We introduce a scheme that combines phonon-mediated quantum logic gates in trapped ions with the benefits of continuous dynamical decoupling. We demonstrate theoretically that a strong driving of the qubit decouples it from external magnetic-field noise, enhancing the fidelity of two-qubit quantum gates. Moreover, the scheme does not require ground-state cooling, and is inherently robust to undesired ac Stark shifts. The underlying mechanism can be extended to a variety of other systems where a strong driving protects the quantum coherence of the qubits without compromising the two-qubit couplings.

In experiments searching for a nonzero electric dipole moment of trapped particles, frequency shifts correlated with an applied electric field can be interpreted as a false signal. One such effect, referred to as the geometric phase effect, is known to occur in a magnetic field that is nonperfectly homogeneous. The increase in sensitivity of experiments demands improved theoretical description of this effect. In the case of fast particles, like atoms at room temperature and low pressure, the validity of established theories was limited to a cylindrical confinement cell in a uniform gradient with cylindrical symmetry. We develop a more general theory valid for an arbitrary shape of the magnetic field as well as for arbitrary geometry of the confinement cell. Our improved theory is especially relevant for experiments measuring the neutron electric dipole moment with an atomic comagnetometer. In this context, we have reproduced and extended earlier numerical studies of the geometric phase effect induced by localized magnetic impurities.

We present two two-step practical entanglement concentration protocols for concentrating an arbitrary three-particle less-entangled $W$ state into a maximally entangled $W$ state assisted with single photons. The first protocol uses the linear optics and the second protocol adopts the cross–Kerr nonlinearity to perform the protocol. In the first protocol, based on the postselection principle, three parties say Alice, Bob, and Charlie in different distant locations can obtain the maximally entangled $W$ state from the arbitrary less-entangled $W$ state with a certain success probability. In the second protocol, the parties are not required to possess the sophisticated single-photon detectors and the concentrated photon pair can be retained after performing this protocol successfully. Moreover, the second protocol can be repeated to get a higher success probability. Both protocols may be useful in practical quantum-information applications.

We investigate how the interplay between a staggered magnetic field and staggered coupling strength affects both ground-state and thermal entanglement. Upon analytically calculating thermodynamic quantities and the correlation functions for such a system, we consider both the global Meyer-Wallach measure of entanglement and the concurrence between pairs of spins. We discover two quantum critical points present in the model and show that the behavior of entanglement at zero temperature reflects each. We discover that increasing the alternating field and alternating coupling strength can actually increase the amount of entanglement present both at zero temperature and for thermal states of the system.

The ${}^q{R}_0\left(0\right)$ rotational transition in the $\tilde{A}{}^2{B}_2\leftarrow \tilde{X}{}^2{A}_1$ system of jet-cooled ${\mathit{NO}}_2$ located around $12536{\mathrm{cm}}^{-1}$ is analyzed using a nonlinear-susceptibility formalism designed to describe the saturated absorption due to two identical counter-propagating radiations in an $n$-level system. An analytical solution of the equations of motion is obtained in the frequency space by considering the pertinent experimental conditions, mainly a high-finesse cavity and a slit-shaped supersonic expansion. Calculation of the nonlinear absorption coefficient requires the summing of all Zeeman-component contributions and a final numerical integration over the frequency detuning assuming a Maxwell-Boltzmann speed distribution. Determination of the experimental absorption coefficients is obtained by converting the shape of the temporal decay of the electromagnetic field amplitude initially captured inside the cavity. The molecular Hamiltonian includes both spin-rotation and hyperfine interactions. Molecular constants relative to the upper level are derived by exploiting Doppler-broadening-free so-called saturated-absorption cavity-ring-down spectroscopy. The dipole moment of the partially assigned hot band is obtained [${\mu }_{\mathrm{band}}=0.0047\left(12\right)$ D] together with the number density and the effective population relaxation rates. The model is validated by varying the intracavity power from 0 to 230 W (i.e., up to a maximum peak irradiance of $240\times {10}^3\mathrm{W}/{\mathrm{cm}}^2$), representing saturation coefficients up to 120. The experimental position, shape, and width of the Lamb and crossover dips are well reproduced. The spatial shape and modulation of the electromagnetic field are discussed.

Efficient self-interference of single photons emitted by a sideband-cooled barium ion is demonstrated. First, the technical tools for performing efficient coupling to the quadrupolar transition of a single ${}^{138}$Ba${}^{+}$ ion are presented. We show efficient Rabi oscillations of the internal state of the ion by using a highly stabilized 1.76-$\mu$m fiber laser resonant with the $S_{1/2}$-$D_{5/2}$ transition. We then show sideband cooling of the ion's motional modes and use it as a means to enhance the interference contrast of the ion with its mirror image to up to 90$\%$. Last, we measure the dependence of the self-interference contrast on the mean phonon number, thereby demonstrating the potential of the setup for single-atom thermometry close to the motional ground state.

