We study the Berry phase of the ground state and excited states in the Lipkin model. Using the Schwinger representation of spin operators, we get the expression for the Berry phase of the ground state in the thermodynamic limit and numerically calculate that of the excited states in finite sizes. We find that the Berry phase of the ground state is nonanalytic, but its derivative (with respect to the control parameter $\alpha$, see text) is finite even in the thermodynamic limit. Whereas, for the excited states, the Berry phase tends to be nonanalytic, and its derivative approaches $\infty$ in the thermodynamic limit and in the vicinity of the critical point of the excited-state quantum phase transition.

We investigate the relation between unextendible product bases (UPB) and Bell inequalities found recently in R. Augusiak et al. [Phys. Rev. Lett. 107, 070401 (2011)]. We, first, review the procedure introduced there that associates to any set of mutually orthogonal product vectors in a many-qubit Hilbert space a Bell inequality. We then show that if a set of mutually orthogonal product vectors can be completed to a full basis, then the associated Bell inequality is trivial, in the sense of not being violated by any nonsignalling correlations. This implies that the relevant Bell inequalities that arise from the construction all come from UPBs, which adds additional weight to the significance of UPBs for Bell inequalities. Then, we provide additional examples of tight Bell inequalities with no quantum violation constructed from UPBs in this way. Finally, it is proven that the Bell inequalities with no quantum violation introduced recently in M. Almeida et al. [Phys. Rev. Lett. 104, 230404 (2010)] are tight for any odd number of parties.

Recent work [J. Dunningham, V. Palge, and V. Vedral, Phys. Rev. A 80, 044302 (2009)] studied entanglement between the spin and momentum components of a single spin-$1/2$ particle and showed that maximal entanglement is obtained only when boosts approach the speed of light. Here we extend the boost scenario to general geometries and show that, intriguingly, maximal entanglement can be achieved with boosts less than the speed of light. Boosts approaching the speed of light may even decrease entanglement. We also provide a geometric explanation for this behavior.

We present a measure of entanglement for pure two-qutrit states which constitutes a probability interpretation for negativity. We also obtain an equivalence relation for the latter, which is used to find a lower bound on its convex-roof extension, for systems with dimensions up to 6. The possibility of the obtained results being valid for higher-dimensional systems is also discussed.

We propose and analyze a quantum repeater architecture in which Rydberg-blocked atomic ensembles inside optical cavities are linked by optical fibers. Entanglement generation, swapping, and purification are achieved through collective laser manipulations of the ensembles and photon transmission. Successful transmission and storage of entanglement are heralded by ionization events rather than by photon detection signals used in previous proposals. We demonstrate how the high charge detection efficiency allows for a shortened average entanglement generation time, and we analyze an implementation of our scheme with ensembles of Cs atoms.

We theoretically investigate high-order harmonic generation in argon gas driven by two orthogonally polarized laser pulses of single color, with the consideration of the macroscopic propagation effect including fundamental and harmonic fields. Both the three-dimensional propagation simulation and a simple analytical model based on a single-atom level demonstrate that the harmonic intensity of any harmonic order varies periodically when the delay between two pulses is scanned. This significant intensity modulation provides an experimentally observable contrast, from which the electron excursion time of a short quantum path associated with a certain harmonic order can be directly read. The result shows that the attosecond time-resolved electron motion can be mapped onto the macroscopic harmonic intensity modulation

The detailed theoretical and experimental analysis of the angular distributions of electrons from interatomic Coulombic decay (ICD) of the Ne dimer in the molecular frame is performed. In the initial state the doubly charged dimer ion has one $2s$ vacancy and one $2p$ vacancy on one atom. After the ICD process the neutral neon atom is ionized and the triply charged molecular ion dissociates into singly and doubly charged atomic ions, ${\text{Ne}}^{2+}\left(2p^{-2}\right)+{\text{Ne}}^{+}\left(2p^{-1}\right)$. From the coincident measurement of kinetic energy release (KER) of the ions and the ICD electron the decay channel can be identified unambiguously. The most detailed experimental data have been obtained for the singlet dicationic state Ne${}^{2+}\left(2p^{-2}\right){[}^{1}D]$. Different KER energies correspond to different internuclear distances at which the ICD process takes place. In experiment the data have been presented for three regions of KER energies, and the corresponding calculations have been performed for three fixed internuclear distances. In calculations we imply that all the electrons in Ne${}_2$ to a good approximation are localized. However, we need to retain the molecular character of the dimer wave functions which opens the possibility for the ICD process. To do it, we calculate at first the Hartree-Fock ground state wave functions of the neutral Ne${}_2$ dimer using the standard procedure for homonuclear diatomic molecules corresponding to the $D_{\infty h}$ symmetry group. For the doubly charged ion Ne${}_2{}^{2+}$ with two vacancies on one atom the symmetry is lowered to $C_{\infty v}$, and we are looking now for the set of one-electron Hartree-Fock wave functions which are localized either on the left or on the right atom as a linear combination of symmetry-adopted wave functions. The theory correctly reproduces the experimental data and predicts the sharp variation of the angular distributions as a function of internuclear distance.

