We present an implementation of a robust quantum random number generator based on the quantum fluctuations of the collective spin of an alkali-metal vapor. The achieved bit rate is limited by the spin relaxation rate of the alkali-metal atoms $1/T_2$ to about 1 kbit/s. However, the same physical scheme, which is impervious to limitations posed by single-photon detectors used in current implementations and rests solely on threshold detection, can be extended to solid state systems with a bit rate higher than 1 Gbit/s.

In this work we present results of integral (ICS) and differential (DCS) cross sections for positron-${\text{CO}}_2$ scattering at low incident energies. Our ICS shows a significant improvement toward the experimental data, especially below 2 eV, and all the way up to the positronium formation threshold (7.8 eV), in comparison to our previous calculations [S. d’A. Sanchez, F. Arretche, M. T. do N. Varella, and M. A. P. Lima, Phys. Scr. T110, 276 (2004)]. Our calculated DCSs show a better resemblance in shape with the quasielastic experimental points of the Detroit group [D. A. Przybyla, W. Addo-Asah, W. E. Kaupilla, C. K. Kwan, and T. S. Stein, Phys. Rev. A 60, 359 (1999)], but the agreement is still not fully satisfactory, indicating a need for further theoretical and experimental investigation.

The deep-rooted concept of “realism”—a viewpoint according to which an external reality exists independent of observation, may have been shaken to some extent by a recent surprising advance in experimental test of a broad class of nonlocal hidden-variable (NLHV) theories [S. Gröblacher, T. Paterek, R. Kaltenbaek, Č. Brukner, M. Żukowski, M. Aspelmeyer, and A. Zeilinger, Nature (London) 446, 871 (2007)], though there exists a detection loophole. In this paper a feasible propose for a loophole-free test of non-local realism is discussed based on the independent readout of two electron spins of donors ${}^{31}\text{P}$ in semiconductors in different cavities entangled through a 4.4 dB bright squeezed coherent light and a postselection based on homodyne measurement. The electron spins are read out by using optically induced spin to charge transduction detected by radio-frequency single electron transistor.

Belief propagation—a powerful heuristic method to solve inference problems involving a large number of random variables—was recently generalized to quantum theory. Like its classical counterpart, this algorithm is exact on trees when the appropriate independence conditions are met and is expected to provide reliable approximations when operated on loopy graphs. In this paper, we benchmark the performances of loopy quantum belief propagation (QBP) in the context of finite-temperature quantum many-body physics. Our results indicate that QBP provides reliable estimates of the high-temperature correlation function when the typical loop size in the graph is large. As such, it is suitable, e.g., for the study of quantum spin glasses on Bethe lattices and the decoding of sparse quantum error correction codes.

We present a modified nanoplasma model to describe the interaction of intense laser pulses with rare-gas clusters of nanometric size. Compared to previous models, the model relies on a better description of the collisional processes and allows one to obtain highly charged ion populations and associated x-ray spectra in accordance with experimental findings. We further study the x-ray emission dependence on pulse duration and cluster size and show that these parameters can be optimized to maximize $\text{He}\alpha$ production.

In this work we study ultracold fermions confined in a two-dimensional optical lattice and we explore the Mott-insulator transition with the Fermi-Hubbard model. On the basis of a mean-field approach, we study the phase diagrams in the presence of a harmonic trapping potential. Local Mott-insulator phases are shown to be generally situated in the center of the trap and correspond to a vanishing variance of the local density. We then study the effects induced by rotation on the Mott-insulator phase transition. In particular, we show that the phase boundary reproduces the edge of the Hofstadter butterfly.

