We report the observation of laser-induced birefringence (LIB) in a wavelength-mismatched cascade system ($J=0\leftrightarrow J=1\leftrightarrow J=0$ transitions) of inhomogeneously broadened ytterbium atoms with strong pump and probe fields. We investigate the transmission spectrum of two circular polarization (${\sigma }_p^{+}$ and ${\sigma }_p^{-}$) components of strong probe field at fixed frequency, depending on the detuning of a circularly polarized $\left({\sigma }_c^{-}\right)$ coupling field from two-photon resonance. We find that the ${\sigma }_p^{+}$ $\left({\sigma }_p^{-}\right)$ polarized component exhibits a narrow electromagnetically induced absorption (transparency) spectrum. Numerical solutions of density matrix equations show qualitative agreement with experimental results. A Doppler-free dispersive LIB signal is obtained by detecting the Stokes parameter of the probe field, enabling us to stabilize the frequency of the coupling laser without frequency modulation.

At present several groups are analyzing astrophysical data in a search for the time variation of the fine-structure constant $\alpha$. Here we discuss how to exclude systematic effects caused by changes in the isotope abundances during the evolution of the universe. We suggest using particular combinations of the transition frequencies of $\mathrm{O}\mathrm{II}$, $\mathrm{Al}\mathrm{II}$, $\mathrm{Al}\mathrm{III}$, $\mathrm{Si}\mathrm{II}$, and $\mathrm{Mn}\mathrm{II}$ as anchors, which are insensitive to $\alpha$ variation and to changes in isotope abundances. These anchors can be used to determine the cosmological redshift. Then, one can use other combinations of frequencies as probes for the time variation of $\alpha$ and another set as probes for the isotopic abundances. In this way it is possible to eliminate one source of systematic errors in the search for $\alpha$ variation and get more information about evolution of the isotopes. On the level of accuracy that has already been reached in the search for $\alpha$ variation it is possible to see $\sim 10\%$ changes in isotope abundances of Si and Ni.

The possibility of repulsive Casimir forces between small metal spheres and a dielectric half-space is discussed. We treat a model in which the spheres have a dielectric function given by the Drude model, and the radius of the sphere is small compared to the corresponding plasma wavelength. The half-space is also described by the same model, but with a different plasma frequency. We find that in the retarded limit, the force is quasioscillatory. This leads to the prediction of stable equilibrium points at which the sphere could levitate in the Earth’s gravitational field. This seems to lead to the possibility of an experimental test of the model. The effects of finite temperature on the force are also studied, and found to be rather small at room temperature. However, thermally activated transitions between equilibrium points could be significant at room temperature.

We present an experimental demonstration of the “optimal” and “universal” quantum entangling process involving qubits encoded in the polarization of single photons. The structure of the “quantum entangling machine” consists of the quantum injected optical parametric amplifier by which the simultaneous realization of the $1\to 2$ universal quantum cloning and of the universal NOT (U-NOT) gate has also been achieved.

The ability to perform a universal set of quantum operations based solely on static resources and measurements presents us with a striking viewpoint for thinking about quantum computation and its powers. We consider the two major models for doing quantum computation by measurements that have hitherto appeared in the literature and show that they are conceptually closely related by demonstrating a systematic local mapping between them. This way we effectively unify the two models, showing that they make use of interchangeable primitives. With the tools developed for this mapping, we then construct more resource-effective methods for performing computation within both models and propose schemes for the construction of arbitrary graph states employing two-qubit measurements alone.

We have measured the frequency splitting between the $(2S,F=1∕2)$ and $(2S,F=3∕2)$ hyperfine sublevels in atomic deuterium by an optical differential method based on two-photon Doppler-free spectroscopy on a cold atomic beam. The result $f_{\mathrm{HFS}}^{\left(D\right)}\left(2S\right)=40924454\left(7\right)\mathrm{Hz}$ is the most precise value for this interval to date. In comparison to previous radio-frequency measurements we have improved the accuracy by a factor of 3. The specific combination $D_{21}=8f_{\mathrm{HFS}}^{\left(D\right)}\left(2S\right)-f_{\mathrm{HFS}}^{\left(D\right)}\left(1S\right)$ of metastable and ground state hyperfine frequency intervals in deuterium derived from our measurement agrees well with the value for $D_{21}$ calculated from quantum electrodynamics.