Photoelectron emission from the valence band of a metal surface due to the grazing incidence of ultrashort laser pulses is studied in the framework of a distorted-wave formulation. We propose a model, named the band-structure-based–Volkov (BSB-V) approximation, which takes into account the contribution of the band structure of the solid. The BSB-V approach is applied to calculate differential electron-emission probabilities for Al(111) and Be(0001) surfaces. A noticeable influence of the electronic band structure was observed in the case of beryllium, while for aluminum such effects were found to play a minor role.

We analyze how the contrast of a quasi-one-dimensional Bose-Einstein-condensation (BEC)-based atomic interferometer is affected by the number density of atoms. In our numerical contrast of a Mach-Zehnder-type BEC-based interferometer a regularly spaced oscillatory structure emerges as a function of both the number density and the recombination time. We attribute this effect to the phase singularities in Husimi representation of the overlapping BEC wave packets caused by the residual nonuniform velocity distribution.

We study the critical behavior near the quantum critical point of strongly interacting bosons placed in an optical lattice. Using the combined Bogoliubov method and the quantum rotor approach, we map the Hamiltonian of strongly interacting bosons onto $U\left(1\right)$ phase action in order to calculate analytically the density profiles as a function of hopping and reduced chemical potential. Our approach allows us to explicitly compute the scaling form of the density $n_s$ for systems confined in two-dimensional square and three-dimensional cubic lattices. We find a good convergence of $n_s$ with the universal scaling function in a wide temperature range that is accessible in the current experiments on the density profiles in cold bosonic systems.

Spatial quantum correlations are an important resource for quantum imaging and future quantum technologies based on high-dimensional entanglement. We experimentally explore spatial correlations in four-photon states generated by a highly efficient parametric down-conversion process in a 2 mm long periodically poled KTP crystal. The four-photon states produced in the crystal contain a contribution due to independent double pairs produced via spontaneous pair emission and a highly correlated four-photon state produced by stimulated emission of a second photon pair. We separate this contribution and introduce a joint spatial density for stimulated pair emission. We observe a maximum visibility $\chi =0.25$ of the four-photon state for a 1.5 nm bandpass filter.

In the framework of the Keldysh technique, we formulate the nonlinear $\sigma$ model for disordered optical media with linear absorption or gain. The effective action for fluctuations of the matrix field about the saddle point acquires an extra term due to the nonconservative nature of the system. We determine the disorder-averaged Green's function correlator, which has a diffusion pole modified by a finite absorption or gain rate. The diffusion coefficient is found to be close to its value for conservative systems in the relevant range of parameters. In the medium with gain, the random-lasing threshold depends on the sample size.

Highly directional backward stimulated scattering has been observed from large-size-gold nanorods suspended in water, pumped with $\sim$816 nm and $\sim$10 ns laser pulses. In comparison with other known stimulated scattering effects, the newly observed effect exhibits the following features. (i) The scattering centers are impurity particles with a size comparable in order of magnitude to the incident wavelength. (ii) There is no frequency shift between the pump wavelength and the stimulated scattering wavelength. (iii) The pump threshold can be significantly lower than that of stimulated Brillouin scattering in pure water. The nonfrequency shift can be explained by the formation of a standing-wave Bragg grating induced by the interference between the forward pump beam and the backward Mie-scattering beam. The low pump threshold results from stronger initial Mie-scattering (seed) signals and the intensity-dependent refractive-index change of the scattering medium enhanced by metallic nanoparticles.

