We present spin-asymmetry parameters (Sherman functions) for elastic electron scattering on zinc and mercury atoms calculated using the relativistic convergent close-coupling method for quasi-two-electron atoms above an inert core. The results for mercury are in excellent agreement with experiment across a wide range of energies. Similarly for zinc, we find excellent agreement between theory and experiment for energies below $9.0$ eV. However, at $11.0$ eV there is a discrepancy between theory and experiment, most likely due to a resonance excitation of a core electron.

We study light-assisted collisions in an ensemble containing a small number ($\sim$3) of cold ${}^{87}$Rb atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated with a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated with light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values $\beta >{10}^{-8}$ cm${}^3$ s${}^{-1}$ and varies rapidly with the trap depth and the parameters of the excitation light.

The recombination step of high-order harmonic generation is investigated with the aim of optimizing the high-energy photon emission yield in this process. The dependence of the recombination probability of a recolliding electron on the parameters of the atomic potential, such as its width and depth and the parameters of the electron wave packet such as its impact parameter, width, and angular momentum, are analyzed. Empirical laws for these parameters are found that ensure optimal recombination and in this way optimal emission of high-energy photons.

Band-specific design of curved light caustics and focusing in optical waveguide arrays is introduced. Going beyond the discrete, tight-binding model, which we examined recently, we show how the exact band structure and the associated diffraction relations of a periodic waveguide lattice can be exploited to phase-engineer caustics with predetermined convex trajectories or to achieve optimum aberration-free focal spots. We numerically demonstrate the formation of convex caustics involving the excitation of Floquet-Bloch modes within the first or the second band and even multiband caustics created by the simultaneous excitation of more than one band. Interference of caustics in abruptly autofocusing or collision scenarios are also examined. The experimental implementation of these ideas should be straightforward since the required input conditions involve phase-only modulation of otherwise simple optical wavefronts. By direct extension to more complex periodic lattices, possibilities open up for band-specific curving and focusing of light inside two-dimensional or even three-dimensional photonic crystals.

Detailed single- and many-particle calculations are carried out for the acceleration of protons employing linearly polarized plane-wave and tightly focused chirped laser pulses of several ten to several hundred femtosecond durations, petawatt peak powers, and relativistic peak intensities. Analytic and numerical methods of calculation are used in the single-particle cases (in vacuum), and particle-in-cell (PIC) simulations (underdense plasma) are employed in the many-particle investigations, without and with electromagnetic particle-particle interactions, respectively. The investigations cover a wide range of cases corresponding to upchirped as well as downchirped pulses.

Results from theoretical investigations are presented which show that protons can be accelerated from rest to a few hundred MeV by a 1-PW chirped radially polarized laser pulse of several hundred femtosecond duration and focused to a waist radius comparable to the radiation wavelength. Single-particle calculations are supported by many-particle and particle-in-cell simulations. Compared with laser acceleration by a similar linearly polarized pulse, the gained energies are less, but have better beam quality. For a suitable initial phase, a particle bunch gets accelerated by the axial component $E_z$ of the laser pulse and, initially focused by the transverse electric field component $E_r$. Beam diffraction finally sets in due to the particle-particle Coulomb repulsion, after interaction with the pulse ceases to exist.

We present an integrable generalization of the Maxwell-Bloch equations describing electromagnetic-field interaction with a two-level system implemented in a dispersive host medium. Two physical onsets of the model are presented. The technique of the inverse scattering transform for the two sets of physical parameters is used to find soliton solutions. It is found that the model possesses both singular and nonsingular soliton solutions. The application of the results for a lossless nonlinear dispersion management of solitons is discussed.

In this work we propose and analyze a scheme where the full spatiotemporal correlation of twin photons or beams generated by parametric down-conversion is detected by using its inverse process, i.e., sum frequency generation. Our main result is that, by imposing independently a temporal delay $\Delta t$ and a transverse spatial shift $\Delta x$ between two twin components of PDC light, the up-converted light intensity provides information on the correlation of the PDC light in the full spatiotemporal domain and should enable the reconstruction of the peculiar X-shaped structure of the correlation predicted previously [A. Gatti et al. Phys. Rev. Lett. 102, 223601 (2009); L. Caspani et al. Phys. Rev. A 81, 033808 (2010); E. Brambilla et al. Phys. Rev. A 82, 013835 (2010)]. Through both a semianalytical and a numerical modeling of the proposed optical system, we analyze the feasibility of the experiment and identify the best conditions to implement it. In particular, the tolerance of the phase-sensitive measurement against the presence of dispersive elements, imperfect imaging conditions, and possible misalignments of the two crystals is evaluated.

We implement a direct detection scheme based on hybrid photodetectors to experimentally investigate high-order correlations for detected photons by means of experimentally accessible quantities. The scheme is self-consistent, allowing the estimation of all the involved parameters (quantum efficiency, number of modes, and average energy). In particular, we show how high-order correlation functions can be exploited to fully characterize bipartite multimode states in regimes realistic for quantum technology, that is, in the mesoscopic photon-number domain and with limited quantum efficiency. Furthermore, we introduce a nonclassicality criterion based on a simple linear combination of high-order correlation functions.

Multiply charged anions possess a repulsive Coulomb barrier (RCB) against electron emission, thus allowing for long-lived metastable species with negative electron binding energies. For the prototypical multianion, bisdisulizole tetra-anion, we demonstrate that electronically excited states supported by the RCB can undergo resonant tunneling. The dynamics of this process was investigated by one-photon photoelectron imaging and femtosecond pump-probe photoelectron spectroscopy and confirmed by theoretical calculations. Efficient resonant tunneling emission of electrons from the excited states of multianions may be common for systems with sufficiently large RCB. This may provide new opportunities to study electron emission dynamics in complex systems.

