The leading-order electron-atom bremsstrahlung is investigated within the rigorous relativistic approach based on the partial-wave representation of the Dirac wave functions in the external atomic field. Approximating the atomic target by an effective local potential, we calculate the double-differential cross section and the polarization correlations in a wide range of the impact energies. Connection between the bremsstrahlung at the hard-photon end point of the spectrum and the continuum-threshold limit of the radiative recombination is studied. A detailed analysis of the screening effect and the energy dependence of the polarization correlations is presented, with the main focus on the high-impact-energy region.

We report on a theoretical investigation of elastic electron scattering from the $(3d^{10}4s){}^{2}S$ ground state and electron-impact excitation of the $(3d^{10}4s){}^{2}S\to (3d^{10}4p){}^{2}P$ resonance transition in copper atoms. The calculations were performed using a semirelativistic and a fully relativistic $B$-spline $R$-matrix (close-coupling) approach. The results for the angle-integrated and angle-differential cross sections are generally in good agreement with those obtained in previous calculations and with numerous experimental data. Where discrepancies with experiment in the differential cross sections remain, the present work generally supports the previous calculations.

The interaction of ion beams with insulators leads to charging-up phenomena, which at present are under investigation in connection with guiding phenomena in nanocapillaries with possible application in nanofocused beams. We studied the charging dynamics of insulating foil targets [Mylar, polypropylene (PP)] irradiated with swift ion beams (C, O, Ag, and Xe at 40, 23, 40, and 30 MeV/u, respectively) via the measurement of the slowing down of fast binary-encounter electrons. Also, sandwich targets (Mylar covered with a thin Au layer on both surfaces) and Mylar with Au on only one surface were used. Fast-electron spectra were measured by the time-of-flight method at the superconducting cyclotron of Laboratori Nazionali del Sud (LNS) Catania. The charge buildup leads to target-material-dependent potentials of the order of 6.0 kV for Mylar and 2.8 kV for PP. The sandwich targets, surprisingly, show the same behavior as the insulating targets, whereas a single Au layer on the electron and ion exit side strongly suppresses the charging phenomenon. The accumulated number of projectiles needed for charging up is inversely proportional to electronic energy loss. Thus, the charging up is directly related to emission of secondary electrons.

We present a joint theoretical and experimental investigation for electric-field effects on ground-state photoionization of Ca. For an electric field with its direction along the $z$ axis, the dominant field-free, doubly excited, odd-parity (i.e., $3\mathit{dnp}$ and/or $3\mathit{dnf}$) resonances of the ${}^{1,3}{L}_{J=1}^o$ (i.e., ${}^{1,3}{P}_{J=1}^o$ and ${}^3{D}_{J=1}^o$) symmetries are coupled with the even-parity (i.e., $3\mathit{dns}$, $3\mathit{dnd}$, and/or $3\mathit{dng}$) resonances of the ${}^{1,3}{L}_{J=0}^e$ (i.e., ${}^1{S}_{J=0}^e$ and ${}^3{P}_{J=0}^e$) and ${}^{1,3}{L}_{J=2}^e$ (i.e., ${}^3{P}_{J=2}^e$, ${}^{1,3}{D}_{J=2}^e$, and ${}^3{F}_{J=2}^e$) symmetries. Using a $B$-spline-based complex-rotation method with spin-dependent interaction, our theoretically calculated spectrum is found to be in good agreement with the observed spectrum from a cross-beam photoionization experiment for field strengths up to 25 kV/cm. We present in detail a number of qualitative features of the field-induced level crossing and avoided crossing in energy between neighboring resonances, their corresponding changes in width, and the resulting variation in resonance structure profiles. A few “hidden” resonances due to strong overlap with more prominant resonances are also identified theoretically.

We investigate the BCS-BEC crossover taking into account an additional three-body interaction, which is essentially the scattering between the Cooper pairs and the newly formed bosons. We show that if the two-body interaction is attractive, the presence of this additional three-body term makes the crossover process a nonreversible one. Starting from a stable Bose-Einstein condensate (BEC) state, crossover to BCS can be achieved; but if the BCS state is the starting point, instead of a stable BEC region, what the system crosses over to is a metastable condensed state.