We study the families of nonlinear modes described by the nonlinear Schrödinger equation with the $\mathcal{PT}$-symmetric harmonic potential $x^2-2i\alpha x$. The nonlinear modes found display a number of interesting features. In particular, we have observed that modes bifurcating from different eigenstates of the underlying linear problem can actually belong to the same family of nonlinear modes. We also show that by proper adjustment of the coefficient $\alpha$ it is possible to enhance the stability of small-amplitude and strongly nonlinear modes compared to the well-studied case of the real harmonic potential.

An efficient broadband sum-frequency generation (SFG) technique using the two cascaded optical parametric processes ${\omega }_3={\omega }_1+{\omega }_2$ and ${\omega }_4={\omega }_1+{\omega }_3$ is proposed. The technique uses chirped quasi-phase-matched gratings, which, in the undepleted pump approximation, make SFG analogous to adiabatic population transfer in three-state systems with crossing energies in quantum physics. If the local modulation period first makes the phase match occur for ${\omega }_3$ and then for ${\omega }_4$ SFG processes then the energy is converted adiabatically to the ${\omega }_4$ field. Efficient SFG of the ${\omega }_4$ field is also possible by the opposite direction of the local modulation sweep; then transient SFG of the ${\omega }_3$ field is strongly reduced. Most of these features remain valid in the nonlinear regime of depleted pump.

A theoretical investigation of photoabsorption and photoionization of Fe${}^{14+}$ extending beyond an earlier frame transformation $R$-matrix implementation is performed using a fully correlated, Breit-Pauli $R$-matrix formulation including both fine-structure splitting of strongly bound resonances and radiation damping. The radiation damping of $2p\to nd$ resonances gives rise to a resonant photoionization cross section that is significantly lower than the total photoabsorption cross section. Furthermore, the radiation-damped photoionization cross section is found to be in good agreement with recent experimental results once a global shift in energy of $\approx$–3.5 eV is applied. These findings have important implications. First, the presently available synchrotron experimental data are applicable only to photoionization processes and not to photoabsorption; the latter is required in opacity calculations. Second, our computed cross section, for which the $L$-shell ionization threshold is aligned with the NIST value, shows a series of $2p\to nd$ Rydberg resonances that are uniformly 3–4 eV higher in energy than the corresponding experimental profiles, suggesting that the actual $L$-shell threshold energy is lower than the value obtained using the current NIST data.

We report on the trapping of ultracold atoms in the magnetic field formed entirely by persistent supercurrents induced in a thin film type-II superconducting square. The supercurrents are carried by vortices induced in the two-dimensional (2D) structure by applying two magnetic field pulses of varying amplitude perpendicular to its surface. This results in a self-sufficient quadrupole trap that does not require any externally applied fields. We investigate the trapping parameters for different supercurrent distributions. Furthermore, to demonstrate possible applications of these types of supercurrent traps we show how a central quadrupole trap can be split into four traps by use of a bias field, as predicted by Zhang et al. [Phys. Rev. A 85, 013404 (2012)].

In the ionization process, longitudinal momentum along the direction of propagation is transferred to the photoelectrons due to the action of the magnetic component of Lorentz force. In a recent experiment by Smeenk et al. [Phys. Rev. Lett. 106, 193002 (2011)], such a transfer is observed in the ionization of argon and neon atoms by circularly polarized light at 800 and 1400 nm in the intensity range of ${10}^{14}–{10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. They accounted for the results by a purely classical model. We present a fully quantum-mechanical calculation of the transfer of longitudinal momentum to the photoelectrons. The results are in agreement with the observations of Smeenk et al. at high intensities, but clear evidence for additional Coulomb interactions emerges at low intensities.

We analyze the performance of the resonator–zero-qubit (RezQu) architecture in which the qubits are complemented by memory resonators and coupled via a resonator bus. Separating the stored information from the rest of the processing circuit by at least two coupling steps and the zero qubit state results in a significant increase in the on-to-off ratio and a reduction in the idling error. Assuming no decoherence, we calculate such idling error, as well as the errors for the move operation and tunneling measurement, and show that the RezQu architecture can provide the high-fidelity performance required for medium-scale quantum information processing.