In order to witness multipartite correlations beyond pairwise entanglement, spin-squeezing parameters are analytically calculated for a spin ensemble in a collective initial state under three different decoherence channels. It is shown that, in analogy to pairwise entanglement, the spin squeezing described by different parameters can suddenly become zero at different vanishing times. This finding shows the general occurrence of sudden vanishing phenomena of quantum correlations in many-body systems, which here is referred to as spin-squeezing sudden death (SSSD). It is shown that the SSSD usually occurs due to decoherence and that SSSD never occurs for some initial states in the amplitude-damping channel. We also analytically obtain the vanishing times of spin squeezing.

The evolution of the entanglement between two oscillators coupled to a common thermal environment is nontrivial. The long time limit has three qualitatively different behaviors (phases) depending on parameters such as the temperature of the bath [J. P. Paz and A. J. Roncaglia, Phys. Rev. Lett. 100, 220401 (2008)]. The phases include cases with nonvanishing long-term entanglement, others with a final disentangled state, and situations displaying an infinite sequence of events of disappearance and revival of entanglement. We describe an experiment to realize these different scenarios in an ion trap. The motional degrees of freedom of two ions are used to simulate the system while the coupling to an extra (central) ion, which is continuously laser cooled, is the gateway to a decohering reservoir. The scheme proposed allows for the observation and control of motional entanglement dynamics and is an example of a class of simulations of quantum open systems in the non-Markovian regime.

We investigate the lower bound obtained from experimental data of a quantum state $\rho$, as proposed independently by O. Gühne et al. [Phys. Rev. Lett. 98, 110502 (2007)] and J. Eisert et al. [New J. Phys. 9, 46 (2007)], and apply it to mixed states of three qubits. The measure we consider is the convex-roof extended three-tangle. Our findings highlight an intimate relation to lower bounds obtained recently from so-called characteristic curves of a given entanglement measure. We apply the bounds to estimate the three-tangle present in recently performed experiments aimed at producing a three-qubit Greenberger-Horne-Zeilinger (GHZ) state. A nonvanishing lower bound is obtained if the GHZ fidelity of the produced states is larger than $3/4$.

An important quantum search algorithm based on the quantum random walk performs an oracle search on a database of $N$ items with $O(\sqrt{\mathit{ph}N})$ calls, yielding a speedup similar to the Grover quantum search algorithm. The algorithm was implemented on a quantum information processor of three-qubit liquid-crystal nuclear magnetic resonance (NMR) in the case of finding 1 out of 4, and the diagonal elements’ tomography of all the final density matrices was completed with comprehensible one-dimensional NMR spectra. The experimental results agree well with the theoretical predictions.

Recent advances in coupled-cluster methods for the treatment of electron correlation in atoms (and molecules) have now made possible the computation of the ionization potentials and electron affinities of the atoms H through Ne with a mean absolute deviation of 0.35 meV with respect to experimental data. In this article, coupled-cluster wave functions are parametrized using orbital and Slater-type geminal basis functions. Many-body electron interactions are accounted for through connected quintuple excitations, and relativistic effects (spin-orbit as well as scalar) and diagonal Born-Oppenheimer corrections are included.

We investigate the suitability of natural orbitals as a basis for describing many-body excitations. We analyze to which extent the natural orbitals describe both bound as well as ionized excited states and show that depending on the specifics of the excited state the ground-state natural orbitals may yield a good approximation. We show that the success of reduced density-matrix functional theory in describing molecular dissociation lies in the flexibility provided by fractional occupation numbers while the role of the natural orbitals is minor.

An investigation of the pressure broadening by helium and hydrogen of six rotational transitions of water has been completed. The six transitions studied included two para water transitions (0${}_{00}\text{–}{1}_{11}$ and 1${}_{11}\text{–}{2}_{02}$) and four ortho water transitions (1${}_{01}\text{–}{1}_{10}$, 2${}_{21}\text{–}{3}_{12}$, 3${}_{03}\text{–}{3}_{12}$ and 3${}_{12}\text{–}{3}_{21}$) in the frequency region 0.55–1.17 THz. This survey was accomplished using the collisional cooling technique which allowed the broadening of each transition to be studied below the water condensation temperature. For each of the transitions studied, the temperature dependence of the pressure broadening by helium showed little dependence on temperature, while the broadening by hydrogen showed a sharp decrease at the lowest temperatures. This behavior was modeled, for each transition broadened by helium and hydrogen, with a power law, or a power law modified with a Boltzmann-like step function, and the results of these fits will be presented. In addition, an extensive investigation of the systematic error in the temperature of the water vapor in the collisional cooling experiment will be discussed. Finally, the impact of these new broadening measurements on models of star formation in the interstellar medium will be outlined.

