We propose a design of an all-optical transistor based on a one-dimensional photonic-crystal cavity doped with a four-level $\mathsf{N}$-type active Raman gain medium. The calculated results show that in a photonic-crystal cavity of this kind transmission and reflection of the probe (Raman) beam are strongly dependent on the optical switching power. Transmission and reflection of the probe beam can be greatly amplified or attenuated. Therefore the optical switching field can serve as a gate field of the transistor to effectively control propagation of the weak probe field. It is shown that the group velocity of the probe pulse can be controlled in the range from subluminal (slow light) to superluminal (fast light).

High-order harmonic generation (HHG) in simultaneous intense near-infrared (NIR) laser light and brilliant x rays above an inner-shell absorption edge is examined. A tightly bound inner-shell electron is transferred into the continuum. Then, NIR light takes over and drives the liberated electron through the continuum until it eventually returns to the cation leading in some cases to recombination and emission of a high-order harmonic (HH) photon that is upshifted by the x-ray photon energy. We develop a theory of this scenario and apply it to $1s$ electrons of neon atoms. The boosted HH light is used to generate a single attosecond pulse in the kiloelectronvolt regime. Prospects for nonlinear x-ray physics and HHG-based spectroscopy involving core orbitals are discussed.

In this paper we present and discuss the relativistic correlation function in a bipartite system of two electrons, originating from the $e^{-}{e}^{-}e^{-}{e}^{-}$ scattering of a polarized electron beam on an unpolarized target. We also calculate and investigate the probabilities of the definite outcomes of spin-projection measurements performed by two observers. The presented results might help in experimentally verifying whether relativistic quantum theory is able to reproduce the behavior of real quantum systems.

It is well known that any given density $\rho \left(\mathbf{x}\right)$ can be realized by a determinantal wave function for $N$ particles. The question addressed here is whether any given density $\rho \left(\mathbf{x}\right)$ and current density $\mathbf{j}\left(\mathbf{x}\right)$ can be simultaneously realized by a (finite kinetic energy) determinantal wave function. In case the velocity field $\mathbf{v}\left(\mathbf{x}\right)=\mathbf{j}\left(\mathbf{x}\right)/\rho \left(\mathbf{x}\right)$ is curlfree, we provide a solution for all $N$, and we provide an explicit upper bound for the energy. If the velocity field is not curl-free, there is a finite energy solution for all $N\ge 4$, but we do not provide an explicit energy bound in this case. For $N=2$ we provide an example of a non-curl-free velocity field for which there is a solution and an example for which there is no solution. The case $N=3$ with a non-curl-free velocity field is left open.

We computed the long-range interactions between two identical polar bialkali molecules in their rovibronic ground level for all ten species involving Li, Na, K, Rb, and Cs, using accurate quantum chemistry results combined with available spectroscopic data. A huge van der Waals interaction is found for eight species in free space. The competition of the van der Waals interaction with the dipole-dipole interaction induced by an external electric field parallel or perpendicular to the intermolecular axis is investigated by varying the electric-field magnitude and the intermolecular distance. Our calculations predict a regime with the mutual orientation of the two molecules but with no preferential direction in the laboratory frame. A mechanism for the stimulated one-photon radiative association of a pair of ultracold polar molecules into ultracold tetramers is proposed, which would open the way towards the optical manipulation of ultracold polyatomic molecules.

A Rydberg-blockade sphere provides an intriguing picture by which a number of collective many-body effects caused by the strong Rydberg-Rydberg interactions can be clearly understood and profoundly investigated. In the present work, we develop a definition for the effective two-atom blockade radius and show that the original spherically shaped blockade surface would be deformed when the real number of atoms increases from two to three. This deformation of the blockade sphere reveals spatially anisotropic and shrunken properties which strongly depend on the interatomic distance. In addition, we also study the optimal conditions for the Rydberg antiblockade effect and make predictions for improving the antiblockade efficiency in few-atom systems.

The droplet crystal phase of a symmetric binary mixture of soft-core bosons is studied by computer simulation. At high temperature each droplet comprises on average equal numbers of particles of either component, but the two components demix below the supersolid transition temperature, i.e., droplets mostly consist of particles of one component. Clustering of droplets of the same component is also observed. Demixing is driven by quantum tunneling of particles across droplets over the system and does not take place in an insulating crystal. This effect provides an unambiguous experimental signature of supersolidity.

In spin-1 collective atomic systems, the spin and nematic-tensor operators constitute the su(3) Lie algebra, whose su(2) subalgebras are shown to give two distinct classes of squeezing which are unitarily equivalent to spin squeezing and spin-nematic squeezing. We explicitly construct a unitary operator that generates an arbitrary squeezed spin-nematic state from an arbitrary spin-1 condensate state. In particular, we demonstrate that squeezed spin states can be generated from a polar state and that squeezed spin-nematic states can be generated from a fully spin-polarized state.

