We present differential, integral, and momentum-transfer cross sections for elastic scattering of low-energy electrons by three butanol isomers, isobutanol, $t$-butanol, and 2-butanol. Our results were calculated with the Schwinger multichannel method in the static-exchange plus polarization approximation for collision energies from $1$ to $50$ eV. The present results are compared with previous calculations and measurements for the remaining $\mathrm{C}{}_4$H${}_9$OH isomer, $n$-butanol [Khakoo et al., Phys. Rev. A 78, 062714 (2008)]. Distinctive behavior is observed in the differential cross sections at collision energies between $5$ and $10$ eV. In particular, whereas $n$-butanol exhibits an $f$-wave scattering pattern, the other isomers exhibit $d$-wave behavior. A similar pattern is found in the related alkanes when comparing straight-chain versus branched isomers. We discuss the possible connection of this behavior to shape resonances that influence the scattering.

Quantum diffraction of fast atoms scattered from the topmost layer of surfaces under grazing angles of incidence can be employed for the analysis of detailed structural properties of insulator surfaces. From comparison of measured and calculated diffraction patterns we deduce the rumpling of the topmost surface layer of LiF(001) (i.e., an inward shift of Li${}^{+}$ ions with respect to ${\mathrm{F}}^{-}$ ions). The effect of thermal vibrations on the measurement of rumpling is accounted for by ab initio calculations of the mean-square vibrational amplitudes of surface ions. At room temperature this leads to a reduction of the apparent rumpling by 0.008 Å. We then obtain a rumpling of ($0.05\pm 0.04$) Å, which improves its accuracy achieved in previous work.

We investigate the phase diagram and the BCS-BEC crossover of a homogeneous three-component ultracold Fermi gas with a U(3)-invariant attractive interaction. We show that the system at sufficiently low temperatures exhibits population imbalance as well as fermionic pairing. We describe the crossover in this system, connecting the weakly interacting BCS regime of the partially population-imbalanced fermion pairing state and the BEC limit with three weakly interacting species of molecules, including pairing fluctuations within a $t$-matrix calculation of the particle self-energies.

We study dynamics of an ultracold atomic BCS superfluid driven toward the BCS superfluid–Fermi-liquid quantum critical point by a gradual decrease of the pairing interaction. We analyze how the BCS superfluid falls out of equilibrium and show that the nonequilibrium gap and Cooper pair size reflect critical properties of the transition. We observe three stages of evolution: adiabatic where the Cooper pair size is inversely proportional to the equilibrium gap, weakly nonequilibrium where it is inversely proportional to the nonequilibrium gap, and strongly nonequilibrium where it decouples from both equilibrium and nonequilibrium gap. These phenomena should stimulate future experimental characterization of nonequilibrium ultracold atomic BCS superfluids.

Extended Bose-Hubbard models with nearest-neighbor interaction describe minimally the effect of long-range interaction on ultracold atoms in deep optical lattices. The rotation of such optical lattices subjects such neutral cold atoms to the effect of an artificial magnetic field. The modification of the phase boundaries of the density-wave and Mott-insulator phases due to this rotation are shown to be related to the edge spectrum of spinorial and scalar Harper equations. Corresponding profiles of the checkerboard vortex states with sublattice modulated superfluid order parameter near the density-wave phase boundary are calculated.

The static fluctuation approximation (SFA) is applied to compute the thermodynamic properties of a trapped two-dimensional (2D) interacting hard-sphere (HS) Bose gas in the weakly and strongly interacting regime. A mean-field approach involving a variational wave function is used to compute the mean-field energy as a function of temperature for each harmonic oscillator (HO) state plugged into the SFA technique. In the variational approach, a parameter $\alpha$ is introduced into the harmonic oscillator wave function in order to take into account the changes in the width when the repulsive interactions between the bosons are increased. In the weakly interacting regime, below the critical temperature, the total energy of all HO states (evaluated by our model) matches the noninteracting result very well. However, beyond the critical temperature, we “fit” our energies to the classical limit for 2D bosons in a trap by using a suitably proposed weighting function. We compare our results to earlier results of mean-field theory. Further, we evaluate the density matrix arising from correlations between the HO orbitals.