We derive expressions for the scattering matrix and the Wigner-Smith delay time, applicable to open systems in two and three spatial dimensions, in terms of a non-Hermitian effective Hamiltonian whose complex eigenvalues give the quasibound state energies of the system. These results are illustrated on an exactly solvable two-dimensional scattering problem and are found to be in excellent agreement with the exact solution.

In this article, we present the results of studies on the quantum mechanical tunneling and reflection of a diatomic, homonuclear molecule with a single bound state incident upon a potential barrier. In the first study, we investigate the tunneling of a molecule using a time-dependent formulation. The molecular wave function is modeled as a Gaussian wave packet, and its propagation is calculated numerically using Crank-Nicholson integration. It is found that a molecule may transition between the bound state and an unbound state numerous times during the process of reflection from or transmission past the barrier. It is also found that, in addition to reflecting and transmitting, the molecule may also temporarily straddle the potential barrier in an unbound state. In the second study, we consider the case of a molecule incident in the bound state upon a step potential with energy less than the step. We show that in the limit where the binding energy $e_0$ approaches zero and the step potential $V_0$ goes to infinity, the molecule cannot remain in a bound state if the center of mass gets closer to the step than an arbitrarily large distance $x_0$ which increases as the magnitude of $e_0$ decreases, as $V_0$ increases, or both. We also show that, for $e_0\to 0^{-}$ and $V_0\to \infty$, if the molecule is incident in the bound state, it is reflected in the bound state with probability equal to unity, when the center of mass reaches the reflection distance $x_0$. We verify that the unbound states exhibit the expected physical behavior. We discuss some surprising results. Connections between our results and investigations done in cold atoms, excitons, Cooper pairs, and Rydberg atoms are discussed.

Photoelectron spectra of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, ionized with ultrashort laser pulses, are measured with a momentum-map-imaging electron spectrometer. Above the photon energy, 1.6 eV, the spectra are essentially structureless and well described by Boltzmann distributions, with temperatures on the order of ${10}^4$ K. This result is similar to previous results for ${\mathrm{C}}_{60}$ using a time-of-flight electron spectrometer, which are confirmed in this study. Comparisons of electron energy distributions recorded for identical laser intensities but for different pulse durations demonstrate unambiguously that a significant fraction of the electrons are emitted in quasithermal processes and results argue strongly against a field ionization or direct multiphoton ionization mechanism. For electron energies above the photon energy, which account for about half the intensity, the whole signal is consistent with this quasithermal emission.

The double photoionization of a He atom enclosed in a ${\mathrm{C}}_{60}$ molecule, He@C${}_{60}$ was investigated using the time-dependent close-coupling method with the ${\mathrm{C}}_{60}$ molecule represented by a spherical shell potential and the He atom treated nonperturbatively. Comparisons are made with the double photoionization of a bare He atom. The ratio of the total double photoionization cross section of He@C${}_{60}$ to that of a bare He atom is found to exhibit oscillations due to reflection of a slow electron. This is confirmed by calculations of the single energy differential cross section that peaks at unequal energy sharing when one of the ejected electrons has an energy comparable to the ${\mathrm{C}}_{60}$ well depth. Triple differential cross sections are found to be shifted in magnitude, but unchanged in shape from the bare He atom.

The electron acceleration in the bubble regime is considered during the intense laser-plasma interaction. The presented ellipsoid cavity model is more consistent than the previous spherical model, and it explains the monoenergetic electron trajectory more accurately. At the relativistic region, the maximum energy of electrons in the ellipsoid model is about $24\%$ more than the spherical model, and this is confirmed by PIC and the measured experimental results reported here. The electron energy spectrum is also calculated, and it is found that the energy distribution ratio of electrons $\Delta E/E$ for the ellipsoid model in the here reported condition is about $11\%$ which is less than the one third that of the spherical model. It is in good agreement with the experimentally measured value in the same condition. In this regime, the parameters of the quasi-monoenergetic electrons output beam can be described more appropriately. In this work, 10 TW from 16.6 TW, 500 mJ, and 30-fs laser pulse was focused on the best matched point above a 2-mm-diameter pulsed He gas jet to obtain a stable ellipsoid bubble. Laser intensity of $1.42\times {10}^{19}$ W cm${}^{-2}$ corresponding to a normalized vector potential of $a_0=2.6$ focused in a 100-$\mu$m${}^2$ spot at the focal point and 1 mm above the edge of the gas jet with an electron density of $1\times {10}^{19}$ cm${}^{-3}$ accelerates electrons to the relativistic velocities. The obtained monoenergetic electron energy spectrum is properly explained by the ellipsoid model introduced here.

