Photoionization cross sections calculated up to the 20th single ionization threshold of triplet $P$ states of planar helium exhibit fluctuations. These are mainly due to a dominant series of resonances which can be associated with an approximate quantum number $F=N-K$ in accordance with three-dimensional full calculations and experimental observations. As the energy increases the dominant role of a single series as sole contributor is apparently lost as new series start to contribute significantly to the cross sections. This would result in an earlier onset of Ericson fluctuations as in the picture of a single dominant series, where the onset is expected around $I_{34}$.

We report integral cross sections for electron-impact excitation of the $4{}^{2}P$ state in copper for incident electrons with energies in the range from threshold to 100 eV. Measurements, based on an optical excitation function procedure, are compared with coupled-channel and coupled-channel-optical method calculations that we have also performed as a part of this study. Agreement between our measurements and theory was generally only modest. The present measurements are also found to be in quite good accord with the early near-threshold integral cross sections of Flynn et al. [C. Flynn, Z. Wei, and B. Stumpf, Phys. Rev. A 48, 1239 (1993)] and the higher energy measurements from Ismail and Teubner [M. Ismail and P. J. O. Teubner, J. Phys. B 28, 4149 (1995)]. Where possible, comparison of our data is also made with earlier theory.

We present measurements and calculations of the trap loss rate for laser-cooled ${}^{87}\text{R}\text{b}$ atoms confined in either a magneto-optic trap (MOT) or a magnetic quadrupole trap when exposed to a room-temperature background gas of Ar. We study the loss rate as a function of trap depth and find that copious glancing elastic collisions, which occur in the so-called quantum diffractive regime and impart very little energy to the trapped atoms, result in significant differences in the loss rate for the MOT compared to a pure magnetic trap due solely to the difference in potential depth. This finding highlights the importance of knowing the trap depth when attempting to infer the total collision cross section from measurements of trap loss rates. Moreover, this variation in trap loss rate with trap depth can be used to extract information about the differential cross section.

On the basis of the Floquet formalism, a two-dimensional model atom in circularly polarized intense laser fields is analyzed. By solving the close-coupling equations, the pole positions of the scattering matrix are calculated on the complex quasienergy Riemann surface, and the pole trajectories with respect to the variation in the laser intensity are obtained for different laser frequency regimes. The behaviors of the pole trajectories indicate that the tunneling ionization is typically observed in the low-frequency regime, while the stabilization of the atom occurs in the high-frequency regime. The mechanisms of the atomic ionization in these two regimes are discussed from a unified point of view. The transition between the different frequency regimes can be explained by the change in the avoided crossings among the adiabatic potential curves for the radial motion of the electron.

We analyze theoretically Josephson oscillations in a mixture of two Zeeman states of a spinor Bose-Einstein condensate in a double-well potential. We find that in the strongly polarized case, the less populated component exhibits a complex dynamics with an anti-Josephson behavior, i.e., oscillates in phase with the more populated one. In the balanced population case, the Josephson oscillations unveil a dependence with the different spin collision channels. This effect could be used to experimentally measure the distinct scattering lengths entering in the description of a spinor condensate. Our numerical results are in close agreement with an analytical description of the binary mixture using a two-mode model.

We have experimentally studied the propagation of two optical fields in a dense rubidium (Rb) gas in the case when an additional microwave field is coupled to the hyperfine levels of Rb atoms. The Rb energy levels form a close-$\Lambda$ three-level system coupled to the optical fields and the microwave field. It has been found that the maximum transmission of the probe field depends on the relative phase between the optical and the microwave fields. We have observed both constructive and destructive interferences in electromagnetically induced transparency. A simple theoretical model and a numerical simulation have been developed to explain the observed experimental results.

Localization properties of particles in one-dimensional incommensurate lattices without interaction are investigated with models beyond the tight-binding Aubry-André (AA) model. Based on a tight-binding $t_1\text{−}{t}_2$ model with finite next-nearest-neighbor hopping $t_2$, we find the localization properties qualitatively different from those of the AA model, signaled by the appearance of mobility edges. We then further go beyond the tight-binding assumption and directly study the system based on the more fundamental single-particle Schrödinger equation. With this approach, we also observe the presence of mobility edges and localization properties dependent on incommensuration.

