A recent $S$-matrix-based QED calculation of energy levels of the lithium isoelectronic sequence is extended to the general case of a valence electron outside an arbitrary filled core. Emphasis is placed on modifications of the lithiumlike formulas required because more than one core state is present, and an unusual feature of the two-photon exchange contribution involving autoionizing states is discussed. The method is illustrated with a calculation of the energy levels of sodiumlike ions, with results for $3s_{1/2}, 3p_{1/2}$, and $3p_{3/2}$ energies tabulated for the range $Z=30–100$. Comparison with experiment and other calculations is given, and prospects for extension of the method to ions with more complex electronic structure discussed.

A method and a code for calculations of diatomic molecules in the external variable electromagnetic field have been developed. The code is applied for calculation of systematics in the electron's electric-dipole-moment search experiment on ThO $H^3{\mathrm{\Delta }}_1$ state related to geometric phases, including dependence on the $\mathrm{\Omega }$ doublet, rotational level, and external static electric field. It is found that systematics decrease cubically with respect to the frequency of the rotating transverse component of the electric field. Calculation confirms that experiment on ThO $H^3{\mathrm{\Delta }}_1$ state is very robust against systematic errors related to geometric phases.

We describe a collective state atomic clock (COSAC) with Ramsey fringes narrowed by a factor of $\sqrt{N}$ compared to a conventional clock—$N$ being the number of noninteracting atoms—without violating the uncertainty relation. This narrowing is explained as being due to interferences among the collective states, representing an effective $\sqrt{N}$-fold increase in the clock frequency, without entanglement. We discuss the experimental inhomogeneities that affect the signal and show that experimental parameters can be adjusted to produce a near ideal signal. The detection process collects fluorescence through stimulated Raman scattering of Stokes photons, which emits photons predominantly in the direction of the probe beam for a high enough optical density. By using a null measurement scheme, in which detection of zero photons corresponds to the system being in a single collective state, we detect the population in a collective state of interest. The quantum and classical noise of the ideal COSAC is still limited by the standard quantum limit and performs only as well as the conventional clock. However, when detection efficiency and collection efficiency are taken into account, the detection scheme of the COSAC increases the quantum efficiency of detection significantly in comparison to a typical conventional clock employing fluorescence detection, yielding a net improvement in stability by as much as a factor of 10.

We present Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. Without any spectral filtering, we observe a visibility of $V=62\pm 4\%$. After correcting for accidental coincidences, we obtain $V=93\pm 6\%$. The observed interference demonstrates the compatibility of the two sources, forming the basis for an efficient quantum interface between different physical systems.

Control over mechanical motion of nanoscale particles is a valuable functionality desired in a variety of multidisciplinary applications, e.g., biophysics, and it is usually achieved by employing optical forces. Hyperbolic metamaterials enable tailoring and enhancing electromagnetic scattering and, as the result, provide a platform for a new type of optical manipulation. Here optical pulling forces acting on a small particle placed inside a hyperbolic metamaterial slab were predicted and analyzed. In order to attract particles to a light source, highly confined extraordinary modes of hyperbolic metamaterial were excited via scattering from an imperfection situated at the slab's interface. This type of structured illumination together with remarkable scattering properties, inspired by the hyperbolic dispersion in the metamaterial, creates optical attraction. Forces acting on high-, low-index dielectric, and gold particles were investigated and it was shown that the pulling effect emerges in all of the cases. The ability to control mechanical motion at nanoscale using auxiliary photonic structures paves the way for investigation of various phenomena, e.g., biochemical reactions, molecular dynamics, and more.

The article addresses the possibility of entanglement-specific infrared-visible sum-frequency generation spectroscopy. In the case of an anisotropic interface, it is possible to employ SSS and PSS polarizations to detect responses not only specific to ${\chi }_{YYY}$ and ${\chi }_{XYY}$ nonlinearities, but also to higher-order ${\chi }_{\left(YYY\right)\left(XYY\right)}$ and ${\chi }_{\left(XYY\right)\left(YYY\right)}$ nonlinearities. Using quantum mechanical studies of the rhenium complex $\left[\mathrm{Re}{\left(\mathrm{OH}\right)}_3{\left(\mathrm{CO}\right)}_3\right]$ as a molecular model, we demonstrate that if such complexes would form an anisotropic orientational distribution at a surface, under the considered geometry and the polarization settings, we may prepare quantum correlated $\mathrm{C}=\mathrm{O}$ vibrational states to emit polarization-entangled photons. Accordingly, we explore the possibility of a polarization-measurement protocol to extract spectral signatures of the entangled states. The response would be informative on intramolecular interactions. As a result, we discuss the possible practical implications in probing dynamics at interfaces, and different opportunities in the preparation of entangled vibrational states of quantified fidelity.