Strong coupling between a two-level system (TLS) and bosonic modes produces dramatic quantum optics effects. We consider a one-dimensional continuum of bosons coupled to a single localized TLS, a system which may be realized in a variety of plasmonic, photonic, or electronic contexts. We present the exact many-body scattering eigenstate obtained by imposing open boundary conditions. Multiphoton bound states appear in the scattering of two or more photons due to the coupling between the photons and the TLS. Such bound states are shown to have a large effect on scattering of both Fock- and coherent-state wave packets, especially in the intermediate coupling-strength regime. We compare the statistics of the transmitted light with a coherent state having the same mean photon number: as the interaction strength increases, the one-photon probability is suppressed rapidly, and the two- and three-photon probabilities are greatly enhanced due to the many-body bound states. This results in non-Poissonian light.

We present an experimental demonstration of sum-frequency generation toward tunable picosecond, broadband radiation pulses in the extreme-ultraviolet regime, supported by adiabatically prepared atomic coherences. We drive a two-photon transition in a dense medium of xenon atoms with a long (nanosecond) pump laser pulse at the Fourier-transform-limited bandwidth and with a wavelength of 225 nm. The frequency of the pump pulse is slightly detuned from exact two-photon resonance. In this configuration, the medium is adiabatically driven by a process of coherent population return. The adiabatic passage process generates a maximal coherent superposition of two quantum states with large energy spacing. An additional, short (picosecond) probe laser pulse at a wavelength of 540 nm beats with the maximal atomic coherence and generates a short (picosecond) signal radiation pulse at 93 nm. As compared to conventional (diabatic) frequency conversion, the adiabatically driven maximal atomic coherence yields enhanced conversion efficiency and significantly enhanced stability.

We report the existence of stable symmetric vortex-type solutions for two-dimensional nonlinear discrete dissipative systems governed by a cubic-quintic complex Ginzburg-Landau equation. We construct a whole family of vortex solitons with a topological charge $S=1$. Surprisingly, the dynamical evolution of unstable solutions of this family does not significantly alter their profile, but instead their phase distribution completely changes; they transform into two-charge swirl-vortex solitons. We dynamically excite this structure showing its experimental feasibility.

Ground-state properties of the repulsive Hubbard model on a cubic lattice are investigated by means of the auxiliary-field quantum Monte Carlo method. We focus on low-density systems with varying on-site interaction $U/t$, as a model relevant to recent experiments on itinerant ferromagnetism in a dilute Fermi gas with contact interaction. Twist-average boundary conditions are used to eliminate open-shell effects and large lattice sizes are studied to reduce finite-size effects. The sign problem is controlled by a generalized constrained path approximation. We find no ferromagnetic phase transition in this model. The ground-state correlations are consistent with those of a paramagnetic Fermi liquid.

Recent experimental progress in producing ultracold polar molecules with a net electric dipole moment opens up possibilities for realizing quantum phases governed by the long-range and anisotropic dipole-dipole interactions. In this work we predict the existence of experimentally observable broken-symmetry states with spontaneous interlayer coherence in cold polar molecule bilayers. These exotic states, which are manifestations of collective bilayer quantum entanglement, appear due to strong repulsive interlayer interactions and exhibit properties of superfluids, ferromagnets, and excitonic condensates.

The process of $e^{+}{e}^{-}$ production in the superposition of a Coulomb and a strong laser field is considered. The pair production rate integrated over the momentum and summed over the spin projections of one of the particles is derived exactly in the parameters of the laser field and in the Born approximation with respect to the Coulomb field. The case of a monochromatic circularly polarized laser field is considered in detail. A very compact analytical expression of the pair production rate and its dependence on the polarization of one of the created particles is obtained in the quasiclassical approximation for the experimentally relevant case of an undercritical laser field. As a result, the polarization of the created electron (positron) is derived.

One of the major efforts in the quantum biological program is to subject biological systems to standard tests or measures of quantumness. These tests and measures should elucidate whether nontrivial quantum effects may be present in biological systems. Two such measures of quantum correlations are the quantum discord and the relative entropy of entanglement. Here, we show that the relative entropy of entanglement admits a simple analytic form when dynamics and accessible degrees of freedom are restricted to a zero- and single-excitation subspace. We also simulate and calculate the amount of quantum discord that is present in the Fenna-Matthews-Olson protein complex during the transfer of an excitation from a chlorosome antenna to a reaction center. We find that the single-excitation quantum discord and single-excitation relative entropy of entanglement are equal for all of our numerical simulations, but a proof of their general equality for this setting evades us for now. Also, some of our simulations demonstrate that the relative entropy of entanglement without the single-excitation restriction is much lower than the quantum discord. The first picosecond of dynamics is the relevant time scale for the transfer of the excitation, according to some sources in the literature. Our simulation results indicate that quantum correlations contribute a significant fraction of the total correlation during this first picosecond in many cases, at both cryogenic and physiological temperatures.

