We have employed the $R$-matrix method to compute elastic (integrated and differential), momentum-transfer, excitation, and ionization cross sections for electron impact on the NH radical. The target states are represented by including correlations via a configuration interaction technique. The results of the static exchange, correlated one-state configuration interaction, and 19-state close-coupling approximation are presented. We have detected a stable anionic bound state ${}^2\Pi$ of NH${}^{-}$ having the configuration $1\sigma {}^{2}2\sigma {}^{2}3\sigma {}^{2}1\pi {}^3$. The data of the momentum-transfer cross section, generated from the differential cross section, is used to compute effective collision frequencies over a wide electron temperature range ($200$–$30$ $000$ $\mathrm{K}$). The ionization cross sections are calculated in the binary-encounter Bethe model in which Hartree-Fock molecular orbitals at self-consistent levels are used to calculate kinetic and binding energies of the occupied molecular orbitals. We have included up to the $g$ partial wave ($l=4$) in the scattering calculations. For this dipolar molecule we have used a Born-closure top-up procedure to account for the higher partial waves for the convergence of the cross section.

We experimentally demonstrate resonant symmetry lifting in a parametrically modulated magneto-optical trap of cold ${}^{85}\mathrm{Rb}$ atoms. This is achieved by applying a weak additional modulation at half the frequency of the strong parametric modulation, which acts as an effective static bias field to the system. We measure the system response by varying the amplitude of the additional fictitious bias as well as the relative phase between the bias and the parametric drive, and the results are in good agreement with theory. The additional modulation provides an additional degree of freedom to control the system, which is useful for investigating system properties such as susceptibility, dynamic response, and related critical phenomena. We also have measured the amplitude of the response to higher harmonics of the additional modulation frequency, which allows more precise understanding of the system dynamics.

Single-photon double $K$-shell ionization of low-$Z$ neutral atoms in the range $12⩽Z⩽23$ is investigated. The experimental method was based on measurements of the high-resolution $K\alpha {}^h$ hypersatellite x-ray spectra following the radiative decay of the $K$-shell double-vacancy states excited by monochromatic synchrotron radiation. The photon energy dependence of the double $K$-shell ionization was measured over a wide range of photon energies from threshold up to and beyond the maximum of the double-to-single photoionization cross section ratios. From the high-resolution x-ray emission spectra the energies and linewidths of the hypersatellite transitions, as well as the $K{\alpha }_1^h$:$K{\alpha }_2^h$ intensity ratios, were determined. The relative importance of the initial-state and final-state electron-electron interactions to the $K$-shell double photoionization is addressed. Physical mechanisms and scaling laws of the $K$-shell double photoionization are examined. A semiempirical universal scaling of the double-photoionization cross sections with the effective nuclear charge for neutral atoms in the range $2⩽Z⩽47$ is established.

We explore pulsed-field control of resonant dipole-dipole interactions between K Rydberg atoms. A laser-based atomic beam chopper is used to reduce the relative velocities of Rydberg atoms excited from an effusive thermal source. Resonant energy transfer (RET) between pairs of atoms is controlled via Stark tuning of the relevant Rydberg energy levels. Resonance line shapes in the electric field dependence of the RET probability are used to determine the effective temperature of the sample. We demonstrate that the relative atom velocities can be reduced to the point where the duration of the electric-field tuning pulses, and not the motion of neighboring atoms, defines the interaction time for each pair within the ensemble. Coherent, transform-limited broadening of the resonance line shape is observed as the tuning pulse duration is reduced below the natural time scale for collisions.

We study the effect of interaction on the temperature change in the process of adiabatic mixing of two components of Fermi gases using the real-space Bogoliubov–de Gennes method. We find that in the process of adiabatic mixing, the competition between the adiabatic expansion and the attractive interaction makes it possible to cool or heat the system depending on the strength of the interaction and the initial temperature of the system. The changes of the temperature in a bulk system and in a trapped system are investigated.

