We report measurements of the isotope shifts of the $\left(3s3p\right){{}^{3}P}_{0,1,2}\text{−}\left(3s4s\right){{}^{3}S}_1$ Mg I transitions for the stable isotopes ${}^{24}\text{M}\text{g}$ $\left(\text{I}=0\right)$, ${}^{25}\text{M}\text{g}$ $\left(\text{I}=5/2\right)$, and ${}^{26}\text{M}\text{g}$ $\left(\text{I}=0\right)$. Furthermore, the ${}^{25}\text{M}\text{g}$ ${{}^{3}S}_1$ hyperfine coefficient $A\left({{}^{3}S}_1\right)=\left(-321.6\pm 1.5\right)\text{MHz}$ is extracted and found to be in excellent agreement with state-of-the-art theoretical predictions giving $A\left({{}^{3}S}_1\right)=-325\text{MHz}$ and $B\left({{}^{3}S}_1\right)\simeq {10}^{-5}\text{MHz}$. Compared to previous measurements, the data presented in this work are improved up to a factor of 10.

The energy spectra of atomic ions are re-examined from the point of view of Thomas-Fermi scaling relations. For the first ionization potential, which sets the energy scale for the true discrete spectrum, Thomas-Fermi theory predicts the following relation: $E_{ioniz}=Z^2{N}^{-2/3}g\left(N/Z\right)$, where $Z$ is the nuclear charge, $N$ is the number of electrons, and $g$ is a function of $N/Z$. This relation does not hold for neutral atoms, but works extremely well in the cationic domain, $Z>N$. We provide an analytic expression for $g$, with two adjustable parameters, which fits the available experimental data for more than 380 ions. In addition, we show that a rough fit to the integrated density of states with a single exponential: $N_{states}=\text{exp}\left(\Delta E/\Theta \right)$, where $\Delta E$ is the excitation energy, leads to a parameter, $\Theta$, exhibiting a universal scaling a la Thomas-Fermi: $\Theta =Z^2{N}^{-4/3}h\left(N/Z\right)$, where $h$ is approximately linear near $N/Z=1$.

The pulse duration of soft x-ray free-electron laser (FEL) radiation is directly measured by time-resolved observation of doubly charged helium ions at 51.8 eV. A wave front splitting autocorrelator produces two correlated FEL pulses with a resolution of better than a femtosecond. In the interesting intensity range from ${10}^{13}$ to ${10}^{16}\text{W}/{\text{cm}}^2$ direct and sequential double ionization contribute to the ion yield which has significant influence on the correlation width, being a general feature at high photon energies. Here, a duration of ${\tau }_L=\left(29\pm 5\right)\text{fs}$ is derived for the soft x-ray pulses at FLASH.

Nonclassical properties of light propagating through the turbulent atmosphere are studied. We demonstrate by numerical simulation that the probability distribution of the transmission coefficient, which characterizes the effects of the atmosphere on the quantum state of light, can be reconstructed by homodyne detection. Nonclassical photonstatistics and, more generally, nonclassical Glauber-Sudarshan functions appear to be more robust against turbulence for weak light fields rather than for bright ones.

We explore the entanglement dynamics of two qubits embedded in one-dimensional (1D) and three-dimensional (3D) photonic crystals. Our 1D results show sudden death and sudden birth of the entanglement, restricted by the initial coherence and the population in the excited state as well as the detuning. The results for the 3D crystal show entanglement death but no birth. We also discuss the entanglement of $N$ qubits in the aforementioned photonic crystals.

We apply a recently demonstrated method for precision spectroscopy on strong transitions in trapped ions to measure both fine-structure components of the $3s\text{−}3p$ transition in ${{}^{24}\text{M}\text{g}}^{+}$ and ${{}^{26}\text{M}\text{g}}^{+}$. We deduce absolute frequency reference data for transition frequencies, isotope shifts, and fine-structure splittings that in particular are useful for comparison with quasar absorption spectra, which test possible space-time variations of the fine-structure constant. The measurement accuracy improves previous literature values, when existing, by more than two orders of magnitude.

