A state-selective investigation of radiative recombination (RR) at threshold energies has been performed by x-ray detection at the electron cooler of the experimental storage ring (ESR) at Gesellschaft für Schwerionenforschung (GSI) for decelerated bare and hydrogenlike uranium ions. The application of the deceleration mode enabled us to observe state-selective RR into the $K$ and $L$ shells of bare and H-like uranium. The observed relative RR intensities into these low-lying bound states exhibit a good agreement with rigorous relativistic predictions, whereas the nonrelativistic approach fails to reproduce the data. Furthermore, the scaled relativistic one-electron treatment seems to be insufficient for the state-selective description of RR into H-like species at extremely low energies, suggesting the importance of electron-electron interactions for the RR process at the threshold.

Through the coincidence measurement of x rays with secondary electrons, x-ray spectra, and their yields have been measured in the interaction of H-like and bare highly charged ions (${\text{I}}^{52+}$ and ${\text{I}}^{53+}$) with different target materials (Be, C, Cu, and W). $K$ x-ray yields are independent of the material, where the $K$ vacancies are almost filled through $K$ x-ray emission. $L$ x-ray yields are affected by the electronic state of the targets and the interacting projectiles, because the states involved in the electron capture depend on the level structure of the projectile and the target and the possible number of electrons captured by the highly charged ions is different depending on the number of electrons in the target states $\left(2n^2\right)$ which are resonant transferred to the projectile states in the dynamics of below-surface neutralization and deexcitation.

Recent studies of quantum phase transitions in coupled atom-cavity arrays have focused on the similarities between such systems and the Bose-Hubbard model. However, the bipartite nature of the atom-cavity systems that make up the array introduces some differences. In order to examine the unique features of the coupled-cavity system, the behavior of a simple two-site model is studied over a wide range of parameters. Four regions are identified, in which the ground state of the system may be classified as a polaritonic insulator, a photonic superfluid, an atomic insulator, or a polaritonic superfluid.

Closed expressions are derived for resonant coherent multidimensional spectra carried out with temporally well-separated pulses. The roles of the pulse carrier frequencies, phases, bandwidths, and envelopes can be readily analyzed. These results are particularly suitable for the design of new pulse sequences and for implementing coherent-control pulse-shaping algorithms for optimizing these signals to meet specific targets.

We study the properties of radiation generated in ultralong fiber lasers and find an interesting link between these optical systems and the theory of weak wave turbulence. Experimental observations strongly suggest that turbulentlike weak interactions between the multitude of laser cavity modes are responsible for practical characteristics of ultralong fiber lasers such as spectra of the output radiation.

We provide a general framework to describe cooling of a micromechanical oscillator to its quantum ground state by means of radiation-pressure coupling with a driven optical cavity. We apply it to two experimentally realized schemes, back-action cooling via a detuned cavity and cold-damping quantum-feedback cooling, and we determine the ultimate quantum limits of both schemes for the full parameter range of a stable cavity. While both allow one to reach the oscillator’s quantum ground state, we find that back-action cooling is more efficient in the good cavity limit, i.e., when the cavity bandwidth is smaller than the mechanical frequency, while cold damping is more suitable for the bad cavity limit. The results of previous treatments are recovered as limiting cases of specific parameter regimes.

The one-way measurement model is a framework for universal quantum computation in which algorithms are partially described by a graph $G$ of entanglement relations on a collection of qubits. A sufficient condition for an algorithm to perform a unitary embedding between two Hilbert spaces is for the graph $G$, together with input and output $\mathit{I}$, $\mathit{O}$ vertices $I,O\subseteq V\left(G\right)$, to have a flow in the sense introduced by Danos and Kashefi [Phys. Rev. A 74, 052310 (2006)]. For the special case of $\left|I\right|=\left|O\right|$, using a graph-theoretic characterization, I show that such flows are unique when they exist. This leads to an efficient algorithm for finding flows by a reduction to solved problems in graph theory.