Quantum phases of ultracold bosons with repulsive interactions in lattices in the presence of quenched disorder are investigated. The disorder is assumed to be caused by the interaction of the bosons with impurity atoms having a large effective mass. The system is described by the Bose-Hubbard Hamiltonian with random on-site energies which have a discrete binary probability distribution. The phase diagram at zero temperature is calculated using several methods like a strong-coupling expansion, an exact numerical diagonalization, and a Bose-Fermi mapping valid in the hard-core limit. It is shown that the Mott-insulator phase exists for any strength of disorder in contrast to the case of continuous probability distribution. We find that the compressibility of the Bose glass phase varies in a wide range and can be extremely low. Furthermore, we evaluate experimentally accessible quantities like the momentum distribution, the static and dynamic structure factors, and the density of excited states. The influence of finite temperature is discussed as well.

We have measured the relative intensity of the photoemission channel leading to the ${\text{Ne}}^{+}\left[2p^2\right]\left({}^{1}D\right)3p{}^{2}P$ final ionic state as a function of excitation photon energy, where square brackets indicate hole states, across the $\left[1s2p\right]\left({}^{3}P\right)3p^2{}^{1}P$ double-excitation resonance. Resonant enhancement is observed in the intensity’s variation curve, we find that the enhancement is definitely seen at the double-excitation resonance, i.e., 902.4 eV. The result confirms the spectator-participator–type resonant Auger transition associates with the $\left[1s2p\right]\left({}^{3}P\right)3p^2$ doubly excited state.

We analytically and numerically study the effects of pulsed control on the decoherence of a qubit coupled to a quantum spin bath. When the environment is critical, decoherence is faster and we show that the control is relatively more effective. Two coupling models are investigated, namely, a qubit coupled to a bath via a single link and a spin-star model, yielding results that are similar and consistent.

We consider a sub-Poissonian single-mode laser with external synchronization and analyze its applicability to the problems of quantum information. Using Heisenberg-Langevin theory, we calculate the quadrature variances of the field emitted by this laser. It is shown that such systems can demonstrate strong quadrature squeezing. Taking into account that the emitted field is temporally multimode, the application of such sources to multichannel quantum teleportation and dense coding protocols is discussed.

How can we quantify the entanglement in a quantum state, if only the expectation value of a single observable is given? This question is of great interest for the analysis of entanglement in experiments, since in many multiparticle experiments the state is not completely known. We present several results concerning this problem by considering the estimation of entanglement measures via Legendre transforms. First, we present a simple algorithm for the estimation of the concurrence and extensions thereof. Second, we derive an analytical approach to estimate the geometric measure of entanglement, if the diagonal elements of the quantum state in a certain basis are known. Finally, we compare our bounds with exact values and other estimation methods for entanglement measures.

We have applied the relativistic Fock-space coupled-cluster theory to calculate $K\alpha$ transition properties of F-like ions for a number of elements throughout the periodic table. A state-specific form of the coupled-cluster method for electron detachment has been used to calculate the $K\alpha$ energies and line strengths. The computed energies are used to determine the transition probabilities for more than 30 elements to study the $Z$ dependencies for $Z=10–79$. This application of a variant of coupled-cluster theories to study the $K\alpha$ transitions should be the most accurate determination of these transition probabilities to date. The most important finding in this paper is the crossover of $K{\alpha }_1$ and $K{\alpha }_2$ at high $Z$. In general, for lighter ions, $K{\alpha }_2$ is higher in magnitude than $K{\alpha }_1$, but at $Z=41–42$ they are switched. In addition, the results demonstrate the variation with $Z$ of the $K{\alpha }_2$ $A$ values along the Periodic Table. The astrophysical significance in the analysis of $K\alpha$ lines, important in x-ray astronomy, is pointed out.

Absolute cross sections for the ionization and fragmentation of water vapor molecules during collisions with 0.5 to 2.0 MeV ${\text{He}}^{+}$ ions, resulting from pure ionization, projectile electron loss, and projectile electron capture processes, have been measured. The masses and charge states of the recoil ions have been detected by time-of-flight spectroscopy in coincidence with the emergent charge states of the projectile. The projectile energy range includes the Bragg peak, where the energy deposition rate by the projectile reaches its maximum. It was observed that the fragmentation patterns depend not only on the projectile characteristics, such as velocity and effective charge state, but also on the particular collision channel involved.