Incompatible measurements in quantum theory always lead to Einstein-Podolsky-Rosen (EPR)-Schrödinger steering. Channel steering, which is a generalized notion of EPR-Schrödinger steering, has been introduced recently. Here we establish a connection between lack of joint measurability and channel steering.

Quantum chemical results for the lowest four potentials of the HeH molecule and the corresponding rotational and radial nonadiabatic coupling matrix elements are reported. Close-coupling calculations of the integral cross sections for the excitation processes $\mathrm{H}\left(1s\right)+\text{He}\to \mathrm{H}(2s,2p)+\text{He}$ are performed on this basis. The calculated cross sections are in a reasonable agreement with the experimental data.

We study three-body recombination in two dimensions for systems interacting via short-range two-body interactions in the regime of large scattering lengths. Using the adiabatic hyperspherical representation, we derive semianalytical formulas for three-body recombination in both weakly and deeply bound diatom states. Our results demonstrate the importance of long-range corrections to the three-body potentials by showing how they alter the low-energy and scattering-length dependence of the recombination rate for both bosonic and fermionic systems, which exhibit suppressed recombination if compared to the three-dimensional case. We verify these results through numerical calculations of recombination for systems with finite-range interactions and supporting a few two-body bound states. We also study finite-range effects for the energies of the universal three-identical-bosons states and find a slow approach to universal predictions as a function of the scattering length.

We propose a strategy of isotope-selective ionization for a binary mixture of isotopologues of homonuclear diatomic molecules, utilizing field-free alignment with a train of femtosecond laser pulses. Field-free alignment can be achieved simultaneously for two isotopologues consisting of two atoms with the same atomic mass number α or β $\left({}^{\alpha }{X}_2\mathrm{and}{}^{\beta }{X}_2\right)$, utilizing a pulse train with their time interval of $T_{\mathrm{com}}^{\mathrm{rev}}=\beta T_{\alpha }^{\mathrm{rev}}=\alpha T_{\beta }^{\mathrm{rev}}$, where $T_{\alpha }^{\mathrm{rev}}$ and $T_{\beta }^{\mathrm{rev}}$ are the rotational revival times of the isotopologues. We demonstrate experimentally that a train of four alignment pulses with their interval of $T_{\mathrm{com}}^{\mathrm{rev}}(\alpha =14,\beta =15)$ creates transiently aligned ${}^{14}{\mathrm{N}}_2$ and antialigned ${}^{15}{\mathrm{N}}_2$ just before $T_{\mathrm{com}}^{\mathrm{rev}}/2$ after the last alignment pulse and vice versa just after $T_{\mathrm{com}}^{\mathrm{rev}}/2$. Highly isotope-selective ${\mathrm{N}}_2$ ionization is achieved at these timings with another femtosecond laser pulse, which induces the nonresonant multiphoton ionization with the cross section remarkably depending on the angle between the molecular axis and the laser-electric-field direction. The ion yield ratio $I\left({}^{15}{N}_2^{+}\right)/I\left({}^{14}{N}_2^{+}\right)$ ranges from 0.49 to 2.00, which is wider than the range obtained with a single alignment pulse [H. Akagi et al., Appl. Phys. B 109, 75 (2012)].

In this paper, we explore the intensity-dependent strong-field double ionization of Ne with orthogonally polarized two-color laser pulses consisting of 800- and 400-nm laser fields. The yield of ${\text{Ne}}^{2+}$ as a function of the relative phase $\mathrm{\Delta }\varphi$ of the two colors experiences a qualitative transition as the laser intensity decreases from the saturation regime to the far-below-saturation regime. In the saturation regime, our simulations well reproduce the recent experimental observations [Phys. Rev. Lett. 112, 193002 (2014)]. Turning to the far-below-saturation regime, however, we find that the observed small knee structure totally disappears and the maximum yield of ${\text{Ne}}^{2+}$ is shifted by a $\pi /2$ phase. This is explained by the competition between the trajectory concentration effect and the $\mathrm{\Delta }\varphi$-dependent ionization rate of the tunneling electron. The possibility of controlling over the two-electron emission direction along the 400-nm field through the laser intensity is also investigated. We show that the two-electron emission direction can be reversed by changing the laser intensity for some vales of $\mathrm{\Delta }\varphi$, while this fails for some other values of $\mathrm{\Delta }\varphi$.