The excitation energies of $ns$, $np$, $nd$, and $nf$ $\left(n\le 6\right)$ states in neutral lithium are evaluated within the framework of relativistic many-body theory. First-, second-, third-, and all-order Coulomb energies and first- and second-order Breit corrections to energies are calculated. All-order calculations of reduced matrix elements, oscillator strengths, transition rates, and lifetimes are given for levels up to $n=4$. Electric-dipole $\left(2s-np\right)$, electric-quadrupole $\left(2s-nd\right)$, and electric-octupole $\left(2s-nf\right)$, matrix elements are evaluated to obtain the corresponding ground-state multipole polarizabilities using the sum-over-states approach. Scalar and tensor polarizabilities for the $2p_{1/2}$ and $2p_{3/2}$ states are also calculated. Magnetic-dipole hyperfine constants $A$ are determined for low-lying levels up to $n=4$. The quadratic Stark shift for the $\left(F=2M=0\right)\leftrightarrow \left(F=1M=0\right)$ ground-state hyperfine transition is found to be $-0.0582\text{Hz}/{\left(\text{kV}/\text{cm}\right)}^2$, in slight disagreement with the experimental value $-0.061\pm 0.002\text{Hz}/{\left(\text{kV}/\text{cm}\right)}^2$. Matrix elements used in evaluating polarizabilities, hyperfine constants, and the quadratic Stark shift are obtained using the all-order method.

We investigate the laser-assisted photoelectric effect from a solid surface. By illuminating a $\mathrm{Pt}\left(111\right)$ sample simultaneously with ultrashort 1.6 and $42\mathrm{eV}$ pulses, we observe sidebands in the extreme ultraviolet photoemission spectrum, and accurately extract their amplitudes over a wide range of laser intensities. Our results agree with a simple model, in which soft x-ray photoemission is accompanied by the interaction of the photoemitted electron with the laser field. This strong effect can definitively be distinguished from other laser surface interaction phenomena, such as hot electron excitation, above-threshold photoemission, and space-charge acceleration. Thus, laser-assisted photoemission from surfaces promises to extend pulse duration measurements to higher photon energies, as well as opening up measurements of femtosecond-to-attosecond electron dynamics in solid and surface-adsorbate systems.

We present an alternative approach to analyze atomic behavior when an external field perturbation is not sudden for a number of states of the field-free system. It is shown that the probability amplitudes for the system to be in these states can be accurately estimated from the closed set of their integral equations. Numerical examples for an electron in a one-dimensional Coulomb potential interacting with (i) laser and (ii) half-cycle pulses are provided. Comparison with exact calculations indicates the strength of the approach.

Bound-state molecules can be photoassociated directly from ultracold free-atom pairs by excitation to a purely repulsive electronic state. The process is explained on the basis of quantum unitarity: the initially free-scattering state is transformed by an impulsive light pulse to a deformed superposition which contains bound-state components. For pulse durations which are short compared to the ultracold dynamics, the maximal rate of photoassociation was found to be determined by the initial stationary distribution of scattering states of the atom pairs. The process was simulated for an ultracold gas of ${}^{87}\text{R}\text{b}$ with a temperature of $T=44\mu \text{K}$ and a density of $\approx {10}^{11}{\text{cm}}^{-3}$. Transform-limited pulses maximize the photoassociation, yielding $\sim 1$ bound molecule per pulse. Coherent control calculated by a local control scheme can increase the photoassociation yield by two orders of magnitude.

Ultrashort pulses of uv and soft x-ray radiation with durations ranging from femtoseconds to attoseconds can be produced as high-order harmonics of the fundamental frequency of a laser beam focused into gas. Applications to fields such as spectroscopy and attosecond metrology require the control and characterization of spectral and spatial properties of the emitted radiation. These are determined by both single atom and macroscopic response of the interaction medium to the laser field. Here we present evidence that microscopic effects have a larger influence than previously thought, and can induce a splitting and a frequency shift of the harmonic lines. These results not only offer a direct diagnostic for high-order harmonic generation, but also enable us to better tune the parameters of the produced radiation, while giving a deeper insight into the fundamental physics underlying this nonlinear optical process.

