the higher-order harmonic generation dynamics in the self-guided region.

We investigate steady-state squeezing in the phase quadratures of the resonance fluorescence emitted from a coherently driven four-level atom of $\Lambda$ configuration. Quantum fluctuation spectra of the optimum-phase quadratures are calculated. We show that if quantum interference between spontaneous emission channels from closed excited states to lower levels of the atom takes place, squeezing in the optimum-phase quadrature can occur in a very wider region of the parameter space than that predicted by previous studies. We also show that the quantum interference can greatly change the atomic coherences and then lead to this interesting result.

The well-known laser-induced Rabi oscillations of a two-level atom are shown to be suppressed under certain conditions when the atom is entering a laser-illuminated region. For temporal Rabi oscillations the effect has two regimes: a first classical-like one, taking place at intermediate atomic velocities, and a second purely quantum case at low velocities. The classical regime is associated with the formation of incoherent internal states of the atom in the laser region, whereas in the quantum, low velocity regime the laser projects the atom onto a pure internal state that can be controlled by detuning. Spatial Rabi oscillations are only suppressed in this low velocity, quantum regime.

Direct experimental evidence of localized few-cycle wave packets with a characteristic spatiotemporal X shape is reported. These wave packets are also referred to as X pulses. For their generation, a Ti:sapphire laser beam was transformed into single and array-shaped Bessel-like beams by using broadband refractive and reflective thin-film axicons. Extremely small conical angles allow a reduction of the spatially induced group velocity dispersion and therefore enhance the contrast. Complete first and second order spatiotemporal autocorrelation maps of sub-10-fs localized wave packets are measured and are compared with numerical simulations. High-power X pulses open further prospects for advanced nonlinear optical experiments like the formation of spatiotemporal solitons.

We propose an implementation of holonomic (geometrical) quantum gates by means of semiconductor nanostructures. Our quantum hardware consists of semiconductor macroatoms driven by sequences of ultrafast laser pulses (all optical control). Our logical bits are Coulomb-correlated electron-hole pairs (excitons) in a four-level scheme selectively addressed by laser pulses with different polarization. A universal set of single- and two-qubit gates is generated by adiabatic change of the Rabi frequencies of the lasers and by exploiting the dipole coupling between excitons

A technique based on the total-angular-momentum representation the smooth exterior complex-scaling procedure, and the three-dimensional finite-element method, is applied to calculations of S, P, and D resonant, so-called doubly excited, states of the helium atom. The resonances are calculated with an accuracy better than ${10}^{-5}$ a.u. The applicability of an extrapolation procedure to complex energies is analyzed.

In a magnetic field, the wave number of a pump laser light polarized along the field was fixed to the isolated ${\mathrm{Cs}}_{2}D{}^1{\Sigma }_u^{+}(v=46,J=54)\leftarrow X{}^1{\Sigma }_g^{+}(v=0,J=55)$ line, and the excitation spectrum of the dissociated $\mathrm{Cs}\left(6p{}^2{P}_{3/2}\right)$ atoms was measured by scanning the wave number of a probe laser light polarized perpendicular to the field. The population of each sublevel $6p{}^2{P}_{{3/2,m}_j}$ of the dissociated atoms was determined from the line intensities in the $m_j$-resolved excitation spectrum. The unequal population between the $6p{}^2{P}_{3/2,+|m_j|}$ and $6p{}^2{P}_{3/2,-|m_j|}$ levels (atomic orientation) was observed and it was enhanced as the magnetic-field strength was increased. The atomic orientation is shown to be induced by the interference between the indirect predissociation, which occurs by a combination of the spin-orbit coupling of the $D{}^1{\Sigma }_u^{+}$ state with the ${\mathrm{}\left(2\right)\mathrm{}}^3{\Pi }_{0u}$ state and the L-uncoupling and Zeeman interactions between the ${\mathrm{}\left(2\right)\mathrm{}}^3{\Pi }_{0u}$ and dissociative ${\mathrm{}\left(2\right)\mathrm{}}^3{\Sigma }_u^{+}$ states, and the dissociation following a direct excitation to the ${\mathrm{}\left(2\right)\mathrm{}}^3{\Sigma }_u^{+}$ state, which is allowed by spin-orbit coupling of the ${\mathrm{}\left(2\right)\mathrm{}}^3{\Sigma }_u^{+}$ state with the $B{}^1{\Pi }_u$ state. It is demonstrated that the atomic orientation is produced by the photodissociation in the presence of an external magnetic field even when all degenerated molecular $M=J,\dots ,0,\dots ,-J$ sublevels are excited by a light polarized linearly along the field.