X-ray pump-probe spectroscopy is studied theoretically. It is shown that two-color—optical$+$x-ray—excitation with constant phase of the pump radiation exhibits strong interference between the one- and two-photon excitation channels. This effect is found to be large for both long and short pump pulses, while the interference vanishes for x-ray pulses longer than one cycle of the pump field. It is predicted that the spectral shape of x-ray absorption is strongly influenced by the absolute phase of the pump light. A strong sensitivity of the x-ray absorption and/or photoionization profile to the phase and detuning of the pump field is predicted, as well as to the duration of the x-ray pulse. Our simulations display oscillations of x-ray absorption as a function of the delay time. This effect allows the synchronization of the x-ray pulse relative to the “comb” of the pump radiation. The interference pattern copies the temporal and space distribution of the pump field. We pay special attention to the role of molecular orientation for the interference effect.

We present integral and differential cross sections for elastic scattering of electrons by OCS molecules for energies between 0.1 and $10\mathrm{eV}$. We employ the Schwinger multichannel method with pseudopotentials in the static-exchange plus polarization approximation. We compare our results with available theoretical and experimental results and find good agreement in general. Our calculated integral cross section is strongly enhanced near zero energy which is an indication of a possible existence of an $s$-wave virtual state.

When a single two-level atom interacts with a pair of Laguerre-Gaussian beams with opposite helicity, this leads to an efficient exchange of angular momentum between the light field and the center-of-mass motion of the atom. When the radial motion is trapped by an additional potential, the wave function of a single localized atom can be split into components that rotate in opposite direction. This suggests a scheme for atom interferometry without mirror pulses.

We present a theoretical analysis of three-dimensional (3D) matter-wave solitons and their stability properties in coupled atomic and molecular Bose-Einstein condensates (BECs). The soliton solutions to the mean-field equations are obtained in an approximate analytical form by means of a variational approach. We investigate soliton stability within the parameter space described by the atom-molecule conversion coupling, the atom-atom $s$-wave scattering, and the bare formation energy of the molecular species. In terms of ordinary optics, this is analogous to the process of sub- or second-harmonic generation in a quadratic nonlinear medium modified by a cubic nonlinearity, together with a phase mismatch term between the fields. While the possibility of formation of multidimensional spatiotemporal solitons in pure quadratic media has been theoretically demonstrated previously, here we extend this prediction to matter-wave interactions in BEC systems where higher-order nonlinear processes due to interparticle collisions are unavoidable and may not be neglected. The stability of the solitons predicted for repulsive atom-atom interactions is investigated by direct numerical simulations of the equations of motion in a full 3D lattice. Our analysis also leads to a possible technique for demonstrating the ground state of the Schrödinger-Newton and related equations that describe Bose-Einstein condensates with nonlocal interparticle forces.

We study the quantum dynamics of a two-mode Bose-Einstein condensate in a time-dependent symmetric double-well potential using analytical and numerical methods. The effects of internal degrees of freedom on the visibility of interference fringes during a stage of ballistic expansion are investigated varying particle number, nonlinear interaction sign and strength, as well as tunneling coupling. Expressions for the phase resolution are derived and the possible enhancement due to squeezing is discussed. In particular, the role of the superfluid–Mott insulator crossover and its analog for attractive interactions is recognized.

Pattern formation in large-aspect-ratio single-mode inhomogeneously broadened lasers is studied by means of the integro-differential Maxwell-Bloch equations. As the inhomogeneous linewidth increases, the neutral stability curve shows a relaxation in the traveling wave selection allowing structures with different sizes to grow. We have performed numerical simulations with a simplified model based on a few discrete groups of atoms at different resonant frequencies to observe transverse dynamics above threshold for good and bad cavity configurations. We obtain, in general, that close to threshold the inhomogeneous broadening leads to a more complex pattern in comparison to the homogeneous broadening case. At higher pumpings, a stabilization in the number of spatial frequencies taking part in the transverse pattern is found. The influence of the laser aperture on temporal dynamics is also studied. It is shown how the typical self-pulsing regime present in single-mode inhomogeneously broadened lasers is destroyed for a wide enough aperture.

We predict a unique effect in a quantum two-level system (TLS) coupled to a resonant cavity. By bringing the TLS in and out of resonance with the cavity by a series of $N$ rectangular bias pulses (the length of the $m\text{th}$ pulse scaling as $1∕\sqrt{m}$), we will coherently excite the $N$-photon state, $\mid N⟩$, of the cavity only if the TLS was initially in an appropriate quantum state (“go” state). Otherwise, the number of photons in the cavity will remain small compared to $N$ (selective amplification). If the TLS was in a coherent superposition of the “go” and “no go” states, the cavity will be in a superposition of states, in which the state $\mid N⟩$ will enter with the same weight as the initial “go” component. The effect is due to $\sqrt{N+1}$ dependence of the Rabi oscillation frequency on the number $N$ of photons in the cavity. It is stable with respect to noise, pulse shape, finite temperature, TLS decoherence, and TLS detuning from resonance with the cavity. The effect can be used as a means to read out a quantum state of a qubit coupled to a resonator. Such coherent amplication of a low-energy signal is most relevant for solid-state quantum information processing, where the energy scale is well below the optical range.