We solve the open question of the existence of four-qubit entangled symmetric states with positive partial transpositions (PPT states). We reach this goal with two different approaches. First, we propose a half-analytical–half-numerical method that allows us to construct multipartite PPT entangled symmetric states (PPTESSs) from the qubit-qudit PPT entangled states. Second, we adapt the algorithm allowing us to search for extremal elements in the convex set of bipartite PPT states [Phys. Rev. A 76, 034304 (2007)] to the multipartite scenario. With its aid we search for extremal four-qubit PPTESSs and show that generically they have ranks $(5,7,8)$. Finally, we provide an exhaustive characterization of these states with respect to their separability properties.

A universally valid Heisenberg uncertainty relation is proposed by combining the universally valid error-disturbance uncertainty relation of Ozawa with the relation of Robertson. This form of the uncertainty relation, which is defined with the same mathematical rigor as the relations of Kennard and Robertson, incorporates both the intrinsic quantum fluctuations and measurement effects.

Repeated closed-loop control operations acting as piecewise-constant Liouville superoperators conditioned on the outcomes of regularly performed measurements may effectively be described by a fixed-point iteration for the density matrix. Even when all Liouville superoperators point to the completely mixed state, feedback of the measurement result may lead to a pure state, which can be interpreted as selective dampening of undesired states. Using a microscopic model, we exemplify this for a single qubit, which can be purified in an arbitrary single-qubit state by tuning the measurement direction and two qubits that may be purified towards a Bell state by applying a special continuous two-local measurement. The method does not require precise knowledge of decoherence channels and works for large reservoir temperatures provided measurement, processing, and control can be implemented in a continuous fashion.

Genuine-multipartite-entanglement (GME) concurrence is a measure of genuine multipartite entanglement that generalizes the well-known notion of concurrence. We define an observable for GME concurrence. The observable permits us to avoid full state tomography and leads to different analytic lower bounds. By means of explicit examples we show that entanglement criteria based on the bounds have a better performance with respect to the known methods.

We propose a simpler and more efficient scheme for the implementation of the multivalued Grover's quantum search. The multivalued search generalizes the original Grover's search by replacing qubits with qudits—quantum systems of $d$ discrete states. The qudit database is exponentially larger than the qubit database and thus it requires fewer particles to control. The Hadamard gate, which is the key elementary gate in the qubit implementation of Grover's search, is replaced by a $d$-dimensional (complex-valued) unitary matrix $\mathbf{F}$, the only condition for which is to have a column of equal moduli elements irrespective of their phases; it can be realized through any physical interaction, which achieves an equal-weight superposition state. An example of such a transformation is the $d$-dimensional discrete Fourier transform, used in earlier proposals; however, its construction is much more costly than that of the far simpler matrix $\mathbf{F}$. We present examples of how such a transform $\mathbf{F}$ can be constructed in realistic qudit systems in a single interaction step.

A laser absorption technique, based upon a pair of offset-frequency-locked extended-cavity diode lasers, was employed to perform line-shape measurements for ${\mathrm{H}}_2{}^{18}\mathrm{O}$ vibration-rotation transitions at 1.38 $\mu$m, with an extremely high spectral fidelity. This important feature was mostly ensured by the highly accurate, absolute, and repeatable frequency axis characterizing each acquired spectrum. We present the outcomes of a specific study of the speed dependence of collision parameters influencing the shape of the ${\mathrm{H}}_2{}^{18}\mathrm{O}$ absorption lines. More particularly, the performance of the quadratic approximation, either used alone or considered in conjunction with the Dicke narrowing effect, was investigated and compared to that of the confluent hypergeometric dependence. Hence, experimental evidences of the partial failure of the quadratic approximation are given and discussed. Finally, we investigate the possible influence of the choice of the speed-dependent line-shape model on the retrieved collisional parameters.

We present a theoretical study of H${}_2$ ionization by a pump-probe scheme consisting on an attosecond pulse train (APT) and a near-infrared (IR) pulse. We focus on the autoionization dynamics of the first series of resonant states of the molecule, the Q${}_1$ doubly excited states. The APT central frequency is tuned to populate the ${}^1{\Sigma }_u^{+}$ resonant states. The trace of autoionization is clearly visible in the two-dimensional (2D) proton-electron coincidence spectra and in the proton kinetic energy spectra. The dynamics of the autoionization process is clearly visible in the movie obtained by plotting the 2D spectrum as a function of the time delay between the APT and IR pulses. An analysis of the final symmetries ${\Sigma }_g$ and ${\Sigma }_u$ allows us to track the origin of the different structures.

When atoms and molecules are irradiated by an x-ray free-electron laser (XFEL), they are highly ionized via a sequence of one-photon ionization and relaxation processes. To describe the ionization dynamics during XFEL pulses, a rate equation model has been employed. Even though this model is straightforward for the case of light atoms, it generates a huge number of coupled rate equations for heavy atoms like xenon, which are not trivial to solve directly. Here, we employ the Monte Carlo method to address this problem and we investigate ionization dynamics of xenon atoms induced by XFEL pulses at a photon energy of 4500 eV. Charge-state distributions, photoelectron and Auger electron spectra, and fluorescence spectra are presented for x-ray fluences of up to ${10}^{13}$ photons/$\mu$m${}^2$. With the photon energy of 4500 eV, xenon atoms can be ionized up to $+$44 through multiphoton absorption characterized by sequential one-photon single-electron interactions.

In dilute mixtures of two atomic gases, interactions between two minority atoms acquire a contribution due to interaction with the majority component. Using thermodynamic arguments, we derive expressions for this induced interaction for both fermions and bosons for arbitrary strength of the interaction between the two components. Implications of the work for the theory of dilute solutions of ${}^3$He in liquid ${}^4$He are discussed.