All possible conformers of the cubic water octamer have been ascertained. Ab initio optimizations were performed on each of 99 structures to determine how many unique structures exist. An analysis of the complete set of structures revealed that there are 14 unique conformers. These 14 conformers were reoptimized with four larger basis sets. The optimized energies were used to establish relative stabilities and an order of stability. Several structural properties of each conformer were then examined. From these data, a correlation was found between the hydrogen bonding characteristics and the relative stabilities of the cubic water octamers.

Energetic, highly charged oxygen ions ${\mathrm{O}}^{q+}$ $(q<~6),$ are copiously produced upon laser field-induced disassembly of highly charged water clusters, $({\mathrm{H}}_2\mathrm{O}{)}_n$ and $({\mathrm{D}}_2\mathrm{O}{)}_n,$ $n\sim 60,$ that are formed by seeding high-pressure helium or argon with water vapor. ${\mathrm{Ar}}_n$ clusters $\left(n\sim 40000\right)$ formed under similar experimental conditions are found to undergo disassembly in the Coulomb explosion regime, with the energies of ${\mathrm{Ar}}^{q+}$ ions showing a $q^2$ dependence. Water clusters, which are argued to be considerably smaller in size, should also disassemble in the same regime, but the energies of fragment ${\mathrm{O}}^{q+}$ ions are found to depend linearly on q which, according to prevailing wisdom, ought to be a signature of hydrodynamic expansion that is expected of much larger clusters. The implication of these observations on our understanding of the two cluster explosion regimes, Coulomb explosion and hydrodynamic expansion, is discussed. Our results indicate that charge state dependences of ion energy do not constitute an unambiguous experimental signature of cluster explosion regime.

A systematic study of closed classical orbits of the hydrogen atom in crossed electric and magnetic fields is presented. We develop a local bifurcation theory for closed orbits, which is analogous to the well-known bifurcation theory for periodic orbits and allows identifying the generic closed-orbit bifurcations of codimension 1. Several bifurcation scenarios are described in detail. They are shown to have as their constituents the generic codimension-1 bifurcations, which combine into a rich variety of complicated scenarios. We propose heuristic criteria for a classification of closed orbits that can serve to systematize the complex set of orbits.

The S-matrix theory formulation of closed-orbit theory recently proposed by Granger and Greene is extended to atoms in crossed electric and magnetic fields. We present a semiclassical quantization of the hydrogen atom in crossed fields, which succeeds in resolving individual lines in the spectrum, but is restricted to the strongest lines of each n manifold. By means of a detailed semiclassical analysis of the quantum spectrum, we demonstrate that it is the abundance of bifurcations of closed orbits that precludes the resolution of finer details. They necessitate the inclusion of uniform semiclassical approximations into the quantization process. Uniform approximations for the generic types of closed-orbit bifurcations are derived, and a general method for including them in a high-resolution semiclassical quantization is devised.

The paper analyzes the interaction of an atomic system with a quantum damped harmonic oscillator. Such an oscillator is the building block in the recently proposed models of bulk dielectrics and may also serve as a simple model of a small dielectric body. Dispersion and losses are taken into account by assuming the oscillator to be coupled to a zero-temperature reservoir consisting of an infinite system of other harmonic oscillators. The Green’s function of the atomic system is calculated perturbatively when coupled to the bath. The self-energy of the atomic electron is obtained by the partial resummation of perturbation diagrams, and thus energy-level shifts of both ground and excited states of the atom due to the presence of the oscillator are determined. Corrections to the decay rates are also obtained and analyzed as functions of the distance of the atom from the oscillator, and of the coupling of the oscillator to the reservoir.

We analyze the problem of sending classical information through qubit channels where successive uses of the channel are correlated. This work extends the analysis of Macchiavello and Palma to the case of a non-Pauli channel—the amplitude damping channel. Using the channel description outlined by Daffer et al., we derive the correlated amplitude damping channel. We obtain a result similar to that obtained by Macchiavello and Palma, that is, under certain conditions on the degree of channel memory, the use of entangled input signals may enhance the information transmission compared to the use of product input signals.

We evaluate the triply differential cross section (TDCS) for photo-double-ionization of helium. We use a final continuum wave function which correlates the motion of the three particles, through an expansion in products of two-body Coulomb functions. This function satisfies a set of appropriate physical conditions in the coalescence points, in addition to the correct asymptotic behavior condition. We analyze the effect of this correlation in the TDCS and compare our results with experimental data.