In this article we present a general security strategy for quantum secret sharing (QSS) protocols based on the scheme presented by Hillery, Bužek, and Berthiaume (HBB) [Phys. Rev. A 59, 1829 (1999)]. We focus on a generalization of the HBB protocol to $n$ communication parties thus including $n$-partite Greenberger-Horne-Zeilinger states. We show that the multipartite version of the HBB scheme is insecure in certain settings and impractical when going to large $n$. To provide security for such QSS schemes in general we use the framework presented by some of the authors [M. Huber, F. Mintert, A. Gabriel, B. C. Hiesmayr, Phys. Rev. Lett. 104, 210501 (2010)] to detect certain genuine $n$-partite entanglement between the communication parties. In particular, we present a simple inequality which tests the security.

We define a negative entanglement measure for separable states which shows how much entanglement one should compensate the unentangled state, at the least, to change it into an entangled state. For two-qubit systems and some special classes of states in higher-dimensional systems, the explicit formula and the lower bounds for the negative entanglement measure (NEM) have been presented, and it always vanishes for bipartite separable pure states. The negative entanglement measure can be used as a useful quantity to describe the entanglement dynamics and the quantum phase transition. In the transverse Ising model, the first derivatives of negative entanglement measure diverge on approaching the critical value of the quantum phase transition, although these two-site reduced density matrices have no entanglement at all. In the one-dimensional (1D) Bose-Hubbard model, the NEM as a function of $t/U$ changes from zero to negative on approaching the critical point of quantum phase transition.

We consider the problem of obtaining the scattering length for a fermion colliding with a dimer, formed from a fermion identical to the incident one and another different fermion. This is done in the universal regime where the range of interactions is short enough that the scattering length $a$ for nonidentical fermions is the only relevant quantity. This is the generalization to fermions with different masses of the problem solved long ago by Skorniakov and Ter-Martirosian for particles with equal masses. We solve this problem analytically in the two limiting cases where the mass of the solitary fermion is very large or very small compared to the mass of the two other identical fermions. This is done for both the value of the scattering length and the function entering the Skorniakov–Ter-Martirosian integral equation, for which simple explicit expressions are obtained.

An experimental technique for the measurement of the total cross section for electron scattering from atoms and molecules at high resolution is described. The total cross sections for electron scattering from Kr in the energy range from 14 meV to 20 eV obtained with the technique are also reported. The present technique employs a combination of the penetrating field technique and the threshold photoionization of rare-gas atoms using synchrotron radiation as an electron source in order to produce a high-resolution electron beam at very low energy. The characteristics of the electron sources were determined by measuring the ionizing photon energy dependence of photoelectron yield. Absolute total cross sections for electron scattering are obtained by the attenuation method. The measured absolute values of the total cross sections for electron scattering from Kr agree with those obtained by other groups down to 175 meV, above which several experimental works have been reported. Below 175 meV, the present results generally agree with theoretical cross sections down to 14 meV. The resonant structures in the total cross sections due to Kr${}^{-}$ ($4p^{5}5s^{2\hspace{0.5em}2}{P}_{3/2}$) and the Kr${}^{-}$ ($4p^{5}5s^{2\hspace{0.5em}2}{P}_{1/2}$) Feshbach resonances are also reported. The resolution of the present setup has been estimated from a fit of the measured profile of the Kr${}^{-}$ ($4p^{5}5s^{2\hspace{0.5em}2}{P}_{3/2}$) resonance by the theoretical curve obtained from the resonant scattering theory.

The magnetic field gradient has been measured with an atom interferometer using the magnetic sublevels of ${}^{87}\mathrm{Rb}$ atoms. The Doppler-insensitive measurement effectively eliminates the contribution from gravity and background vibration noise, and the differential measurement also can reject some systematic errors. A resolution of $300$ pT/mm has been demonstrated with a $90$-s integration time and a spatial resolution of $1.4$ mm. The gradiometer was then used to measure the magnetic field gradient in an ultrahigh-vacuum environment. The technique will also be very useful to subtract the systematic error arising from the magnetic field inhomogeneity in precision atom-interferometry experiments, such as gravity measurement.

The resolution of optical far-field microscopes is classically diffraction-limited to half the illumination wavelength. We show experimentally that this fundamental limit does not apply in the multiple scattering regime. We used tomographic diffractive microscopy at 633 nm to image two pairs of closely spaced rods (with a width and interdistance of 50 nm) of widely different diffractive properties. Using an inversion algorithm accounting for multiple scattering, only the pair of highly diffracting rods could be clearly visualized with a resolution similar to that of an atomic force microscope.