We present the full expression for energy conservation for partially coherent electromagnetic fields from the randomly fluctuating and statistically stationary magnetic as well as electric sources. While the polarization gives the dominant effect in most realistic situations, magnetization could also give a non-negligible effect in various physical situations where the magnetic field is strong. The formula we derive contains terms not only from electric and magnetic sources alone but also from the interference between them. We also confirm that this conservation is valid for correlation-induced spectral changes as was proved in the previous study for the electric source alone.

The scalar-complex-source model is used to develop an accurate description of highly focused radially, azimuthally, linearly, and circularly polarized monochromatic vector beams. We investigate the power and full beam widths at half maximum of vigorous Maxwell equation solutions. The analytical expressions are employed to compare the vector complex source beams with the real beams produced by various high-numerical-aperture (NA) focusing systems. We find a parameter set for which the spatial extents of the analytical beams are the same as those of experimentally realized ones. We ensure the same shape of the considered beams by investigating an overlap of the complex source beams with high-NA beams. We demonstrate that the analytical expressions are good approximations for realistic highly focused beams.

We illustrate how to get the geometric phase from eigenfunctions in the vicinity of an exceptional point in a dielectric microcavity whose non-Hermitian character comes from the outgoing-wave boundary condition. It is shown that the geometrical phase $\pm \pi$ can be obtained either from total variation of the inner product of eigenfunctions or from a continuous change of phase plot, not of intensity plot, during a double cyclic parameter variation encircling the exceptional point. One can use either of the two ways by properly choosing the arbitrary phase of the calculated eigenfunctions.

Theoretical predictions for the dynamic structure factor of a harmonically trapped Fermi superfluid near the Bose-Einstein condensate–Bardeen-Cooper-Schrieffer (BEC-BCS) crossover are compared with recent Bragg spectroscopy measurements at large transferred momenta. The calculations are based on a random-phase (or time-dependent Hartree-Fock-Gorkov) approximation generalized to the strongly interacting regime. Excellent agreement with experimental spectra at low temperatures is obtained, with no free parameters. Theoretical predictions for zero-temperature static structure factor are also found to agree well with the experimental results and independent theoretical calculations based on the exact Tan relations. The temperature dependence of the structure factors at unitarity is predicted.

A closed-form finite conductivity correction factor for the ideal Casimir force is proposed, based on exponential basis functions. Our method can facilitate experimental verifications of theories in the study of the Casimir force. A theoretical analysis is given to explain why our method is accurate at both large and small separation gaps. Numerical computations have been performed to confirm that our method is accurate in various experimental configurations. Our approach is widely applicable to various Casimir force interactions between metals and dielectrics. Our study can be extended to the study of the repulsive Casimir force as well.

We investigate the low-temperature thermodynamics of the unitary Fermi gas by introducing a model based on the zero-temperature spectra of both bosonic collective modes and fermonic single-particle excitations. We calculate the Helmholtz free energy and from it we obtain the entropy, the internal energy, and the chemical potential as a function of the temperature. By using these quantities and the Landau’s expression for the superfluid density we determine analytically the superfluid fraction, the critical temperature, the first sound velocity, and the second sound velocity. We compare our analytical results with other theoretical predictions and experimental data of ultracold atoms and dilute neutron matter.

We discuss the possibility of exponential quantum localization in systems of ultracold bosonic atoms with repulsive interactions in open optical lattices without disorder. We show that exponential localization occurs in the maximally excited state of the lowest energy band. We establish the conditions under which the presence of the upper energy bands can be neglected, determine the successive stages and the quantum phase boundaries at which localization occurs, and discuss schemes to detect it experimentally by visibility measurements. The discussed mechanism is a particular type of quantum localization that is intuitively understood in terms of the interplay between nonlinearity and a bounded energy spectrum.

We present a modal approach to calculate finite-temperature Casimir interactions between two periodically modulated surfaces. The scattering formula is used and the reflection matrices of the patterned surfaces are calculated by decomposing the electromagnetic field into the natural modes of the structures. The Casimir force gradient from a deeply etched silicon grating is evaluated using the modal approach and compared to experiment for validation. The Casimir force involving a two-dimensional periodic structure is computed and deviations from the proximity force approximation are examined.