We examine the effect of the intra- and interspecies scattering lengths on the dynamics of a two-component Bose-Einstein condensate, particularly focusing on the existence and stability of solitonic excitations. For each type of possible soliton pairs, stability ranges are presented in tabulated form. We also compare the numerically established stability of bright-bright, bright-dark, and dark-dark solitons with our analytical prediction and with that of Painlevé analysis of the dynamical equation. We demonstrate that tuning the interspecies scattering length away from the predicted value (keeping the intraspecies coupling fixed) breaks the stability of the soliton pairs.

We report on light shift and broadening in the atomic-motion-induced Ramsey narrowing of dark resonances prepared in alkali-metal vapors contained in wall-coated cells without buffer gas. The atomic-motion-induced Ramsey narrowing is due to the free motion of the polarized atomic spins in and out of the optical interaction region before spin relaxation. As a consequence of this effect, we observe a narrowing of the dark resonance linewidth as well as a reduction of the ground states’ light shift when the volume of the interaction region decreases at constant optical intensity. The results can be intuitively interpreted as a dilution of the intensity effect similar to a pulsed interrogation due to the atomic motion. Finally the influence of this effect on the performance of compact atomic clocks is discussed.

The quantum $N$-body problem is studied in the context of nonrelativistic quantum mechanics with a one-dimensional deformed Heisenberg algebra of the form $[\mathrm{x̂},\mathrm{p̂}]=i(1+\beta {\mathrm{p̂}}^2)$, leading to the existence of a minimal observable length $\sqrt{\beta }$. For a generic pairwise interaction potential, analytical formulas are obtained that allow estimation of the ground-state energy of the $N$-body system by finding the ground-state energy of a corresponding two-body problem. It is first shown that in the harmonic oscillator case, the $\beta$-dependent term grows faster with increasing $N$ than the $\beta$-independent term. Then, it is argued that such a behavior should also be observed with generic potentials and for $D$-dimensional systems. Consequently, quantum $N$-body bound states might be interesting places to look at nontrivial manifestations of a minimal length, since the more particles that are present, the more the system deviates from standard quantum-mechanical predictions.

We present and discuss different protocols for preparing an arbitrary quantum state of a qubit using only a restricted set of measurements, with no unitary operations at all. We show that an arbitrary state can indeed be prepared, provided that the available measurements satisfy certain requirements. Our results shed light on the role that measurement-induced back-action plays in quantum feedback control and the extent to which this back-action can be exploited in quantum-control protocols.

We present quantum electrodynamic (QED) calculations within the picture of bound-state QED for the frequency-dependent Breit interaction between electrons, the vacuum polarization, and the electron self-energy correction starting from the Dirac-Coulomb Hamiltonian for the ionization potentials of the group 1, 2, 11, 12, 13, and 18 elements of the periodic table, and down to the superheavy elements up to nuclear charge $Z=120$. The results for the $s$-block elements are in very good agreement with earlier studies by Labzowsky et al. [Phys. Rev. A 59, 2707 (1999)]. We discuss the influence of the variational versus perturbative treatment of the Breit interaction for valence-space ionization potentials. We argue that the lowest-order QED contributions become as important as the Breit interaction for ionization potentials out of the valence $s$ shell.

Theoretical ground-state configurations of lanthanide ions calculated with the Cowan suite of codes are presented. Theoretical $4d$-$4f$ and $4p$-$4d$ spectra of Pd-like to Rb-like lanthanide ions calculated using the relativistic flexible atomic code are also shown. The effects of configuration interaction are investigated, and the results compare favorably with experiments in which, for increasing nuclear charge, strong emission peaks are observed to move toward shorter wavelength. The application of these strong emitters as extreme ultraviolet radiation sources, a topic of emerging interest, is discussed.

An atom moving in a spatially periodic field experiences a temporally periodic perturbation and undergoes a resonance transition between atomic internal states when the transition frequency is equal to the atomic velocity divided by the field period. We demonstrated that spin nutation was induced by this resonant transition in a polarized rubidium (Rb) atomic beam passing through a magnetic lattice. The lattice was produced by current flowing through an array of parallel wires crossing the beam. This array structure, reminiscent of a multiwire chamber for particle detection, allowed the Rb beam to pass through the lattice at a variety of incident angles. The dephasing of spin nutation was reduced by varying the incident angle.

