We perform a theoretical analysis to interpret the spectra of purely long-range helium dimers produced by photoassociation (PA) in an ultracold gas of metastable helium atoms. The experimental spectrum obtained with the PA laser tuned closed to the $2{}^{3}S_1\leftrightarrow 2{}^{3}P_0$ atomic line has been reported in a previous paper. Here, we first focus on the corrections to be applied to the measured resonance frequencies in order to infer the molecular binding energies. We then present a calculation of the vibrational spectra for the purely long-range molecular states, using adiabatic potentials obtained from perturbation theory. With retardation effects taken into account, the agreement between experimental and theoretical determinations of the spectrum for the $0_u^{+}$ purely long-range potential well is very good. The results yield a determination of the lifetime of the $2{}^{3}P$ atomic state.

We have observed strong amplification with a gain of $14.5{\text{cm}}^{-1}$ for the $4d\to 4p$ transition of the Ni-like La ions at a wavelength of $8.8\text{nm}$ pumped by a chirped pulse amplification Nd:glass laser with an energy of $18\mathrm{J}$. In this experiment, the laser pulses consist of a prepulse and a main pulse with durations of $200\text{ps}$ and $7\text{ps}$, separated by $250\text{ps}$. The keV x-ray spectroscopy and theoretical calculation indicate that the plasma temperature increased effectively by reducing the prepulse intensity, and that the substantial gain is generated in a small overcritical region heated by the electron heat transport.

We report two key distribution schemes achieved by swapping quantum entanglement. Using two Bell states, two bits of secret key can be shared between two distant parties that play symmetric and equal roles. We also address eavesdropping attacks against the schemes.

We analyze the formation, propagation, and interaction of stable two-frequency $(2+1)$-dimensional solitons, formed in a Raman media driven near maximum molecular coherence. The propagating light is trapped in the two transverse dimensions.

We consider the ground-state properties of mixed Bose-Einstein condensates of ${}^{87}\mathrm{Rb}$ and ${}^{85}\mathrm{Rb}$ atoms in the isotropic pancake trap for both signs of the interspecies scattering length. In the case of the repulsive interspecies interaction, there are the axially symmetric and symmetry-breaking ground states. The threshold for the symmetry-breaking transition, which is related to appearance of a zero dipole mode, is found numerically. For attractive interspecies interactions, the two condensates assume symmetric ground states for the numbers of atoms up to the collapse instability of the mixture.

Oscillators driven by a degenerate wave mixing process are bistable in the phase of the generated radiation. In systems with a large Fresnel number, domains of opposite phase form therefore spontaneously. A simple model predicts a real field in which phase domains are separated by Ising-type walls. In this paper we show experimentally (using complex field reconstruction from measurements) and theoretically (by an extended model) that the optical field can be real as well as complex valued and that complex field fronts are related to the front curvature.

The nonstationary pump-probe interaction between short laser pulses propagating in a resonant optically dense coherent medium is considered. Special attention is paid to the case where the density of two-level particles is high enough that a considerable part of the energy of relatively weak external laser fields can be coherently absorbed and reemitted by the medium. Thus, the field of the medium reaction plays a key role in the interaction processes, which leads to collective behavior of an atomic ensemble in the strongly coupled light-matter system. Such behavior results in fast excitation interchanges between the field and a medium in the form of optical ringing, which is analogous to polariton beating in solid-state optics. This collective oscillating response, which can be treated as successive beats between light wave packets of different group velocities, is shown to significantly affect the propagation and amplification of the probe field under its nonlinear interaction with a nearly copropagating pump pulse. Depending on the probe-pump time delay, the probe transmission spectra show the appearance of either a specific doublet or coherent dip. The widths of these features are determined by the density-dependent field-matter coupling coefficient and increase during the propagation. Besides that, the widths of the coherent features, which appear close to resonance in the broadband probe spectrum, exceed the absorption-line width, since under the strong-coupling regime, the frequency of optical ringing exceeds the rate of incoherent relaxation. Contrary to stationary strong-field effects, the density- and coordinate-dependent transmission spectra of the probe manifest the importance of collective oscillations and cannot be obtained in the framework of a single-atom model.

Two-photon interference of multimode two-photon pairs produced by an optical parametric oscillator has been observed with an unbalanced interferometer. The time correlation between the multimode two photons has a multipeaked structure. This property of the multimode two-photon state induces two-photon interference depending on the delay time.