We introduce an alternative way to derive the generalized form of the master equation recently presented by J. P. Pekola et al. [Phys. Rev. Lett. 105, 030401 (2010)] for an adiabatically steered two-level quantum system interacting with a Markovian environment. The original derivation employed the effective Hamiltonian in the adiabatic basis with the standard interaction picture approach but without the usual secular approximation. Our approach is based on utilizing a master equation for a nonsteered system in the first superadiabatic basis. It is potentially efficient in obtaining higher-order equations. Furthermore, we show how to select the phases of the adiabatic eigenstates to minimize the local adiabatic parameter and how this selection leads to states which are invariant under a local gauge change. We also discuss the effects of the adiabatic noncyclic geometric phase on the master equation.

We investigate the detection of entanglement in $n$-partite quantum states. We obtain practical separability criteria to identify genuinely entangled and nonseparable mixed quantum states. No numerical optimization or eigenvalue evaluation is needed, and our criteria can be evaluated by simple computations involving components of the density matrix. We provide examples in which our criteria perform better than all known separability criteria. Specifically, we are able to detect genuine $n$-partite entanglement which has previously not been identified. In addition, our criteria can be used in today’s experiments.

In this paper we consider the quantum phase transition in the Ising model in the presence of a transverse field in one, two, and three dimensions from a multipartite entanglement point of view. Using exact numerical solutions, we are able to study such systems up to 25 qubits. The Meyer-Wallach measure of global entanglement is used to study the critical behavior of this model. The transition we consider is between a symmetric Greenberger-Horne-Zeilinger–like state to a paramagnetic product state. We find that global entanglement serves as a good indicator of quantum phase transition with interesting scaling behavior. We use finite-size scaling to extract the critical point as well as some critical exponents for the one- and two-dimensional models. Our results indicate that such multipartite measure of global entanglement shows universal features regardless of dimension $d$. Our results also provide evidence that multipartite entanglement is better suited for the study of quantum phase transitions than the much-studied bipartite measures.

We investigate the physically allowed probabilities for transforming one $N$-partite $W$-class state to another by means of local operations assisted with classical communication. Recently,   S. Kintaş and S. Turgut [J. Math. Phys. 51, 092202 (2010)] obtained an upper bound for the maximum probability of transforming two such states. Here, we provide a simple sufficient and necessary condition for when this upper bound can be satisfied and, thus, when optimality of state transformation can be achieved. Our discussion involves obtaining lower bounds for the transformation of arbitrary $W$-class states and showing precisely when this bound saturates the bound of Kintaş and Turgut. Finally, we consider the question of transforming symmetric $W$-class states and find that, in general, the optimal one-shot procedure for converting two symmetric states requires a nonsymmetric filter by all the parties.

We present a unified formalism for threshold quantum secret sharing using graph states of systems with prime dimension. We construct protocols for three varieties of secret sharing: with classical and quantum secrets shared between parties over both classical and quantum channels.

We present a detailed theoretical analysis for the distillation of one copy of a mixed two-mode continuous-variable entangled state using beam splitters and coherent photon-detection techniques, including conventional on-off detectors and photon-number-resolving detectors. The initial Gaussian mixed-entangled states are generated by transmitting a two-mode squeezed state through a lossy bosonic channel, corresponding to the primary source of errors in current approaches to optical quantum communication. We provide explicit formulas to calculate the entanglement in terms of logarithmic negativity before and after distillation, including losses in the channel and the photon detection, and show that one-copy distillation is still possible even for losses near the typical fiber channel attenuation length. A lower bound for the transmission coefficient of the photon-subtraction beam splitter is derived, representing the minimal value that still allows to enhance the entanglement.

We report on two-center convergent close-coupling calculations of positron-lithium collisions. The target is treated as one active electron interacting with an inert ion core. The positronium formation channels are taken into account explicitly utilizing both negative- and positive-energy Laguerre-based states. A large number of channels and high partial waves are used to ensure the convergence of the cross sections. We find the Ramsauer-Townsend minimum in total and elastic cross sections at an impact energy $E$ of about 0.0016 eV. As found previously for H and He, the contributions to the breakup cross section from both the Li and the Ps centers become the same as the threshold is approached.