We experimentally investigate light scattering from a Bose-Einstein condensate which is positioned inside an optical ring cavity. With a cavity linewidth at the recoil limit, the resulting superradiant scattering behavior is strongly influenced by the cavity detuning. The experimental observations are interpreted with a quantum mean-field description and with a rate-equation model. We discuss applications in the context of quantum phase transitions, Dicke subradiance, and nonconventional superfluidity.

A strong light pulse propagating in a nonlinear Kerr medium produces a change in the refractive index, which makes light travel at different speeds inside and outside the pulse. By tuning the pulse velocity, an analog black hole horizon can be obtained in a suitable frequency window. In this paper, we develop a quantum theory of light propagation for this system, including the frequency dispersion of the refractive index of the medium by coupling the electromagnetic field to matter polarization fields. In a configuration with a single black hole horizon, the spectrum of spontaneously emitted particles presents some similarities with Hawking radiation. Furthermore, even in horizonless systems spontaneous vacuum emission is still possible due to the dispersive nature of the medium, yet with dramatically different spectral properties.

Optical imaging methods are typically restricted to a resolution of the order of the probing light wavelength ${\lambda }_p$ by the Rayleigh diffraction limit. This limit can be circumvented by making use of multiphoton detection of correlated $N$-photon states, having an effective wavelength ${\lambda }_p/N$. However, the required $N$-photon detection usually renders these schemes impractical. To overcome this limitation, recently, so-called optical centroid measurements have been proposed which replace the multiphoton detectors with an array of single-photon detectors. Complementary to the existing approximate analytical results, we explore the approach using numerical experiments by sampling and analyzing detection events from the initial-state wave function. This allows us to quantitatively study the approach also beyond the constraints set by the approximate analytical treatment, to compare different detection strategies, and to analyze other classes of input states.

We explore the possibility of exciting the so-called quartic solitons in specially designed slot waveguides based on silicon and silica or silicon nanocrystals. This requires the excitation of the structure with quasi-transverse-magnetic polarized pulses—for which the Raman effect is absent—and at a specific infrared wavelength for which only the second- and fourth-order group velocity coefficients are nonvanishing. Pulses launched in these conditions will generate a spectrally localized continuum coming from the spectral interference of many quartic solitons.

We present an approach associated with the Jacobi matrix method to calculate a three-body wave function that describes the double continuum of an atomic two-electron system. In this approach, a symmetrized product of two Coulomb waves is used to describe the asymptotic wave function, while a smooth cutoff function is introduced to the dielectronic potential that enters its integral part in order to have a compact kernel of the corresponding Lippmann-Schwinger-type equation to be solved. As an application, the integral equation for the ($e^{-},e^{-},{\mathrm{He}}^{2+}$) system is solved numerically; the fully fivefold differential cross sections (FDCSs) for ($e,3e$) processes in helium are presented within the first-order Born approximation. The calculation is performed for a coplanar geometry in which the incident electron is fast ($\sim 6$ keV) and for a symmetric energy sharing between both slow ejected electrons at excess energy of 20 eV. The experimental and theoretical FDCSs agree satisfactorily both in shape and in magnitude. Full convergence in terms of the basis size is reached and presented.

Magic wavelengths for the Li, Na, and K alkali-metal atoms are determined using the circularly polarized light for the $ns$-$n{p}_{1/2,3/2}$ transitions, with $n$ denoting their ground-state principal quantum numbers, by studying their differential ac dynamic polarizabilities. These wavelengths for all possible sublevels are given linearly as well as circularly polarized lights and are further compared with the available results for the linearly polarized light. The present study suggests that it is possible to carry out state-insensitive trapping of different alkali-metal atoms using the circularly polarized light.

We investigate the way in which the pattern of fringes in a coherent pair of two-dimensional Bose condensed clouds of ultracold atoms traveling in opposite directions subject to a harmonic trapping potential can seed the irreversible formation of internal excitations in the clouds, notably solitons and vortices. We identify underdamped, overdamped, and critically damped regimes in the dipole oscillations of the condensates according to the balance of internal and center-of-mass energies of the clouds. We carry out simulations of the collision of two clouds with respect to different initial phase differences in these regimes to investigate the creation of internal excitations. We distinguish the behavior of this system from previous studies of quasi-one-dimensional BECs. In particular we note that the nature of the internal excitations is only essentially sensitive to an initial phase difference between the clouds in the overdamped regime.