The rovibrational states of the metastable anionic hydrogen molecule $\text{D}_2{}^{-}$ were studied in photofragmentation experiments at 532 nm employing a fast (keV) ion beam. By a three-dimensional fragment momentum imaging technique, the photodetachment and the dissociation of the system into neutral atoms were investigated at different times after production of the molecular ions. The present data are sensitive to both the rotational states and the vibrational wave functions of the anions and suggest that the level of rotational excitation, while still exceptionally high, is somewhat lower than estimated in recent theoretical predictions.

Under certain conditions, the quantum $\delta$-kicked harmonic oscillator displays quantum resonances. We consider an atom-optical realization of the $\delta$-kicked harmonic oscillator and present a theoretical discussion of the quantum resonances that could be observed in such a system. Having outlined our model of the physical system we derive the values at which quantum resonances occur and relate these to potential experimental parameters. We discuss the observable effects of the quantum resonances using the results of numerical simulations. We develop a physical explanation for the quantum resonances based on symmetries shared between the classical phase space and the quantum-mechanical time evolution operator. We explore the evolution of coherent states in the system by reformulating the dynamics in terms of a mapping over an infinite two-dimensional set of coefficients from which we derive an analytic expression for the evolution of a coherent state at quantum resonance.

We propose a possible way to realize nuclear fusion, via an approach which involves aligned molecules. We compare results from quantum and classical simulations of two nuclei originating from an HT molecule moving under a superintense (field intensities approaching ${10}^{23}\text{W}/{\text{cm}}^2$) and ultrashort (few-femtosecond, single-cycle) laser pulse. Classical simulations show that when the field is strong enough, it may make the nuclei collide with high kinetic energy needed to overcome the Coulomb barrier, therefore leading to fusion. A quantum-mechanical calculation solving the Schrödinger equation displays analogous behavior and reveals additional information about the wave packet evolution around the collision point.

We study the interactions between two atomic species in a binary Bose-Einstein condensate to revisit the conditions for miscibility, oscillatory dynamics between the species, steady-state solutions, and their stability. By employing a variational approach for a quasi-one-dimensional, two-atomic species condensate, we obtain equations of motion for the parameters of each species: amplitude, width, position, and phase. A further simplification leads to a reduction of the dynamics into a simple classical Newtonian system where components oscillate in an effective potential with a frequency that depends on the harmonic trap strength and the interspecies coupling parameter. We develop explicit conditions for miscibility that can be interpreted as a phase diagram that depends on the harmonic trap’s strength and the interspecies coupling parameter. We numerically illustrate the bifurcation scenario whereby nontopological, phase-separated states of increasing complexity emerge out of a symmetric state as the interspecies coupling is increased. The symmetry-breaking dynamical evolution of some of these states is numerically monitored and the associated asymmetric states are also explored.

We explore the structure and dynamics of dipolar Bose-Einstein condensates (DBECs) near their threshold for instability. Near this threshold a DBEC may exhibit nontrivial biconcave density distributions, which are associated with instability against collapse into “angular roton” modes. Here we discuss experimental signatures of these features. In the first, we infer local collapse of the DBEC from the experimental stability diagram. In the second, we explore the dynamics of collapse and find that a nontrivial angular distribution is a signature of the DBEC possessing a biconcave structure.

We relate short-range correlations in ultracold-atom Fermi gases to the entropy of the system over the entire temperature, $T$, vs coupling strength, $-1/k_{F}a$, plane. In the low-temperature limit the entropy is dominated by phonon excitations and the correlations increase as $T^4$. In the Bose-Einstein condensate (BEC) limit, we calculate a boson model within the Bogoliubov approximation to show explicitly how phonons enhance the fermion correlations. In the high-temperature limit, we show from the virial expansion that the correlations decrease as $1/T$. The correlations therefore reach a maximum at a finite-temperature. We infer the general structure of the isentropes of the Fermi gas in the $\left(T,-1/k_{F}a\right)$ plane, and the temperature dependence of the correlations in the unitary, BEC, and BCS limits. Our results compare well with measurements of the correlations via photoassociation experiments at higher temperatures.