We analyze in detail the heating of bosonic atoms in an optical lattice due to incoherent scattering of light from the lasers forming the lattice. Because atoms scattered into higher bands do not thermalize on the time scale of typical experiments, this process cannot be described by the total energy increase in the system alone (which is determined by single-particle effects). The heating instead involves an important interplay between the atomic physics of the heating process and the many-body physics of the state. We characterize the effects on many-body states for various system parameters, where we observe important differences in the heating for strongly and weakly interacting regimes, as well as a strong dependence on the sign of the laser detuning from the excited atomic state. We compute heating rates and changes to characteristic correlation functions based on both perturbation-theory calculations and a time-dependent calculation of the dissipative many-body dynamics. The latter is made possible for one-dimensional systems by combining time-dependent density-matrix-renormalization-group methods with quantum trajectory techniques.

We investigate the effects of correlations on the properties of the ground state of the rotating harmonically trapped Bose gas by adding Bogoliubov fluctuations to the mean-field ground state of an $N$-particle single-vortex system. We demonstrate that the fluctuation-induced correlations lower the energy compared to that of the mean-field ground state, that the vortex core is pushed slightly away from the center of the trap, and that an unstable mode with negative energy (for rotations slower than a critical frequency) emerges in the energy spectrum, thus pointing to a better state for slow rotation. We construct mean-field ground states of zero-, one-, and two-vortex states as a function of rotation rate and determine the critical frequencies for transitions between these states, as well as the critical frequency for appearance of a metastable state with an off-center vortex and its image vortex in the evanescent tail of the cloud.

We address the existence and the controlled stability of two-dimensional solitons in modulated Bessel lattices (MBL) induced by a superposition of nondiffracting Bessel beams. We show that variation of the modulation parameter of the lattice and the initial transverse momentum of the soliton significantly modify the behavior of the solitons. We find that, under suitable and well-identified conditions, solitons propagating in the MBL exhibit six regimes of transverse mobility: stationary, oscillatory, rotating, unbounded or escape, transitional, and unstable. These results report propagating solitons that can develop these dynamics of transverse motion.

We fabricate L3 cavities in a silicon nitride photonic crystal, with cavity modes in the visible range. We demonstrate a noninvasive method to characterize the cavities: the sample is illuminated on its edge and the upward-scattering spectrum is measured. We determine the modes’ wavelengths, polarizations, and quality factors. We find modes between 700 and 830 nm with quality factors up to 280, polarized along or perpendicular to the cavity. The tuning of the cavity characteristics by means of its geometric parameters (holes sizes and shift) is discussed. The results are analyzed and compared with numerical simulations.

We propose a quantum algorithm for finding eigenvalues of non-unitary matrices. We show how to construct, through interactions in a quantum system and projective measurements, a non-Hermitian or non-unitary matrix and obtain its eigenvalues and eigenvectors. This proposal combines ideas of frequent measurement, measured quantum Fourier transform, and quantum state tomography. It provides a generalization of the conventional phase estimation algorithm, which is limited to Hermitian or unitary matrices.

A full quantum treatment shows that coupled electronic and nuclear fluxes exhibit a strong sensitivity to a small mass change in a vibrating molecule. This has been exemplified with the existing isotopes of ${\mathrm{H}}_2$${}^{+}$ as well as few fictitious ones. We find that the fluxes undergo a significant change as one goes from one isotope of reduced mass $\mu$ to another. Other well-defined observables are likewise affected. It turns out that as a general rule, the heavier the isotope, the larger the flux, the smaller the dispersion, and the longer the revival period. While we were able to confirm analytically that the time at the first turning point scales as $\sqrt{\mu }$ and that the revival period changes linearly with $\mu$, the mechanism of other observables remains subtle as the result of quantum interference highlighted by the pronounced difference observed on the dispersion pattern.

We study the tunneling decay of a Bose-Einstein condensate from tilted optical lattices within the mean-field approximation. We introduce a method to calculate ground and excited resonance eigenstates of the Gross-Pitaevskii equation, based on a grid relaxation procedure with complex absorbing potentials. This algorithm works efficiently in a wide range of parameters where established methods fail. It allows us to study the effects of the nonlinearity in detail in the regime of resonant tunneling, where the decay rate is enhanced by resonant coupling to excited unstable states.