Bounds on quantum probabilities and expectation values are derived for experimental setups associated with Bell-type inequalities. In analogy to the classical bounds, the quantum limits are experimentally testable and therefore serve as criteria for the validity of quantum mechanics.

We report a combined theoretical and experimental study of the spectral and polarization dependence of near-resonant radiation coherently backscattered from an ultracold gas of ${}^{85}\mathrm{Rb}$ atoms. Measurements in a $\pm 6\text{MHz}$ range about the $5s{}^{2}S_{1∕2}\to 5p{}^{2}P_{3∕2}$, $F=3\to F^{\prime }=4$ hyperfine transition are compared with simulations based on a realistic model of the experimental atomic density distribution. In the simulations, the influence of heating of the atoms in the vapor, magnetization of the vapor, finite spectral bandwidth, and other nonresonant hyperfine transitions are considered. Good agreement is found between the simulations and measurements.

Radiation from a chaotic cavity filled with gain medium is considered. A set of coupled equations describing the photon density and the population of gain medium is proposed and solved. The spectral distribution and fluctuations of the radiation are found. The full noise is a result of a competition between positive correlations of photons with equal frequencies (due to stimulated emission and chaotic scattering) which increase fluctuations, and a suppression due to interaction with a gain medium which leads to negative correlations between photons. The latter effect is responsible for a pronounced suppression of the photonic noise as compared to the linear theory predictions.

We show that, in the context of von Neumann measurement theory, the condition of strong measurement is the classical limit of the measuring device, and vice versa. This means that a strong measurement can be performed only by using a measuring device prepared in a classical limit state, and vice versa. Since it is the strong measurement that conforms with the postulate that the outcome of a single measurement is one of the eigenvalues of the corresponding operator, the aforementioned postulate is valid only in the classical limit for a von Neumann measurement.

We present studies on collision induced dissociation of $\mathrm{C}_2{}^{-}$ with Ar at an impact energy of $15\text{keV}$. The ${\mathrm{C}}^{-}$ fragment ion kinetic-energy release (KER) distribution is measured and is used to compute the KER in the center of mass (c.m.) frame $\left({\text{KER}}_{\mathit{c.m.}}\right)$. We employ the reflection method to deduce an effective repulsive potential-energy curve for the molecular anion that is otherwise difficult to evaluate from quantum computational methods. The nuclear wave packet of the molecular ion in the initial ground state is computed by the semiclassical WKB method using the potential-energy curve of the ${}^2\sum _g^{+}$ ground electronic state calculated by an ab initio quantum computation method. The ground-state nuclear wave packet is reflected on a parametrized repulsive potential-energy curve where the parameters are determined by fitting the measured ${\text{KER}}_{\mathit{c.m.}}$ with the calculated KER distribution.

It was found experimentally with increasing the laser ellipticity that the intensity of the 45th order harmonic from ${\mathrm{O}}_2$ decreases slower than that from ${\mathrm{N}}_2.$ The difference is attributed to the fact that the recombination probability is suppressed for ${\mathrm{O}}_2$ but enhanced for ${\mathrm{N}}_2$ in a linearly polarized field. Simulation results obtained by extending the Lewenstein model to molecules agreed qualitatively with the experimental discoveries.

Under a standard set of assumptions for a hidden-variable model for quantum events we show that all observables must commute simultaneously. This seems to be an ultimate statement about the inapplicability of the usual hidden-variable model for quantum events. And, despite Bell’s complaint that a key condition of von Neumann’s was quite unrealistic, we show that these conditions, under which von Neumann produced the first no-go proof, are entirely equivalent to those introduced by Bell and Kochen and Specker. As these conditions are also equivalent to those under which the Bell-Clauster-Horne inequalities are derived, we see that the experimental violations of the inequalities demonstrate only that quantum observables do not commute.

We give a rigorous analytical derivation of low-temperature behavior of the Casimir entropy in the framework of the Lifshitz formula combined with the Drude dielectric function. An earlier result that the Casimir entropy at zero temperature is not equal to zero and depends on the parameters of the system is confirmed, i.e., the third law of thermodynamics (the Nernst heat theorem) is violated. We illustrate the resolution of this thermodynamical puzzle in the context of the surface impedance approach by several calculations of the thermal Casimir force and entropy for both real metals and dielectrics. Different representations for the impedances, which are equivalent for real photons, are discussed. Finally, we argue in favor of the Leontovich boundary condition which leads to results for the thermal Casimir force that are consistent with thermodynamics.

