Motivated by a recent experimental work [S. Lederer and H. Winter, Phys. Rev. A 73, 054901 (2006)] we study the contribution of the $3d$ shell electrons to the energy loss of $100\mathrm{keV}$ protons scattered off from the Cu(111) surface. To describe this process we use a multiple collision formalism, where the interaction of the projectile with $3d$ electrons is described by means of a sequence of single encounters with atoms belonging to the first atomic layer. In order to compare the theoretical energy loss with the experimental data, we add the contribution of valence electrons, which is evaluated in linear response theory using a response function that incorporates information on the surface band structure. For completeness, the energy lost by protons is also calculated within a jellium model that includes $3d$ and valence electrons with equal footing. Fair agreement between theory and experiment exists when the $3d$ shell is taken into account in the calculation.

A sequence of two femtosecond pulses—a strong driving $\pi$-polarized pulse and a weak propagating $\sigma$-polarized pulse—excites resonantly the $S_{1∕2}\to P_{1∕2}$ transition of an atomic system. Strong interference effects take place in the system between absorption and emission paths leading to a substantial amplification of the $\sigma$ pulse. We study the influence of the fine structure on the medium gain when the contribution of the off-resonant $P_{3∕2}$ level is taken into account. A drastic reduction of the medium gain is obtained. This effect is explained within the bright-state–dark-state formalism where the strong driving pulse creates a wave packet that can be trapped in a state—the bright state—leading to a significant reduction of the gain for the $\sigma$ pulse. Finally, we also show that periodical gain dependence with the driving pulse energy exhibits a significant change in its period value (compared with expected Rabi oscillations).

We investigate the multiphoton states generated by high-gain optical parametric amplification of a single injected photon, polarization encoded as a “qubit.” The experiment configuration exploits the optimal phase-covariant cloning in the high gain regime. The interference fringe pattern showing the nonlocal transfer of coherence between the injected qubit and the mesoscopic amplified output field involving up to 4000 photons has been investigated. A probabilistic method to extract full information about the multiparticle output wave function has been implemented.

We propose a scheme to overcome phase damping decoherence by periodically performing open loop tracking control and projective measurement. Although it is impossible to stabilize a qubit subject to Markovian dynamics only by open loop coherent control, one can attain a “softened” control goal with the help of periodical projective measurement. The “softened” control objective in our scheme is to keep the state of the controlled qubit to stay near a reference pure state with a high probability for a sufficiently long time. Two suboptimal control problems are given in the sense of trace distance and fidelity, respectively, and they are eventually reduced to the design of a period $T$. In our scheme, one can choose the period $T$ as long as possible if the “softened” control goal is attained. This is in contrast to the observation that quantum Zeno effect takes place only if measurements are performed in a very frequent manner, i.e., the period $\mathit{T}$ must be extremely small.

We propose a scheme able to entangle at the steady state a nanomechanical resonator with a microwave cavity mode of a driven superconducting coplanar waveguide. The nanomechanical resonator is capacitively coupled with the central conductor of the waveguide and stationary entanglement is achievable up to temperatures of tens of milliKelvin.

We present density functional theory calculations of ${\text{C}}_{60}^{q-}$ and ${\text{C}}_{70}^{q-}$ $\left(q=0-2\right)$ isomers that contain zero to three pairs of adjacent pentagons. The first- and second-electron affinities for the archetype structures of ${\text{C}}_{60}$ and ${\text{C}}_{70}$ are close to the experimental results, while isomers with pentagon adjacencies have significantly higher values. The results are rationalized in view of the numbers and locations of pentagon adjacencies in the fullerene cages and the spherical aromaticity for the magic number ${\text{C}}_{70}^{2-}$ with a closed $\left(\text{l}=5\right)$ electronic shell in the spherical gas model.