An effect is demonstrated where off-resonant optical pumping is enhanced by spin-exchange collisions, which enables a high degree of spin orientation to be obtained in alkali-metal atoms. The experiment with room-temperature cesium vapor employs radio-frequency spectroscopy under conditions of the nonlinear Zeeman effect that allow the populations of the individual Zeeman sublevels to be determined. The improved efficiency of atomic orientation leads to a significant reduction of population relaxation out of the state with maximum magnetic quantum number $m$ (stretched state), which is observed as a narrowing of radio-frequency resonances. Spin-exchange collisions play a crucial role in this effect although the applied atomic density $\sim$10${}^{11}$ cm${}^{-3}$ corresponds to collision rates not greater than several tens of Hz. A simplified theoretical model has been developed to reproduce the measurement results numerically.

The spectrum of the two-photon Rabi Hamiltonian is found, proceeding in full analogy with the solution of the standard (one-photon) Rabi Hamiltonian, published by D. Braak [Phys. Rev. Lett. 107, 100401 (2011)]. The Hamiltonian is rewritten as a set of two differential equations. Symmetries that get hidden after further treatment are found. One can plainly see how the Hilbert space splits into four disjunct subspaces, categorized by four values of the symmetry parameter $c=\pm 1,\pm i$. There were only two values, $\pm 1$, for the standard Rabi model. The values of $c$ are in fact the eigenvalues of the respective parity operators. Four analytic functions are introduced by a recurrence scheme for the coefficients of their series expansion. All their roots yield the complete spectrum of the Hamiltonian. Eigenstates in Bargmann space can also be found.

We theoretically predict efficient optical second-harmonic generation (SHG) from a micrometer-thick slab consisting of a quadratic nonlinear anisotropic medium whose linear principal permittivities have, at the fundamental wavelength, real parts of different signs (indefinite permittivity) and magnitude smaller than 1. We show that, by illuminating the slab with a $p$-polarized fundamental wave (with intensity of a few $\mathit{MW}/{\mathrm{cm}}^2$), highly efficient scattering of the second-harmonic field occurs in conditions at which the slab is linearly fully transparent for the fundamental wave. The high efficiency of the SHG process stems from the enhancement of the longitudinal field, perpendicular to the slab surface, produced by the very small value of the slab dielectric permittivities. We investigate the role played by medium losses, showing that, even in the strong-absorption regime, the described process yields a second-harmonic field which is much stronger than that produced by a standard (not indefinite) nonlinear slab.

The terahertz transmission properties of symmetrical and asymmetrical annular apertures arrays (AAAs) are investigated both experimentally and numerically. It is found that only odd-order resonant modes are observed for the symmetrical structures but both odd- and even-order resonances can be shown for the asymmetrical structures. Breaking the symmetry of AAAs by gradually displacing the H-shaped AAAs with U-shaped AAAs allows an intensity modulation depth of $98.7\%$ of the second-order resonance. Simulation results verify the experimental conclusions well. This result provides a tremendous opportunity for terahertz-wavelength tunable filtering, sensing, and near-field imaging.

We analyze a family of measures of general quantum correlations for composite systems, defined in terms of the bipartite entanglement necessarily created between systems and apparatuses during local measurements. For every entanglement monotone $E$, this operational correspondence provides a different measure $Q_E$ of quantum correlations. Examples of such measures are the relative entropy of quantumness, the quantum deficit, and the negativity of quantumness. In general, we prove that any so-defined quantum correlation measure is always greater than (or equal to) the corresponding entanglement between the subsystems, $Q_E\ge E$, for arbitrary states of composite quantum systems. We analyze qualitatively and quantitatively the flow of correlations in iterated measurements, showing that general quantum correlations and entanglement can never decrease along von Neumann chains, and that genuine multipartite entanglement in the initial state of the observed system always gives rise to genuine multipartite entanglement among all subsystems and all measurement apparatuses at any level in the chain. Our results provide a comprehensive framework to understand and quantify general quantum correlations in multipartite states.

Macroscopic behavior such as the lack of interference patterns has been attributed to “decoherence,” a word with several possible definitions such as (1) the loss of off-diagonal density matrix elements, (2) the flow of information to the environment, (3) the loss of complementary information, and (4) the loss of the ability to create entanglement in a measurement. In this article, we attempt to unify these distinct definitions by providing general quantitative connections between them, valid for all finite-dimensional quantum systems or quantum processes. The most important application of our results is to the understanding of quantum discord, a measure of the nonclassicality of the correlations between quantum systems. We show that some popular measures of discord measure the information missing from the purifying system and hence quantify security, which can be stated operationally in terms of distillable secure bits. The results also give some strategies for constructing discord measures.