We discuss the security implications of noise for quantum coin tossing protocols. We find that if quantum error correction can be used, so that noise levels can be made arbitrarily small, then reasonable security conditions for coin tossing can be framed so that results from the noiseless case will continue to hold. If, however, error correction is not available (as is the case with present day technology), and significant noise is present, then tossing a single coin becomes problematic. In this case, we are led to consider random n-bit string generation in the presence of noise, rather than single-shot coin tossing. We introduce precise security criteria for n-bit string generation and describe an explicit protocol that could be implemented with present day technology. In general, a cheater can exploit noise in order to bias coins to their advantage. We derive explicit upper bounds on the average bias achievable by a cheater for given noise levels.

Electric-dipole matrix elements for $\mathrm{ns}-n^{\prime }p,$ $\mathrm{nd}-n^{\prime }p,$ and $6d-4f$ transitions in Rb are calculated using a relativistic all-order method. A third-order calculation is also carried out for these matrix elements to evaluate the importance of the high-order many-body perturbation theory contributions. The all-order matrix elements are used to evaluate lifetimes of $\mathrm{ns}$ and $\mathrm{np}$ levels with $n=6,7,8\mathrm{}$ and $\mathrm{nd}$ levels with $n=4,5,6\mathrm{}$ for comparison with experiment and to provide benchmark values for these lifetimes. The dynamic polarizabilities are calculated for $\mathrm{ns}$ states of rubidium. The resulting lifetime and polarizability values are compared with available theory and experiment.

Knowledge of the monochromaticity of an x-ray source is increasingly important in fundamental experiments and critical applications. The bandwidth of an x-ray beam, selected from a synchrotron radiation spectrum for example, ultimately defines the limiting resolution of the synchrotron source. The development of x-ray technology through the use of characteristic line sources of fixed line shape has impaired the development of understanding of this parameter. The bandwidth is particularly relevant with the modern trend towards the use of synchrotron sources in conjunction with monochromating devices, where the monochromaticity of the x-ray beam is not known a priori. The ability to control the bandwidth of the beam can lead to significant new experiments. We have observed the effect of the x-ray bandwidth on precise but relatively scaled measurements of the mass attenuation coefficient of molybdenum made on the absorption edge. We derive an expression describing the effect of the x-ray bandwidth on these measurements and invert this to determine the bandwidth of a highly monochromatized $20-\mathrm{keV}$ synchrotron x-ray beam to be $1.57\mathrm{eV}\pm 0.03\mathrm{eV}.$ The technique presented here determines the bandwidth, a parameter critically dependent on the x-ray optical elements in the beam, in such a manner as to require no knowledge of these elements. We demonstrate that the x-ray bandwidth has significant effects upon measured edge energies, mass attenuation coefficients, x-ray anomalous-fine structure and x-ray absorption near-edge structure (XANES). In particular, the observed x-ray bandwidth necessitates a correction of up to $1.4\%$ in the measurement of the mass attenuation coefficient of molybdenum on the absorption edge and is shown to shift the observed absorption-edge energy location by up to $0.5\mathrm{eV}.$

Multiphoton ionization of linear molecules is studied using the strong-field S-matrix approach. Numerical calculations for the angular distribution and the energy spectrum of the photoelectron as well as the total ionization rates and yields in an intense linearly polarized laser pulse are performed. Results are obtained for molecules aligned along the polarization axis and for ensembles of molecules having a random orientation of the molecular axis with respect to the polarization direction. Signatures of the molecular geometry and the orbital symmetry are identified and discussed with reference to molecular imaging and alignment. It is found that these signatures are clearly marked for the diatomic molecules ${\mathrm{C}}_2,$ ${\mathrm{N}}_2,$ and ${\mathrm{O}}_2,$ but are much weaker for the group of linear carbon clusters due to the polyatomic character as well as to the competing contributions from different valence shells of these molecules. Finally, predictions of the present theory for the dependence of the total ionization rates and yields on the orientation of the molecule are compared with other theoretical models and recent experimental data.