We prove that the inequality used recently by Wilk and Włodarczyk [Phys. Rev. A 79, 062108 (2009)] to find a better lower bound in the uncertainty relations for the Rényi entropies is invalid. Thus, the problem of improving the bound given in our article [Phys. Rev. A 74, 052101 (2006)] remains unsolved.

Experimental results are reported for elastic differential and integral cross sections for electrons scattering from ${\mathrm{CF}}_3$I. These measurements were made at ten incident electron energies in the range 10–50 eV, with a scattered electron angular range of ${20}^{°}$–${135}^{°}$. Where possible, comparison is made to the only other comprehensive experimental set of results available in the literature [M. Kitajima et al., J. Phys. B 35, 3257 (2002)] and to calculated cross sections from the Schwinger multichannel with pseudopotentials method [M. H. Bettega et al., J. Phys. B 36, 1263 (2003)]. In general, quite good agreement is found between the present results and those of the earlier studies.

In this work we report the results of calculation for quantum-mechanical rotational transitions in molecular hydrogen, ${\mathrm{H}}_2$, induced by an ultracold ground-state antihydrogen atom ${\overline{\mathrm{H}}}_{1s}$. The calculations are accomplished using a nonreactive close-coupling quantum-mechanical approach. The ${\mathrm{H}}_2$ molecule is treated as a rigid rotor. The total elastic-scattering cross section ${\sigma }_{\mathrm{el}}(\epsilon )$ at energy $\epsilon$, state-resolved rotational transition cross sections ${\sigma }_{{\mathit{jj}}^{\text{'}}}(\epsilon )$ between states $j$ and $j^{\text{'}}$, and corresponding thermal rate coefficients $k_{{\mathit{jj}}^{\text{'}}}(T)$ are computed in the temperature range $0.004\hspace{0.5em}\mathrm{K}\lesssim T\lesssim$ 4 K. Satisfactory agreement with other calculations (variational) has been obtained for ${\sigma }_{\mathrm{el}}(\epsilon )$.

The emission of electromagnetic radiation by relativistic bare heavy ions penetrating ordinary matter is investigated. Our main aim is to determine the bremsstrahlung which we define as the radiation emitted when the projectile does not break up. It pertains to collisions without nuclear contact (“ultraperipheral collisions”). Requirement of coherent action of the nucleons in order to keep the penetrating projectile intact limits bremsstrahlung to relatively soft photons. The spectrum shows a resonance structure with peak position near $2\gamma$ times the position of the giant dipole resonance, that is, near $25\gamma$ MeV for a lead ion ($\gamma \equiv E/{\mathit{Mc}}^2$ is the Lorentz factor of the projectile of energy $E$ and mass $M$). The maximum exceeds the bremsstrahlung from a hypothetical structureless, pointlike particle of the same charge and mass as the incoming nucleus, but rapid depletion follows on the high-energy side of the peak. As a result of its relative softness, bremsstrahlung never dominates the energy-loss process for heavy ions. As to the emission of electromagnetic radiation in collisions with nuclear break-up, it appears modest when pertaining to incoherent action of the projectile nucleons in noncontact collisions. In collisions with nuclear contact, though, substantial radiation is emitted. It overshoots the bremsstrahlung. However, despite the violence of contact events, the associated photon emission only exceeds the radiation from a hypothetical structureless pointlike nucleus [emitted energy per unit photon-energy interval essentially constant up to $(\gamma -1){\mathit{Mc}}^2$] at relatively low photon energies (for lead roughly below $0.2\gamma$ GeV, a limit which is about an order of magnitude above the position of the bremsstrahlung peak). Results are presented for bare lead ions penetrating a solid lead target at energies of $158$ GeV/n ($\gamma =170$) and beyond.

We report the experimental observation of an optically controllable shift of the central frequency of a laser pulse, using a scheme based on dynamical electromagnetically induced transparency. This is evidence of frequency shift controllable by a coherent process. Original theoretical results are in agreement with the experimental data.

We show that Rydberg states in an ultracold gas can be excited with strongly preferred nearest-neighbor distance if densities are well below saturation. The scheme makes use of an echo sequence in which the first half of a laser pulse excites Rydberg states while the second half returns atoms to the ground state, as in the experiment of Raitzsch et al. [Phys. Rev. Lett. 100, 013002 (2008)]. Near the end of the echo sequence, almost any remaining Rydberg atom is separated from its next-neighbor Rydberg atom by a distance slightly larger than the instantaneous blockade radius halfway through the pulse. These correlations lead to large deviations of the atom-counting statistics from a Poissonian distribution. Our results are based on the exact quantum evolution of samples with small numbers of atoms. Finally, we demonstrate the utility of the $\omega$ expansion for the approximate description of correlation dynamics through an echo sequence.