The nonclassical behavior of a two-level system coupled to a harmonic oscillator is investigated in the ultrastrong coupling regime. We revisit the variational solution of the ground state and find that the existing solutions do not account accurately for nonclassical effects such as squeezing. We suggest a trial wave function and demonstrate that it has an excellent accuracy for the quantum correlation effects as well as for the energy.

We propose the quantum simulation of the Dirac equation with potentials, allowing the study of relativistic scattering and Klein tunneling. This quantum relativistic effect permits a positive-energy Dirac particle to propagate through a repulsive potential via the population transfer to negative-energy components. We show how to engineer scalar, pseudoscalar, and other potentials in the $1+1$ Dirac equation by manipulating two trapped ions. The Dirac spinor is represented by the internal states of one ion, while its position and momentum are described by those of a collective motional mode. The second ion is used to build the desired potentials with high spatial resolution.

Drawing from the Bohmian formulation of the time-dependent Schrödinger equation, we present a self-consistent hydrodynamical method to describe electron dynamics in strong field light-matter interactions. Prototypical implementation is made for one-dimensional H atom embedded in short and intense laser pulses. The method provides very accurate electron densities and yields quantum trajectories that shed light on the electron dynamics, beyond the strong-field approximation.

We propose a scheme for generating Rashba spin-orbit coupling and a perpendicular Zeeman field simultaneously for cold fermionic atoms in a harmonic trap through the coupling between atoms and laser fields. The realization of Rashba spin-orbit coupling and a perpendicular Zeeman field provides opportunities for exploring many topological phenomena using cold fermionic atoms. We focus on the intrinsic anomalous Hall effect and show that it may be observed through the response of atomic density to a rotation of the harmonic trap.

We consider Casimir pressure on a slab in a configuration consisting of various dielectrics with planar symmetry. In such configurations, one usually calculates the Casimir pressure (force) on a particular slab assuming that lengths of all other slabs remain unchanged. Alternatively, one can consider a multilayer system with a fixed total length. With this restriction only, the length of each slab can eventually be changed under the Casimir pressure that will try to minimize the total Casimir energy of the system. Here we calculate the Casimir pressure on the slab in such a “constrained” configuration and compare the results with the standard approach. It turns out that, by applying different boundary conditions, one can obtain significantly different Casimir pressures on the same object. In particular, when the thicknesses of the slab and surrounding layers are on the nanometer scale, the Casimir pressure on the slab can change from strongly squeezing in the case of fixed thicknesses of surrounding slabs to strongly relaxing in the case when only the length of the total system remains fixed.

Radiative electron capture by highly-charged, heavy ions is re-investigated within the framework of the first-order perturbation theory and impulse approximation. In this study, we focus especially on the polarization properties of the emitted radiation and on the effects which arise from the binding of the target electrons. In order to explore such effects, detailed calculations have been carried out for relativistic collisions of bare uranium ${\mathrm{U}}^{92+}$ projectiles with neutral low- and medium-$Z$ atoms. From these calculations, it is shown that the linear polarization of K-REC (L-REC) photons [photons originating from radiative electron capture into the $K$ shell ($L$ shell)] can be very sensitive to the momentum distribution of bound electrons if the capture occurs selective for the different shells of the target. The most pronounced effect can be observed for collision studies as performed with well-defined impact parameters or upon suppression of the capture from the outer target shells.

The effect of the static polarization of the ${\mathrm{C}}_{60}$ fullerene shell by the $A^{+}$ ion remainder in response to photoionization of the atom $A$ encapsulated in ${\mathrm{C}}_{60}$ ($A@{\mathrm{C}}_{60}$) has been unraveled and found to be generally significant.

Our previously developed finite-element discrete-variable representation in prolate spheroidal coordinates is extended to two-electron systems with a study of double ionization of ${\mathrm{H}}_2$ with fixed nuclei. Particular attention is paid to the development of fast and accurate methods for treating the electron-electron interaction. The use of exterior complex scaling in the implementation offers a simple way of enforcing Coulomb boundary conditions for the electronic double continuum. While the angular distributions calculated in this study are found to be completely consistent with our earlier treatments that employed single-center expansions in spherical coordinates, we find that the magnitude of the integrated cross sections are sensitive to small changes in the initial-state wave function. The present formulation offers significant advantages with respect to convergence and efficiency and opens the way to calculations on more complicated diatomic targets.

We investigate the photon-assisted tunneling of a Bose-Einstein condensate in a periodically driven double-well potential. It is shown that the energy levels become nonequidistant due to the presence of the interatomic interaction, thereby leading to the occurrences of the fractional photon resonances. Depending on whether the resonance frequency shifts with the interatomic interaction, two distinct types of resonances are identified: nonshifting resonances and shifting resonances. In particular, certain nonshifting fractional resonances show an odd-even sensitivity to the atom number.

We study the dependency of the quantum spin dynamics on the particle number in a system of ultracold spin-1 atoms within the single-spatial-mode approximation. We find, for all strengths of the spin-dependent interaction, convergence toward the mean-field dynamics in the thermodynamic limit. The convergence is, however, particularly slow when the spin-changing collisional energy and the quadratic Zeeman energy are equal; that is, deviations between quantum and mean-field spin dynamics may be extremely large under these conditions. Our estimates show that quantum corrections to the mean-field dynamics may play a relevant role in experiments with spinor Bose-Einstein condensates. This is especially the case in the regime of few atoms, which may be accessible in optical lattices. Here, spin dynamics is modulated by a beat note at large magnetic fields due to the significant influence of correlated many-body spin states.