The resonant Auger effect of atomic neon exposed to high-intensity x-ray radiation in resonance with the $1s\to 3p$ transition is discussed. High intensity here means that the x-ray peak intensity is sufficient $\left(\sim {10}^{18}\text{W}/{\text{cm}}^2\right)$ to induce Rabi oscillations between the neon ground state and the $1s^{-1}3p$ $\left({}^{1}P\right)$ state within the relaxation lifetime of the inner-shell vacancy. For the numerical analysis presented, an effective two-level model, including a description of the resonant Auger decay process, is employed. Both coherent and chaotic x-ray pulses are treated. The latter are used to simulate radiation from x-ray free-electron lasers based on the principle of self-amplified spontaneous emission. Observing x-ray-driven atomic population dynamics in the time domain is challenging for chaotic pulse ensembles. A more practical option for experiments using x-ray free-electron lasers is to measure the line profiles in the kinetic energy distribution of the resonant Auger electron. This provides information on both atomic population dynamics and x-ray pulse properties.

We report on the dissociative ionization of a large polyatomic molecule induced by quantum control of tunneling ionization. The directionally asymmetric tunneling ionization induced by phase-controlled two-color $(\omega +2\omega )$ intense $(1.0\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2)$ laser fields with 130-fs pulses led to selective ionization of oriented molecules, which reflect the geometric nature of the highest occupied molecular orbital. We conclude that in the case of 1-iodohexane the molecular Ammosov-Delone-Krainov model for tunneling ionization is qualitatively valid even for the large polyatomic molecule, and that the nonadiabatic multielectron dynamics, which lead to fragmentation, play a much more important role in molecular cations than in neutral molecules.

We study the ground-state properties of a one-dimensional Bose gas with $N$-body attractive contact interactions. By using the explicit form of the bright soliton solution of a generalized nonlinear Schrödinger equation, we compute the chemical potential and the ground-state energy. For $N=3$, a localized soliton wave function exists only for a critical value of the interaction strength: in this case the ground state has an infinite degeneracy that can be parametrized by the chemical potential. The stabilization of the bright soliton solution by an external harmonic trap is also discussed, and a comparison with the effect of $N$-body attractive contact interactions in higher dimensions is presented.

We demonstrate generation of hyperentangled (time-bin and polarization) photon pairs via four-wave mixing in a microstructure-fiber Sagnac interferometer. The two-photon interference visibility in the time-bin (polarization) degree of freedom is $88\%\pm 2\%$ $(84\%\pm 1\%)$ without subtraction of accidental coincidences, and Bell’s inequality is violated by 27 standard deviations at a $1\mathrm{kHz}$ coincidence rate.

We numerically study both linear and nonlinear surface modes in semi-infinite chirped two-dimensional photonic lattices in the framework of an effective discrete nonlinear model. We demonstrate that the lattice chirp can dramatically change the conditions for the mode localization near the surface even in the linear limit, and we find surface modes, in linear lattices, and the families of discrete surface solitons, in nonlinear lattices. In a sharp contrast to one-dimensional discrete surface solitons, we demonstrate that the mode threshold power in two-dimensional lattices is lowered by the action of both the surface and lattice chirp. By manipulating the lattice chirp, we can control the mode position and its localization.

We investigate numerically the recently proposed technique of pulse self-compression by filamentation in noble gases. We show that propagation of a 30 fs infrared pulse containing a few mJ of energy leads to a few-cycle pulse in xenon, krypton, argon, and neon. We describe the different mechanisms and stages of self-compression in the different gases and show that neon, with the highest ionization potential, allows compression to the shortest durations and highest peak intensities. We discuss the process by which an unavoidable frequency modulation of the self-compressed filament simultaneously allows the generation of isolated attosecond (as) pulses via high-order harmonic generation and limits the conversion efficiency of the as pulses.