The influence of higher-order optical nonlinearities on the self-guiding of femtosecond pulses in the atmosphere is investigated theoretically. ${\chi }^{\left(5\right)}$ susceptibility, even small, is shown to significantly affect the propagation dynamics and enhance the robustness of femtosecond filaments by lowering the threshold intensity at which the beam saturates and by enlarging the self-channeling range.

We present a fully operational and consistent approach to complementarity. In contrast to previous approaches, in this proposal the duality relations emerge exclusively from the outcomes of simultaneous measurements performed on every run of the experiment and under the same experimental conditions. This can be done without assuming any definite relationship between the measurement performed and the complementary observables being studied.

We suggest a method of generating distillable entanglement from mixed states unitarily, by utilizing the flexibility of dimension of occupied Hilbert space. We present a model of a thermal spin state entering a beam splitter generating entanglement. It is the truncation of the state that allows for entanglement generation. The output entanglement is investigated for different temperatures and it is found that more randomness—in the form of higher temperature—is better for this setup.

Static dipole polarizabilities for the neutral and ionized $M$, $M^{+}$, and $M^{2+}$ group 2 elements from $M=\mathrm{Ca}$ to Ra are predicted from relativistic coupled-cluster calculations using finite field techniques together with carefully optimized Gaussian-type basis sets. Useful relations between the dipole polarizability and the ionization potential are established.

The photoabsorption spectrum of Ne in the excitation energy region of the $\left[1s2s\right]\left({}^{3,1}S\right)n\mathcal{l}{n}^{\prime }{\mathcal{l}}^{\prime }$ doubly excited states has been measured using high brilliant soft x-ray undulator radiation, where square brackets indicate hole states. The observed absorption spectrum was analyzed with the aid of theoretical calculations based on the multiconfiguration Dirac-Fock (MCDF) method. The computation has revealed that the observed features are successfully assigned with several series of the resonant double excitations, such as the $\left[1s2s\right]\left({}^{3,1}S\right)3snp$ $(3⩽n⩽6)$ and the $\left[1s2s\right]\left({}^{3}S\right)3pns$ $(4⩽n⩽6)$ series. The $\left[1s2s\right]n\mathcal{l}{n}^{\prime }{\mathcal{l}}^{\prime }$ double excitation resonance leading to the lowest $\left[1s2s\right]\left({}^{3}S\right)3s3p$ state was analyzed using the well-known parametrized Fano profile. The energy as well as the width of the resonance for such highly correlated multiply excited state have been determined and the possible decay channels for the $\left[1s2s\right]\left({}^{3}S\right)3s3p$ state were also discussed.

The properties of photoassociation (PA) spectra near the intercombination line (the weak transition between and states) of group II atoms are theoretically investigated. As an example, we have carried out a calculation for calcium atoms colliding at ultralow temperatures of , , and . Unlike in most current photoassociation spectroscopy, the Doppler effect can significantly affect the shape of the investigated lines. Spectra are obtained using and short-range potentials and long-range van der Waals and resonance dipole potentials. The similar van der Waals coefficients of ground and excited states cause the PA to differ greatly from those of strong, allowed transitions with resonant dipole interactions. The density of spectral lines is lower, the Condon points are at relatively short range, the reflection approximation for the Franck-Condon factors is not applicable, and the spontaneous decay to bound ground-state molecules is efficient. Finally, the possibility of efficient production of cold molecules is discussed.

Phase effects in a quantum control experiment on the two-photon transition $4s\leftarrow \leftarrow 3s$ of atomic sodium are studied. In our experiment, we combine the generation of phase-locked interferometrically generated pulses with pulse-shaping techniques to study in detail the interplay of optical and quantum-mechanical phases. Subtle differences in the physical realization of the pulse sequences produce large difference in the control objective—the $4s$ population. The control mechanisms of more complex pulse sequences generated by periodic and discontinuous phase-modulation functions are investigated systematically and analyzed on the basis of quantum-mechanical interference.