The theory of spontaneous emission presented by Weisskopf and Wigner [V. Weisskopf and E. Wigner, Z. Phys. 63, 54 (1930)] provides an excellent approximation of the actual decay that atoms undergo on optically allowed transitions. However, the theory cannot be rigorously correct since it leads to a Lorentzian spectrum that extends to negative frequencies. Within the rotating-wave approximation, we derive a closed-form expression for the spectrum that is valid for all frequencies.

We discuss a correlated two-photon imaging apparatus that is capable of producing images that are free of the effects of odd-order aberration introduced by the optical system. We show that both quantum-entangled and classically correlated light sources are capable of producing the desired spatial-aberration cancellation.

We have investigated the energy correlation of the two electrons from nonsequential double ionization of helium atoms in 800-nm laser fields at intensities below the recollision threshold by quantum calculations. The circular arcs structure of the correlated electron momentum spectra reveals a resonant double-ionization process in which the two electrons transit from doubly excited states into continuum states by simultaneously absorbing and sharing excess energy in integer units of the photon energy. The effectiveness of Coulomb repulsion plays a decisive role in controlling the electronic dynamics in continuum states and is responsible for the dominant back-to-back electron emission and two intensity-independent cutoffs in the two-electron energy spectra.

A simple set of algebraic equations is derived for the exact low-temperature thermodynamics of one-dimensional multicomponent strongly attractive fermionic atoms with enlarged SU$(N)$ spin symmetry and Zeeman splitting. Universal multicomponent Tomonaga-Luttinger liquid (TLL) phases are thus determined. For linear Zeeman splitting, the physics of the gapless phase at low temperatures belongs to the universality class of a two-component asymmetric TLL corresponding to spin-neutral $N$-atom composites and spin-$(N-1)/2$ single atoms. The equation of state which we obtained provides a precise description of multicomponent composite fermions and opens up the study of quantum criticality in one-dimensional systems of $N$-component Fermi gases with population imbalance.

Recent experiments have shown that nature violates noncontextual inequalities regardless of the state of the physical system. So far, all these inequalities involve measurements of dichotomic observables. We show that state-independent quantum contextuality can also be observed in the correlations between measurements of observables with genuinely continuous spectra, highlighting the universal character of the effect.

We demonstrate electromagnetic quantum states of single photons and of correlated photon pairs exhibiting “hybrid” entanglement between spin and orbital angular momentum. These states are obtained from entangled photon pairs emitted by spontaneous parametric down conversion by employing a $q$ plate for coupling the spin and orbital degrees of freedom of a photon. Entanglement and contextual quantum behavior (that is also nonlocal, in the case of photon pairs) is demonstrated by the reported violation of the Clauser-Horne-Shimony-Holt inequality. In addition, a classical analog of the hybrid spin-orbit photonic entanglement is reported and discussed.

The $I_{3322}$ inequality is the simplest bipartite two-outcome Bell inequality beyond the Clauser-Horne-Shimony-Holt (CHSH) inequality, consisting of three two-outcome measurements per party. In the case of the CHSH inequality the maximal quantum violation can already be attained with local two-dimensional quantum systems; however, there is no such evidence for the $I_{3322}$ inequality. In this paper a family of measurement operators and states is given which enables us to attain the maximum quantum value in an infinite-dimensional Hilbert space. Further, it is conjectured that our construction is optimal in the sense that measuring finite-dimensional quantum systems is not enough to achieve the true quantum maximum. We also describe an efficient iterative algorithm for computing quantum maximum of an arbitrary two-outcome Bell inequality in any given Hilbert space dimension. This algorithm played a key role in obtaining our results for the $I_{3322}$ inequality, and we also applied it to improve on our previous results concerning the maximum quantum violation of several bipartite two-outcome Bell inequalities with up to five settings per party.

Measurement devices could benefit from entangled correlations to yield a measurement sensitivity approaching the physical Heisenberg limit. Building upon previous magnetometric work using pseudoentangled spin states in solution-state NMR, we present two conceptual advancements to better prepare and interpret the pseudoentanglement resource. We apply these to a 13-spin cat state to measure the local magnetic field with a 12.2 sensitivity increase over an equivalent number of isolated spins.

X-ray absorption of atomic cesium is measured in the $L$-edge region, using a beryllium-window cell for cesium vapor. For comparison, absorption in Xe gas in the same energy region is remeasured with improved signal-to-noise ratio. By combining deconvolution and modeling, the edge profiles are studied to determine the threshold energies and the shape of the edge apex with exponential slope of the high-energy flank. In both elements, multielectron excitations show the same basic ordering in compact groups, largely independent of the core-hole subshell, following the energy sequence of coexcited valence and subvalence orbitals. The main effect of $6s$ electron in Cs, apart from $2(s,p)6s$ excitation, is the enhancement of single- as well as some multielectron resonant channels. The spectra of both elements show the “polarization effect”: a convex basic curvature of the $L_2$ and $L_3$ segments, a concave $L_1$ segment. Previously, Kutzner demonstrated a convincing theoretical explanation of the effect on Xe in a relativistic random-phase approximation with relaxation involving overlap integrals with continuum [Rad. Phys. Chem. 70, 95 (2004)].