We determine the effects of quantum fluctuations about the $T=0$ mean-field solution of the BCS-BEC crossover in a dilute Fermi gas using the functional integral method. These fluctuations are described in terms of the zero-point motion of collective modes and the virtual scattering of gapped quasiparticles. We calculate their effects on various measurable properties, including chemical potential, ground-state energy, the gap, the speed of sound and the Landau critical velocity. At unitarity, we find excellent agreement with quantum Monte Carlo and experimental results. In the BCS limit, we show analytically that we obtain Fermi liquid interaction corrections to thermodynamics including the Hartree shift. In the Bose-Einstein condensation (BEC) limit, we show that the theory leads to an approximate description of the reduction of the scattering length for bosonic molecules and also obtain quantum depletion of the Lee-Yang form. At the end of the paper, we describe a method to include feedback of quantum fluctuations into the gap equation, and discuss the problems of self-consistent calculations in satisfying Goldstone’s theorem and obtaining ultraviolet finite results at unitarity.

An accurate description of a radially polarized fundamental Gaussian beam is presented on the basis of complex-source-point spherical waves (CSPSWs). In contrast to other descriptions based on the perturbative Lax series, the expressions for the electromagnetic field components of this description have explicit and simple mathematical forms. Numerical calculations show that both paraxial and fifth-order corrected beam descriptions have large relative error when the diffraction angle is large, while the accurate description based on the CSPSW approach proposed here can give field expressions which satisfy Maxwell’s equations with great accuracy.

We compute lower bounds for the exact quantum query complexity of a ternary function $f$. The lower bound is of order $O\mathbf{\left(}{\text{log}}_3\left(n\right)\mathbf{\right)}$. In case $f$ is symmetric on a sphere then the lower bound is of order $O\left(\sqrt{n}\right)$. This work is a natural continuation of the work of Beals, Buhrman, Cleve, Mosca, and de Wolf on lower limits for binary functions.

We explore the finite-temperature phase diagram of the anisotropic $XY$ spin chain using the quantum Chernoff bound metric on thermal states. The analysis of the metric elements allows one to easily identify, in terms of different scaling with temperature, quasiclassical and quantum-critical regions. These results extend recent ones obtained using the Bures metric and show that different information-theoretic notions of distance can carry the same sophisticated information about the phase diagram of an interacting many-body system featuring quantum-critical points.

The major goal of high-precision studies of rovibrational states in the hydrogen molecular ions is to provide an alternative way for improving the electron-to-proton mass ratio, or the atomic mass of electron. By now the complete set of relativistic and radiative corrections have been obtained for a wide range of rovibrational states of $\text{H}_2{}^{+}$ and ${\text{HD}}^{+}$ up to order $R_{\infty }{\alpha }^4$. In this work we complete calculations of various contributions to the $R_{\infty }{\alpha }^4$ order by computing the relativistic corrections to the binding energy of electron.

The asymptotic ratios of double-to-single photoionization cross sections of plasma-embedded helium atoms at very high but nonrelativistic photon energies are calculated for different shielding parameters that are based on the Debye model for weakly coupled plasmas. The ratio of double-to-single photoionization for free atom is determined as 1.644%, in good agreement with the synchrotron measurements. The asymptotic ratio of double-to-single photoionization cross sections for different screening parameters are reported.

We present a quantum-theoretical treatment of biphoton generation in single-resonant type-II parametric down-conversion. The nonlinear medium is continuously pumped and is placed inside a cavity which is resonant for the signal field, but nonresonant for the idler deflected by an intracavity polarizing beam splitter. The intensity of the classical pump is assumed to be sufficiently low in order to yield a biphoton production rate that is small compared to the cavity loss rate. Explicit expressions are derived for the rate of biphoton generation and for the biphoton wave function. The output spectra of the signal and idler field are determined, as well as the second-order signal-idler cross-correlation function which is shown to be asymmetric with respect to the time delay. Due to frequency entanglement in the signal-idler photon pair, the idler spectrum is found to reveal the longitudinal mode structure of the cavity, even though the idler field is not resonant.