The collective atomic recoil lasing is studied for an ultracold and collisionless atomic gas in a partially coherent pump with a colored noise. Compared to white noise, correlations in colored noise are found to be able to greatly enhance or suppress the growth rate above or below a critical detuning. Effects on cooperative scattering of light for noise correlation time, noise intensity, and pump-probe detuning are discussed. This result is consistent with our simulation and linear analysis about the evolution equations in the regions of instability.

Using a rigorous quantum model a comprehensive study of physical properties of entangled photon pairs generated in spontaneous parametric downconversion in chirped periodically poled crystals is presented. Spectral, temporal, as well as spatial characteristics of photon pairs are analyzed. Spectral bandwidths, photon-pair flux, and entanglement area can be effectively controlled by chirping. Quantification of entanglement between photons in a pair is given. Splitting of entanglement area in the transverse plane accompanied by spectral splitting has been revealed. Using the model temperature dependencies of the experimental intensity profiles reported in literature have been explained.

Stimulated Raman scattering induced by a femtosecond pulse formed by two pulses, polarized orthogonally with respect to each other is reported. High-frequency and low-frequency components generated at the center of the pulse through self-phase modulation temporally overlap. The two components induce a coherent molecular rotation through stimulated Raman scattering. The amplitude of this rotation in this way is larger than that of traditional transient stimulated Raman scattering in which linearly, elliptically, and circularly polarized pulses are used.

We construct a generalized phase-space representation (GPSR) based on the idea of Einstein-Podolsky-Rosen quantum entanglement, i.e., we generalize the Torres-Vega-Frederick phase-space representation to the entangled case, which is characteristic of the features when two particles’ relative coordinate, total momentum operators, and their conjugative variables, respectively, operate on the GPSR. This representation is complete and nonorthogonal. The Weyl-ordered form of the density operator of GPSR is derived, and its identification with the generalized Husimi operator is recognized, which clearly exhibit its statistical behavior. The minimum uncertainty relation obeyed by the GPSR is also demonstrated.

Fundamental low-energy limits to the accuracy of quantum clock and stopwatch models in which the clock hand motion is activated by the presence of a particle in a region of space have been studied in the past, but their relevance for actual atomic clocks had not been assessed. In this work we address the effect of slow atomic quantum motion on Rabi and Ramsey resonance fringe patterns, as a perturbation of the results based on classical atomic motion. We find the dependence of the fractional error of the corresponding atomic clocks on the atomic velocity and interaction parameters.

Quantum detection processes in quantum field theory (QFT) must play a key role in the description of quantum-field correlations, such as the appearance of entanglement, and of causal effects. We consider the detection in the case of a simple QFT model with a suitable interaction to exact treatment, consisting of a quantum scalar field coupled linearly to a classical scalar source. We then evaluate the response function to the field quanta of two-level pointlike quantum model detectors, and analyze the effects of the approximation adopted in standard detection theory. We show that the use of the RWA, which characterizes the Glauber detection model, leads in the detector response to nonlocal terms corresponding to an instantaneously spreading of source effects over the whole space. Other detector models, obtained with nonstandard or no application of RWA, give instead local responses to field quanta, apart from source-independent vacuum contribution linked to preexisting correlations of zero-point field.

This paper proposes a scheme for quantum cryptography with a nonmaximally entangled qubit pair. In a two-way configuration similar to quantum super dense coding, a two-bit key can be distributed by a round trip of one particle of the qubit pair and received by an appropriate joint measurement with the other. The security of our scheme is based on the quantum-mechanical impossibility of local unitary transformation between certain nonmaximally entangled states. Although an eavesdropper, Eve, can always identify the quantum state and intercept the key, it is impossible for her to resend the state with a unity probability and the legitimate parties inevitably detect her. Our protocol is also capable to quantum secure direct communication.