Recent advances in realizing artificial gauge fields on optical lattices promise experimental detection of topologically nontrivial energy spectra. Self-similar fractal energy structures generally known as Hofstadter butterflies depend sensitively on the geometry of the underlying lattice, as well as the applied magnetic field. The recent demonstration of an adjustable lattice geometry [L. Tarruell, D. Greif, T. Uehlinger, G. Jotzu, and T. Esslinger, Nature (London) 483, 302 (2012)] presents a unique opportunity to study this dependence. In this paper, we calculate the Hofstadter butterflies that can be obtained in such an adjustable lattice and find three qualitatively different regimes. We show that the existence of Dirac points at zero magnetic field does not imply the topological equivalence of spectra at finite field. As the real-space structure evolves from the checkerboard lattice to the honeycomb lattice, two square-lattice Hofstadter butterflies merge to form a honeycomb lattice butterfly. This merging is topologically nontrivial, as it is accomplished by sequential closings of gaps. Ensuing Chern number transfer between the bands can be probed with the adjustable lattice experiments. We also calculate the Chern numbers of the gaps for qualitatively different spectra and discuss the evolution of topological properties with underlying lattice geometry.

A tunable double optomechanically induced transparency (OMIT) with a squeezed field is investigated in a system consisting of an optomechanical cavity coupled to a charged nanomechanical resonator via Coulomb interaction. Such a double OMIT can be achieved by adjusting the strength of the Coulomb interaction and can be observed even with a single-photon squeezed field at finite temperature. Since it is robust against cavity decay but very sensitive to some parameters, such as the environmental temperature, the model under consideration can be applied as a quantum thermometer for precision measurement of the environmental temperature within the reach of current techniques.

In this work we study numerically and analytically the interaction of two qubits in a one-dimensional waveguide, as mediated by the photons that propagate through the guide. We develop strategies to assert the Markovianity of the problem, the effective qubit-qubit interactions, and their individual and collective spontaneous emission. We prove the existence of collective Lamb shifts that affect the qubit-qubit interactions and the dependency of coherent and incoherent interactions on the qubit separation. We also develop the scattering theory associated with these models and prove single-photon spectroscopy does probe the renormalized resonances of the single- and multiqubit models, in sharp contrast to earlier toy models in which individual and collective Lamb shifts cancel.

Photon absorption spectroscopy is a powerful tool for uncovering the structure of atoms, molecules, and solids. Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures related to the structural and dynamical properties. Recently, Ott et al. [Science 340, 716 (2013)] successfully transferred Fano profile into Lorentzian line shape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward in quantum phase control. However, here we show experimentally that an autoionizing state can have both Fano and Lorentzian behavior naturally, depending on the process involved. This study utilized the inverse process of photon absorption ionization, i.e., electron ion recombination with photon emission, making sure the resonant autoionizing state is not modified or decorated by the laser fields. Our result implies that excitation of the state through different paths—for example, one photon versus multiphoton excitation, or even one step versus multistep excitation—can lead to different Fano profiles for the same resonant state. We also report an experimental determination of the energy shifts in the recombination photon-intensity peaks due to the interference between the resonant and nonresonant processes.

Quantum paradoxes show that quantum statistics can exceed the limits of positive joint probabilities for physical properties that cannot be measured jointly. It is therefore impossible to describe the relations between the different physical properties of a quantum system by assigning joint realities to their observable values. Instead, recent experimental results obtained by weak measurements suggest that nonclassical correlations could be expressed by complex valued quasiprobabilities, where the phases of the complex probabilities express the action of transformations between the noncommuting properties [H. F. Hofmann, New J. Phys. 13, 103009 (2011)]. In these relations, negative probabilities necessarily emerge whenever the physical properties involved are related to each other by half-periodic transformations, since such transformations are characterized by action phases of $\pi$ in their complex probabilities. It is therefore possible to trace the failure of realist assumptions back to a fundamental and universally valid relation between statistics and dynamics that associates half-periodic transformations with negative probabilities.

We determine and analyze the quantum phases and time delays in photoionization and photorecombination of valence $3p$ and $3s$ electrons of argon using the Kohn-Sham local-density-functional approach. The time-dependent local-density approximation is used to account for the electron correlation. Resulting attosecond Wigner-Smith time delays show very good agreement with the recent experiment on argon that measured the delay in $3p$ photorecombination [S. B. Schoun et al., Phys. Rev. Lett. 112, 153002 (2014)].

An understanding of the possible ways in which interactions can produce fundamentally new emergent many-body states is a central problem of condensed-matter physics. We ask if a Fermi sea can arise in a system of bosons subject to contact interaction. Based on exact diagonalization studies and variational wave functions, we predict that such a state is likely to occur when a system of two-component bosons in two dimensions, interacting via a species-independent contact interaction, is exposed to a synthetic magnetic field of strength that corresponds to a filling factor of unity. The fermions forming the SU(2) singlet Fermi sea are bound states of bosons and quantized vortices, formed as a result of the repulsive interaction between bosons in the lowest Landau level.