An investigation of field-free molecular alignment produced by elliptically polarized laser pulses is reported. Experiments are conducted in ${\text{CO}}_2$ at room temperature. A noninvasive all-optical technique, based on the cross defocusing of a probe pulse, is used to measure the alignment along two orthogonal directions which is sufficient to provide a three-dimensional characterization. The field-free molecular alignment produced by a laser of elliptical polarization is in good agreement in terms of amplitude and shape with theoretical predictions. It turns out to be almost equivalent to the superposition of the effects that one would obtain with two individual cross-polarized pulses. The investigation highlights notably the occurrence of field-free two-direction alignment alternation for a suitably chosen degree of ellipticity. The analogy between this specific ellipticity and the well-known “magic angle” used in time-resolved spectroscopy to prevent rotational contributions is discussed.

We have performed quantum Monte Carlo simulations to investigate the superfluid behavior of one- and two-dimensional interfaces separating checkerboard solid domains. The system is described by the hard-core Bose-Hubbard Hamiltonian with nearest-neighbor interaction. In accordance with Burovski et al. [Phys. Rev. Lett. 94, 165301 (2005)] we find that (i) the interface remains superfluid in a wide range of interaction strength before it undergoes a superfluid-insulator transition; (ii) in one dimension, the transition is of the Kosterlitz-Thouless type and is accompanied by the roughening transition, driven by proliferation of charge-$1∕2$ quasiparticles; (iii) in two dimensions, the transition belongs to the three-dimensional $\mathrm{U}\left(1\right)$ universality class and the interface remains smooth. Similar phenomena are expected for domain walls in quantum antiferromagnets.

Numerical solutions of the Hartree-Fock (HF) equation of polyatomic molecules have been obtained by an extension of the numerical density-functional method of Becke and Dickson [J. Chem. Phys. 89, 2993 (1988); 92, 3610 (1990)]. A finite-difference method has been used to solve Poisson’s equation for the Coulomb and exchange potentials and to evaluate the action of the Laplace operator on numerical orbitals expanded on an interlocking multicenter quadrature grid. Basis-set-limit HF results for an atom and diatomic and triatomic molecules are presented with the total energies and the highest occupied orbital energies converged to within ${10}^{-5}$ Hartree without any extrapolation.

A combined experimental and theoretical study of the energy loss of protons in fluorides is presented. The measurements were performed in fresh $\mathrm{Al}{\mathrm{F}}_3$ evaporated in situ on self-supported C foils, extending the low-energy range from 25 down to $0.7\mathrm{keV}$. The transmission method is used in combination with time-of-flight spectrometry. The experimental stopping power increases almost linearly with the mean projectile velocity, showing a velocity threshold at about $0.1\mathrm{a.u.}$ These features are well reproduced by a model based on quantum scattering theory that takes into account the velocity distribution and the excitation of the active $2p$ electrons in the ${\mathrm{F}}^{-}$ anions, and the properties of the electronic bands of the insulators. The model also reproduces the threshold behavior measured previously on LiF.

We present a geometric approach to the characterization of separability and entanglement in pure Gaussian states of an arbitrary number of modes. The analysis is performed adapting to continuous variables a formalism based on single subsystem unitary transformations that has been recently introduced to characterize separability and entanglement in pure states of qubits and qutrits [S. M. Giampaolo and F. Illuminati, Phys. Rev. A 76, 042301 (2007)]. In analogy with the finite-dimensional case, we demonstrate that the $1\times M$ bipartite entanglement of a multimode pure Gaussian state can be quantified by the minimum squared Euclidean distance between the state itself and the set of states obtained by transforming it via suitable local symplectic (unitary) operations. This minimum distance, corresponding to $a$, uniquely determined, extremal local operation, defines an entanglement monotone equivalent to the entropy of entanglement, and amenable to direct experimental measurement with linear optical schemes.