We analyze energy spectra of photoelectrons ejected by strong laser pulses, using the manifestly gauge-invariant approach. Gauge-invariant expressions for the ionization amplitude can be obtained by grouping together all terms of the same order in atomic potential in the standard strong-field approximation (SFA) expansion. We calculate energy spectra of photoelectrons for the hydrogen atom, and compare them with results based on numerical solutions of the time-dependent Schrödinger equation. Agreement, at least qualitative, is observed for an electric-field amplitude higher than 1 a.u., and for larger energies of ionized electrons, basically beyond the region, where the standard SFA is usually applied.

We report a fully fibered source emitting cross time-bin-entangled photons at 1540 nm from type-II spontaneous parametric down-conversion. Compared to standard time-bin-entanglement realizations, the preparation interferometer requires no phase stabilization, simplifying its implementation in quantum key distribution experiments. Bell-type tests of such a cross time-bin state are performed in the Franson configuration and lead to two-photon interference raw visibilities greater than 95$\%$, which are only limited by the dark counts in the detectors and imperfections in the analysis system. Just by trusting the randomness of the beam splitters, the correlations generated by the source can be proved of nonclassical origin even in a passive implementation. The obtained results confirm the suitability of this source for time-bin-based quantum key distribution.

We consider a quantum-mechanical analysis of spontaneous emission in terms of an effective two-level system with a vacuum decay rate ${\Gamma }_0$ and transition angular frequency ${\omega }_A$. Our analysis is in principle exact, even though presented as a numerical solution of the time evolution including memory effects. The results so obtained are confronted with previous discussions in the literature. In terms of the dimensionless lifetime $\tau =t{\Gamma }_0$ of spontaneous emission, we obtain deviations from exponential decay of the form $O(1/\tau )$ for the decay amplitude as well as the previously obtained asymptotic behaviors of the form $O(1/{\tau }^2)$ or $O(1/\tau {\ln }^2\tau )$ for $\tau \gg 1$. The actual asymptotic behavior depends on the adopted regularization procedure as well as on the physical parameters at hand. We show that for any reasonable range of $\tau$ and for a sufficiently large value of the required angular frequency cutoff ${\omega }_c$ of the electromagnetic fluctuations, i.e., ${\omega }_c\gg {\omega }_A$, one obtains either a $O(1/\tau )$ or a $O(1/{\tau }^2)$ dependence. In the presence of physical boundaries, which can change the decay rate with many orders of magnitude, the conclusions remains the same after a suitable rescaling of parameters.

The assumption of local realism, in a Bell locality scenario, imposes nontrivial conditions on the Shannon entropies of the associated probability distributions, expressed by linear entropic Bell inequalities. In principle, these entropic inequalities provide necessary but not sufficient criteria for the existence of a local hidden variable model reproducing the correlations, as, for example, the paradigmatic nonlocal Popescu-Rohrlich (PR) box is entropically not different from a classically correlated box. In this paper we show that for the $n$-cycle scenario, entropic inequalities completely characterize the set of local correlations. In particular, every nonsignaling box which violates the Clauser-Horne-Shimony-Holt (CHSH) inequality—including the PR box—can be locally modified so that it also violates the entropic version of CHSH inequality. As we show, any nonlocal probabilistic model when appropriately mixed with a local model, violates an entropic inequality, thus evidencing a very peculiar kind of nonlocality. As the $n$-cycle captures equally well both the notion of local realism introduced by Bell and that of noncontextuality presented by the Kochen-Specker theorem, the results are also valid for noncontextuality scenarios.

The electric dipole moments of various neutral elementary particles, such as neutron, neutrinos, certain hypothetical dark matter particles, and others, are predicted to exist by the standard model of high-energy physics and various extensions of it. However, the predicted values are beyond the present experimental capabilities. We propose to simulate and emulate the electric dipole moment of neutral relativistic particles and the ensuing effects in the presence of an electrostatic field by emulation of an extended Dirac equation in ion traps.

We demonstrate that perfect transfer of path-entangled photons as well as of single-photon states is possible in a certain class of spin inspired optical systems—the so-called $J_x$ photonic lattices. In these fully integrable optical arrangements, perfect cyclic transitions from correlated states to totally anticorrelated states can naturally occur. Moreover we show that the bunching and antibunching response of path-entangled photons can be preengineered at will in such coupled optical arrangements. We elucidate these effects via pertinent examples.