Nondegenerate forward four-wave mixing in hot atomic vapors has been shown to produce strong quantum correlations between twin beams of light [McCormick et al., Opt. Lett. 32, 178 (2007)], in a configuration which minimizes losses by absorption. In this paper, we look at the role of the phase-matching condition in the trade-off that occurs between the efficiency of the nonlinear process and the absorption of the twin beams. To this effect, we develop a semiclassical model by deriving the atomic susceptibilities in the relevant double-$\Lambda$ configuration and by solving the classical propagation of the twin-beam fields for parameters close to those found in typical experiments. These theoretical results are confirmed by a simple experimental study of the nonlinear gain experienced by the twin beams as a function of the phase mismatch. The model shows that the amount of phase mismatch is key to the realization of the physical conditions in which the absorption of the twin beams is minimized while the cross coupling between the twin beams is maintained at the level required for the generation of strong quantum correlations. The optimum is reached when the four-wave mixing process is not phase matched for fully resonant four-wave mixing.

A scheme for spatial multimode quantum memory is developed such that spatial-temporal structure of a weak signal pulse can be stored and recalled via cavity-assisted off-resonant Raman interaction with a strong angular-modulated control field in an extended $\Lambda$-type atomic ensemble. It is shown that effective multimode storage is possible when the Raman coherence spatial grating involves wave vectors with different longitudinal components relative to the paraxial signal field. The possibilities of implementing the scheme in the solid-state materials are discussed.

We show how communication without a physical system carrying the information from the sender to the receiver can be realized. The main point is that sending the vacuum state is not nothing. This extends the recent work by Salih et al. [Phys. Rev. Lett. 110, 170502 (2013)].

We develop a theory of damping of low-energy, collective excitations in a quasi-two-dimensional, homogenous, dipolar Bose gas at zero temperature, via processes whereby an excitation decays into two excitations with lower energy. We find that owing to the nature of the low-energy spectrum of a quasi-two-dimensional dipolar gas, such processes cannot occur unless the momentum of the incoming quasiparticle exceeds a critical value ${\mathbf{k}}_{\text{crit}}$. We find that as the dipolar interaction strength is increased, this critical value shifts to larger momenta. Our predictions can be directly verified in current experiments on dipolar Bose condensates using Bragg spectroscopy, and provide valuable insight into the quantum many-body physics of dipolar gases.

We present a scheme of a compact atomic gravimeter based on atom interferometry. Atoms are maintained against gravity using a sequence of coherent accelerations performed by the Bloch oscillations technique. We demonstrate a sensitivity of $4.8\times {10}^{-8}$ with an integration time of 4 min. Combining this method with an atomic elevator allows us to measure the local gravity at different positions in the vacuum chamber. This method can be of relevance to improve the measurement of the Newtonian gravitational constant $G$.

A wide class of non-Markovian completely positive master equations can be formulated on the basis of quantum collisional models. In this phenomenological approach the dynamics of an open quantum system is modeled through an ensemble of stochastic realizations that consist in the application at random times of a (collisional) completely positive transformation over the system state. In this paper, we demonstrate that these kinds of models can be embedded in bipartite Markovian-Lindblad dynamics consisting of the system of interest and an auxiliary one. In contrast with phenomenological formulations, here the stochastic ensemble dynamics an the interevent time interval statistics are obtained from a quantum measurement theory after assuming that the auxiliary system is continuously monitored in time. Models where the system intercollisional dynamics is non-Markovian [B. Vacchini, Phys. Rev. A 87, 030101(R) (2013)] are also obtained from the present approach. The formalism is exemplified through bipartite dynamics that lead to non-Markovian system effects such as an environment-to-system backflow of information.

Results on Auger formation of the ${\left({}^3\mathrm{He}\mu p\right)}_J^{2+}$ molecule (and its generalization to any isotopes of hydrogen and helium) in collisions of muonic hydrogen with helium atom and ion in the ground and singly excited state are presented for all rotational states $J$ of the molecule. The corresponding reaction rates are calculated in the energy range 0.1–50 eV using a one-level adiabatic approximation for the $2p\sigma$ state of the three-body system ${}^3{\mathrm{He}}^{2+}-\mu -p$ and the dipole approximation for the interaction of the three-body system with helium electrons. It is found that reaction rates corresponding to the formation of ${\left({}^3\mathrm{He}\mu p\right)}_1^{2+}$, ${\left({}^4\mathrm{He}\mu d\right)}_2^{2+}$, and ${\left({}^{3,4}\mathrm{He}\mu t\right)}_2^{2+}$ molecules are significantly enhanced by shape resonances in $\left(h\mu \right){}_{1s}+{\mathrm{He}}^{2+}$ collisions (where $h=p,d,t$ and ${\mathrm{He}}^{2+}={}^{3,4}\mathrm{He}{}^{2+}$) situated between 7 and 33 eV. In particular, reaction rates for the formation of the ${\left({}^3\mathrm{He}\mu p\right)}_1^{2+}$ molecule in the vicinity of the resonant energy (12.9 eV) are almost two orders of magnitude greater than those presented by Aristov et al. [Sov. J. Nucl. Phys. 33, 564 (1981)]. Average reaction rates for the formation of the molecules in triple H-H′-He gas mixtures (H and H′ denote different hydrogen isotopes) at temperatures between 30 and 5000 K, as well as in H-He plasma at temperature $T\le 50\mathrm{eV}$, are also presented. Rotational ground states of the molecules $J=0$, from which nuclear synthesis is preferred, are considerably populated both in triple mixtures and in H-He plasma. The results may be interesting from the point of view of the experimental study of muon-catalyzed nuclear fusion.