Quasiclassical correction to the differential cross section of high-energy electron-positron photoproduction in the electric field of a heavy atom is obtained with the exact account of the field. This correction is responsible for the charge asymmetry $\mathcal{A}$ in this process. When the transverse momentum of at least one of the produced particles is much larger than the electron mass $m$, the charge asymmetry can be as large as tens percent. We also estimate the contribution $\tilde{\mathcal{A}}$ to the charge asymmetry coming from the Compton-type diagram. For heavy nuclei, this contribution is negligible. For light nuclei, $\tilde{\mathcal{A}}$ is noticeable only when the angle between the momenta of the electron and positron is of order of $m/\omega$ ($\omega$ is the photon energy) while the transverse momenta of both particles are much larger than $m$.

Energies, lifetimes, and wave-function compositions have been computed for all levels of $4p^{6}4d,4p^{6}4f,$ and $4p^{5}4d^2$ using single and double excitations from a multireference set (SD-MR) to generate expansions for the multiconfiguration Dirac-Hartree-Fock (MCDHF) approximation. An extended version of the general relativistic atomic structure package, grasp2K, was used to deal with configuration state functions with as many as six open shells and with configurations containing as many as three $f$ electrons. $E$1, $E$2, $M$1 transition probabilities are reported for transitions between the levels as supplemental material. Results are compared with other theory and with experiment, when available.

We consider a Dirac one-electron atom placed in a weak, static, uniform magnetic field. We show that, to the first order in the strength $B$ of the perturbing field, the only electric multipole moment induced by the field in the ground state of the atom is the quadrupole one. Using the Sturmian expansion of the generalized Dirac-Coulomb Green function [Szmytkowski, J. Phys. B 30, 825 (1997); Szmytkowski, J. Phys. B 30, 2747(E) (1997)], we derive a closed-form expression for an induced electric quadrupole moment. The result contains the generalized hypergeometric function ${}_3{F}_2$ of the unit argument. Earlier calculations by other authors, based on a nonrelativistic model of the atom, predicted in the low-field region the quadratic dependence of the induced electric quadrupole moment on $B$.

We analyze $d$-wave resonances in atom-atom scattering in the presence of harmonic confinement by employing a higher-partial-wave pseudopotential. Analytical results for the scattering amplitude and transmission are obtained and compared to corresponding numerical ones, which employ the Lennard-Jones potential. Qualitative agreement is observed for weak confinement. For strong confinement the pseudopotential does not capture the $s$- and $d$-wave interference phenomena, yielding an asymmetric Fano profile for the transmission resonance.

It is a common perception that evanescent waves do not contribute to the far field. On scattering, however, the evanescent incident field may contribute to both the homogeneous and the evanescent scattered waves. With the method of angular spectrum representation of plane waves, we calculate the ratio between the contributions of the evanescent and homogeneous incident waves to the scattered far field for a δ-correlated scattering medium. The result may be applicable to the scattered near field of the scatterer as well as to the intermediate and far zones.

We consider the dynamics of atomic and field coherent states in the nonresonant Dicke model. At weak coupling an initial product state evolves into a superposition of multiple field coherent states that are correlated with the atomic configuration. This process is accompanied by the buildup and decay of atom-field entanglement and leads to the periodic collapse and revival of Rabi oscillations. We provide a perturbative derivation of the underlying dynamical mechanism that complements the rotating wave approximation at resonance. The identification of two different time scales explains how the dynamical signatures depend on the sign of detuning between the atomic and field frequency and predicts the generation of either atomic or field cat states in the two opposite cases. We finally discuss the restrictions that the buildup of atom-field entanglement during the collapse of Rabi oscillations impose on the validity of semiclassical approximations that neglect entanglement.

We investigate surface plasmon polariton waves at the interface between metals or metamaterials and gyrotropic dielectrics. Due to gyrotropy, the surface wave changes its polarization, propagation constant, and transverse profile. With the increase of gyrotropy the localization decreases leading to the existence of the critical value of the gyrotropic coefficient above which there are no localized surface-plasmons at the interface. A method of experimental investigation of gyrotropic films based on the surface-plasmon excitation is proposed.

In this Brief Report an improved u(2) algebraic model is proposed to study both stretching and bending vibrational spectra of a bent triatomic molecule. The model with fewer parameters is used to reproduce the observed spectra of hydrogen sulfide (H${}_2$S) with better precision than the u(4) algebraic model. Furthermore, the tripartite entanglement dynamics is investigated for vibrationally localized states in H${}_2$S. It is shown that the entanglement of a highly excited state in the bending mode displays better quasiperiodicity than that in the stretch. Those are useful for molecular vibrations and multipartite quantum entanglement.