The largest contributions to the $n=2$ Lamb shift, fine structure interval and $2s$ hyperfine structure of muonic hydrogen are calculated by accurate numerical evaluations of the Dirac equation rather than by a perturbation expansion in powers of $1/c$, in the framework of nonrelativistic quantum electrodynamics. Previous calculations and the validity of the perturbation expansion for light elements are confirmed. The dependence of the various effects on the nuclear size and model are studied.

The ionization potentials, excitation energies, and electron affinity of superheavy element 120 and the polarizabilities of its neutral and ionized states are calculated. Relativity is treated within the four-component Dirac-Coulomb formalism; Breit or Gaunt terms are added in some cases. Electron correlation is included via the intermediate Hamiltonian Fock-space coupled cluster method for the spectra and ionization potentials and via the single reference coupled cluster singles and doubles with perturbative triples [CCSD(T)] approach for the electron affinities and polarizabilities. To assess the accuracy of the results, the atomic properties of the lighter homologues, Ba and Ra, are also calculated. Very good agreement with available experimental values is obtained, lending credence to the predictions for element 120. The atomic properties in group 2 are largely determined by the valence $ns$ orbital, which experiences relativistic stabilization and contraction in the heavier group-2 elements. As a result, element 120 is predicted to have a relatively high ionization potential (5.851 eV), similar to that of Sr, and rather low electron affinity (0.021 eV) and polarizability (163 a.u.), comparable to those of Ca. The adsorption enthalphy of element 120 on Teflon, which is important for possible future experiments on this atom, is estimated as 35.4 kJ/mol, the lowest among the elements considered here.

We study the conditions for attaining crystalline order in the stationary state of a continuously driven, open many-body system. Specifically, we consider resonant optical excitations of atoms in a one-dimensional lattice to the Rydberg states interacting via the van der Waals potential. Strong blockade of excitations at neighboring lattice sites steers the system toward a crystalline state while competing with the fluctuations associated with relaxation. We analyze the stationary state of the many-body system and the dynamics of its buildup employing numerically exact time-dependent density-matrix renormalization-group simulations for two- and three-level excitation schemes. We also present an approximate rate equation model which provides qualitative conditions for attaining crystalline order.

Using cold ${}^{87}$Rb atoms trapped in a one-dimensional (1D)-optical lattice, atomic interferometers involving coherent superpositions between different Wannier-Stark atomic states are realized. Two different kinds of trapped interferometer schemes are presented: a Ramsey-type interferometer sensitive both to clock frequency and external forces, and a symmetric accordion-type interferometer, sensitive to external forces only. We evaluate the limits in terms of sensitivity and accuracy of those schemes and discuss their application as force sensors. As a first step, we apply these interferometers to the measurement of the Bloch frequency and the demonstration of a compact gravimeter.

Miscellaneous magnetic systems are currently being intensively investigated because of their potential applications in modern technologies. Nonetheless, a many-body dynamical description of complex magnetic systems may be cumbersome, especially when the system exhibits a geometrical frustration. This paper deals with simulations of the classical $XY$ model on a three-dimensional triangular lattice with anisotropic couplings, including an analysis of the phase diagram and a Bogoliubov description of the dynamical stability of mean-field stationary solutions. We also discuss the possibilities of the realization of Bose-Hubbard models with complex tunneling amplitudes in shaken optical lattices without breaking the generalized time-reversal symmetry and the opposite, i.e., real tunneling amplitudes in systems with the time-reversal symmetry broken.

We study the physical mechanism and the information velocity of fast light based on the time-domain propagator governing light propagation in a dispersive system. We show that the output pulse can be regarded as the superposition of multiple “copies” of the input pulse, each traveling at a well-defined signal velocity no greater than the speed of light in vacuum, and the interference between the front edges of these copies generates the peak of the output pulse. The information velocity is defined as the ratio of the propagation distance to the mini-mum time required to complete any given information transfer process, directly reflecting how fast the signal of interest, i.e., the part of the pulse that carries the information of interest, propagates. With the peak of the output pulse representing the information, as is typical in slow- and fast-light applications, we find that the information velocity is significantly smaller than the speed of a discontinuity in the pulse, which was traditionally proposed to be the signal velocity of fast light. The analysis can also be applied to slow-light propagation.