Using coupled equations for the bosonic and fermionic order parameters, we construct families of gap solitons (GSs) in a nearly one-dimensional Bose-Fermi mixture trapped in a periodic optical-lattice (OL) potential, the boson and fermion components being in the states of the Bose-Einstein condensation and Bardeen-Cooper-Schrieffer superfluid, respectively. Fundamental GSs are compact states trapped, essentially, in a single cell of the lattice. Full families of such solutions are constructed in the first two band gaps of the OL-induced spectrum, by means of variational and numerical methods, which are found to be in good agreement. The families include both intragap and intergap solitons, with the chemical potentials of the boson and fermion components falling in the same or different band gaps, respectively. Nonfundamental states, extended over several lattice cells, are constructed too. The GSs are stable against strong perturbations.

We examine the conditions that give rise to metastable, persistent currents in a trapped Bose-Einstein condensate. A necessary condition for the stability of persistent currents is that the trapping potential is not a monotonically increasing function of the distance from the trap center. Persistent currents also require that the interatomic interactions are sufficiently strong and repulsive. Finally, any off-center vortex state is shown to be unstable, while a driven gas shows hysteresis.

A quantum superposition of two coherent states of light with small amplitude can be obtained by subtracting a photon from a squeezed vacuum state. In experiments this preparation can be made conditioned on the detection of a photon in the field from a squeezed light source. We propose and analyze an extended measurement strategy which allows generation of high fidelity coherent state superpositions with larger amplitude.

The influence of disordering of one-dimensional quasi-phase-matched structures in nonlinear-optical crystals on the spectral characteristics of parametric processes is studied in the general form, taking into consideration arbitrary spatial distribution of the second-order susceptibility, sum- and difference-frequency interactions in the forward and backward directions, and continuous-wave and pulsed pumping. It is shown that, at the initial low-gain stages of all processes, the impact of the medium spatial structure can be described in terms of a single functional parameter, nonlinear transfer function (T-function). Expressed in terms of dimensionless phase mismatches, the T-function is considered as a universal characteristic of the structured medium. The method of its direct measurement is proposed, based on spontaneous parametric down-conversion. Examples of the T-function approach are demonstrated for short-pulse second harmonic generation, as well as for generation and detection of spectral-shaped terahertz signals.

The distribution of spacings between the lasing frequencies for an ensemble of random lasers in the two-mode regime was computed. The random lasers are implemented as open chaotic cavities filled with an active medium. The spectral properties of the passive cavities are modeled with non-Hermitian random matrices. The spacing distribution is found to depend on the relation between the gain-profile width and the mean spacing of the passive-cavity modes. The distribution displays mode repulsion and, under certain conditions, agrees with the Wigner surmise. The role of mode competition is discussed.

The giant (or shape) resonance in the photodetachment cross section of ${\text{D}}^{-}$ above the $n=2$ threshold has been subjected to high-resolution vacuum ultraviolet spectroscopy performed by Doppler tuning of ions stored in the ASTRID storage ring. In order to prevent changes in the overlap between laser and ion beams over the large kinetic-energy range employed in the experiment, a new ion-beam-positioning method based on a quadrupole-shunt technique was applied. The study presents an accurate measurement of parameters for the resonance, which is also denoted ${{}_2\left\{0\right\}}_2^{+}{{}^{1}P}^o$. The resonance has a width of 26(2) meV while the asymmetry parameter $q$ of the Fano profile is 3.2(0.4), which is at variance with the most recent theoretical calculations.

A quantum transition can be seen as a result of interference between various pathways (e.g., Feynman paths), which can be labeled by a variable $f$. An attempt to determine the value of $f$ without destroying the coherence between the pathways produces a weak value of $\overline{f}$. We show $\overline{f}$ to be an average obtained with an amplitude distribution which can, in general, take negative values, which, in accordance with the uncertainty principle, need not contain information about the actual range of $f$ which contributes to the transition. It is also demonstrated that the moments of such alternating distributions have a number of unusual properties which may lead to a misinterpretation of the weak-measurement results. We provide a detailed analysis of weak measurements with and without post-selection. Examples include the double-slit diffraction experiment, weak von Neumann and von Neumann–like measurements, traversal time for an elastic collision, phase time, and local angular momentum.