A comprehensive calculation of electron impact scattering by NF${}_3$ molecule is reported in this article. Total cross sections were presented over electron impact energies from 1 eV to 5 keV. The ab initio R-matrix method was employed through QUANTEMOL-N software for low-energy calculations and spherical complex optical potential formalism for intermediate to high-energy calculations. The consistency of data at the overlapping energy (∼14 eV) from these theories allows us to predict total cross sections for such an extensive energy range. Electronic excitation, rotational excitation, and differential cross sections were calculated using the $R$-matrix method at low incident energies. In the present study, resonances were detected at 5.6 and 6.2 eV, indicating the probability of anions formation by the electron attachment process and further decay to neutral and negative ion fragments. In general, the present results show overall agreement with previous theories and experiments, wherever available. A maiden attempt to calculate $e$-NF${}_3$ rotational excitation cross sections is also undertaken.

Diagrammatic many-body theory is used to calculate the scattering phase shifts, normalized annihilation rates $Z_{\mathrm{eff}}$, and annihilation $\gamma$ spectra for positron collisions with the hydrogenlike ions He${}^{+}$, Li${}^{2+}$, B${}^{4+}$, and F${}^{8+}$. Short-range electron-positron correlations and longer-range positron-ion correlations are accounted for by evaluating nonlocal corrections to the annihilation vertex and the exact positron self-energy. The numerical calculation of the many-body theory diagrams is performed using $B$-spline basis sets. To elucidate the role of the positron-ion repulsion, the annihilation rate is also estimated analytically in the Coulomb-Born approximation. It is found that the energy dependence and magnitude of $Z_{\mathrm{eff}}$ are governed by the Gamow factor that characterizes the suppression of the positron wave function near the ion. For all of the H-like ions, the correlation enhancement of the annihilation rate is found to be predominantly due to corrections to the annihilation vertex, while the corrections to the positron wave function play only a minor role. Results of the calculations for $s$-, $p$-, and $d$-wave incident positrons of energies up to the positronium-formation threshold are presented. Where comparison is possible, our values are in excellent agreement with the results obtained using other, e.g., variational, methods. The annihilation-vertex enhancement factors obtained in the present calculations are found to scale approximately as $1+(1.6+0.46\ell )/Z_i$, where $Z_i$ is the net charge of the ion and $\ell$ is the positron orbital angular momentum. Our results for positron annihilation in H-like ions provide insights into the problem of positron annihilation with core electrons in atoms and condensed matter systems, which have similar binding energies.

In atomic physics, adiabatic evolution is often used to achieve a robust and efficient population transfer. Many adiabatic schemes have also been implemented in optical waveguide structures. Recently there has been increasing interest in the influence of decay and absorption, and their engineering applications. Here it is shown that even a small decay can significantly influence the dynamical behavior of a system, above and beyond a mere change of the overall norm. In particular, a small decay can lead to a breakdown of adiabatic transfer schemes, even when both the spectrum and the eigenfunctions are only sightly modified. This is demonstrated for the generalization of a stimulated Raman adiabatic passage scheme that has recently been implemented in optical waveguide structures. Here the question how an additional absorption in either the initial or the target waveguide influences the transfer property of the scheme is addressed. It is found that the scheme breaks down for small values of the absorption at a relatively sharp threshold, which can be estimated by simple analytical arguments.

Subwavelength confinement of light in nonlinear hyperbolic metamaterials due to formation of spatial solitons has attracted much recent attention because of its seemingly counterintuitive behavior. In order to achieve self-focusing in a hyperbolic wire medium, a nonlinear self-defocusing Kerr medium must be used as a dielectric host. Here we demonstrate that this behavior finds a natural explanation in terms of the analog of gravity. A wave equation describing the propagation of extraordinary light inside hyperbolic metamaterials exhibits (2+1)-dimensional Lorentz symmetry. The role of time in the corresponding effective three-dimensional Minkowski space-time is played by the spatial coordinate aligned with the optical axis of the metamaterial. Nonlinear optical Kerr effect “bends” this space-time resulting in effective gravitational force between extraordinary photons. In order for the effective gravitational constant to be positive, a negative self-defocusing Kerr medium must be used as a host. If gravitational self-interaction is strong enough, the spatial soliton may collapse into a black hole analog.