We develop a numerical technique to describe few-body systems. Correlated Gaussian basis functions are used to expand the channel functions in the hyperspherical representation. The method is proven to be robust and efficient compared to other numerical techniques. The method is applied to few-body systems with short-range interactions, including several examples for three- and four-body systems. Specifically, for the two-component, four-fermion system, we extract the coefficients that characterize its behavior at unitarity.

Nondipole photodetachment parameters for the $3p$ subshell of ${\text{Cl}}^{-}$ are calculated using relativistic random-phase approximation (RRPA). Interchannel-coupling effects on nondipole photodetachment parameters for the $3p$ subshell are studied with different degrees of truncation of the RRPA. It is inferred from the present investigations that the shape resonance in the photodetachment cross-section profile and the positions of the Cooper minima are sensitive to interchannel coupling. An analysis of the angular distribution of the photoelectrons reveals how the Cooper minima in the dipole and the quadrupole channels introduce a complex energy dependence, with peaks and zeroes, of the nondipole asymmetry parameter $\zeta$.

We propose a method to stop a pulse of particles with different velocities by making them collide with an accelerated wall with trajectory $\sim \sqrt{t}$. Classical and quantum-mechanical cases are discussed, and numerical simulations are provided that show the efficiency of the method.

We have theoretically investigated the vibrational population transfer in the presence of manifolds of rotational levels in a model 15-level ${\text{Li}}_2$ molecular system by stimulated Raman adiabatic passage with intense unchirped femtosecond laser pulses. We have considered five rotational levels $J_g=0,2,\dots ,8$ associated with the initial vibrational level $v_g=0$ of the ground electronic state $X{{}^1\Sigma }_g^{+}$, five rotational levels $J_i=1,3,\dots ,9$ associated with the vibrational level $v_i=1$ of the intermediate excited electronic state $A{{}^1\Sigma }_u^{+}$, and five rotational levels $J_f=0,2,\dots ,8$ associated with the final target vibrational levels $v_f=1,2$ of the $X{{}^1\Sigma }_g^{+}$ state of the ${\text{Li}}_2$ alkali dimer. The pump and Stokes laser pulses are taken to have the same temporal (Gaussian) shapes, pulse widths, and linear parallel polarizations. The laser wavelengths are in the (visible) range of 700–740 nm. The (peak) intensities of the pulses are of the order of $1\times {10}^{12}–2.5\times {10}^{13}\text{W}/{\text{cm}}^2$ and the pulse widths are of the order of 20–50 fs. We have solved the 15 coupled time-dependent Schrödinger equations to study the population transfer to the target-excited vibrational levels. We have observed that, even with “mushrooming” of closely spaced rotational levels, efficient and selective population transfer to excited vibrational levels in ${\text{Li}}_2$ alkali dimer can be achieved for counterintuitive pulse sequence by controlling the pump and Stokes laser parameters. We have endeavored to explain the results within the framework of adiabatic picture.

We study Anderson localization of ultracold atoms in weak one-dimensional speckle potentials using perturbation theory beyond Born approximation. We show the existence of a series of sharp crossovers (effective mobility edges) between energy regions where localization lengths differ by orders of magnitude. We also point out that the correction to the Born term explicitly depends on the sign of the potential. Our results are in agreement with numerical calculations in a regime relevant for experiments. Finally, we analyze our findings in the light of a diagrammatic approach.

By direct numerical simulation of the time-dependent Gross-Pitaevskii equation, we study different aspects of the localization of a noninteracting ideal Bose-Einstein condensate (BEC) in a one-dimensional bichromatic quasiperiodic optical-lattice potential. Such a quasiperiodic potential, used in a recent experiment on the localization of a BEC [Roati et al., Nature (London) 453, 895 (2008)], can be formed by the superposition of two standing-wave polarized laser beams with different wavelengths. We investigate the effect of the variation of optical amplitudes and wavelengths on the localization of a noninteracting BEC. We also simulate the nonlinear dynamics when a harmonically trapped BEC is suddenly released into a quasiperiodic potential, as done experimentally in a laser speckle potential [Billy et al., Nature (London) 453, 891 (2008)]. We finally study the destruction of the localization in an interacting BEC due to the repulsion generated by a positive scattering length between the bosonic atoms.

We propose a scheme of electromagnetically induced left-handedness in a dense atomic vapor medium. By applying an incoherent pump field to manipulate the populations in atomic levels, and a strong coherent field to create quantum coherence, the magnetic part of the probe field can be amplified instead of strong absorption as usual materials. The system has negative permittivity and permeability simultaneously with transparent propagation of the electric part of the probe field. It is shown that the pumping rate of incoherent field, the intensity and frequency detuning of the coherent field, exert effects on the properties of the negative refractive index of left-handed material.

We report experimental evidence of soliton modulation instability in erbium-doped fiber lasers. An alternate type of spectral sideband generation was always experimentally observed on the soliton spectrum of the erbium-doped soliton fiber lasers when energy of the formed solitons reached beyond a certain threshold value. Following this spectral sideband generation, if the pump power of the lasers was further increased, either a new soliton would be formed or the existing solitons would experience dynamical instabilities, such as the period-doubling bifurcations or period-doubling route to chaos. We point out that the mechanism for this soliton spectral sideband generation is the modulation instability of the solitons in the lasers. We further show that, owing to the internal energy balance of a dissipative soliton, modulation instability itself does not destroy the stable soliton evolution in a laser cavity. It is the strong resonant wave coupling between the soliton and dispersive waves that leads to the dynamic instability of the solitons.

We present the design, characterization, and modeling of a specific optical metamaterial, and employ it to manipulate the light polarizations at optical frequencies. Experimental results reveal that the maximum polarization conversion efficiency, i.e., the energy portion converted from $s$ to $p$ polarization after reflection, can be as high as 96% at the wavelength of $\sim 685\text{nm}$. Simulations and analytical results, which are in reasonable agreements with the experimental results, reveal that the underlying physics are governed by the particular electric and magnetic resonances in the optical metamaterial.

We study superfluid dissipation due to phase slips for a Bose-Einstein condensate flowing through a repulsive barrier inside a torus. The barrier is adiabatically raised across the annulus, while the condensate flows with a finite quantized angular momentum. At a critical height, a vortex moves from the inner region and reaches the barrier to eventually circulate around the annulus. At a higher critical height, an antivortex also enters into the torus from the outer region. Both vortex and antivortex decrease the total angular momentum by leaving behind a $2\pi$ phase slip. When they collide and annihilate or orbit along the same loop, the condensate suffers a global $2\pi$ phase slip, and the total angular momentum decreases by one quantum. In hydrodynamic regime, the instability sets in when the local superfluid velocity equals the sound speed inside the barrier region.

Incoherent scattering of photons off two remote atoms with a $\Lambda$-level structure is used as a basic Young-type interferometer to herald long-lived entanglement of an arbitrary degree. The degree of entanglement, as measured by the concurrence, is found to be tunable by two easily accessible experimental parameters. Fixing one of them to certain values unveils an analog to the Malus’ law. An estimate of the variation in the degree of entanglement due to uncertainties in an experimental realization is given.

Estimates of the quantum accuracy threshold often tacitly assume that it is possible to interact arbitrary pairs of qubits in a quantum computer with a failure rate that is independent of the distance between them. None of the many physical systems that are candidates for quantum computing possess this property. Here we study the performance of a concatenated error-detection code in a system that permits only nearest-neighbor interactions in one dimension. We make use of a message-passing scheme that maximizes the number of errors that can be reliably corrected by the code. Our numerical results indicate that arbitrarily accurate universal quantum computation is possible if the probability of failure of each elementary physical operation is below approximately ${10}^{-5}$. This threshold is three orders of magnitude lower than the highest known.

We present a precise measurement of the lifetime of the $6p{{}^{2}P}_{1/2}^o$ excited state of a single trapped ytterbium ion $\left({\text{Yb}}^{+}\right)$. A time-correlated single-photon counting technique is used, where ultrafast pulses excite the ion and the emitted photons are coupled into a single-mode optical fiber. By performing the measurement on a single atom with fast excitation and excellent spatial filtering, we are able to eliminate common systematics. The lifetime of the $6p{{}^{2}P}_{1/2}^o$ state is measured to be $8.12\pm 0.02\text{ns}$, in good agreement with previous experimental measurements and some theoretical values.