We control the interspecies interaction in a two-species atomic quantum mixture by tuning the magnetic field at a Feshbach resonance. The mixture is composed by fermionic ${}^{40}\mathrm{K}$ and bosonic ${}^{87}\mathrm{Rb}$. We observe effects of the large attractive and repulsive interaction energy across the resonance, such as collapse or a reduced spatial overlap of the mixture, and we accurately locate the resonance position and width. Understanding and controlling instabilities in this mixture opens the way to a variety of applications, including formation of heteronuclear molecular quantum gases.

We demonstrate high-resolution high-contrast nonlinear optical spectroscopy based on phase-modulated ultrashort pulses. The problem of causing selective nonlinear optical excitation at one frequency while minimizing background is framed in terms of low autocorrelation binary sequences. We investigate phase functions with up to 608-bit resolution for their ability to induce two-photon excitation or stimulated Raman scattering, and find the highest theoretical and experimental signal-to-noise ratios yet reported. We confirm our findings by measuring experimental two-photon excitation spectra.

We study the effects of amplitude and phase damping decoherence in $d$-dimensional one-way quantum computation. We focus our attention on low dimensions and elementary unidimensional cluster state resources. Our investigation shows how information transfer and entangling gate simulations are affected for $d⩾2$. To understand motivations for extending the one-way model to higher dimensions, we describe how basic qudit cluster states deteriorate under environmental noise of experimental interest. In order to protect quantum information from the environment, we consider encoding logical qubits into qudits and compare entangled pairs of linear qubit-cluster states to single qudit clusters of equal length and total dimension. A significant reduction in the performance of cluster state resources for $d>2$ is found when Markovian-type decoherence models are present.

We propose a method to characterize and quantify multipartite entanglement for pure states. The method hinges upon the study of the probability density function of bipartite entanglement and is tested on an ensemble of qubits in a variety of situations. This characterization is also compared to several measures of multipartite entanglement.

The generation of three-mode continuous-variable entanglement in a quasiperiodically optical superlattice is studied theoretically in this paper. This work is based on the previous experiment result in which three-color light generated from a quasiperiodically optical superlattice through a stimulated parametric down-conversion cascaded with a sum-frequency process. The degree of quadrature phase amplitude correlations, a nonclassical characteristic, among the three mode was discussed by a sufficient inseparability criterion for continuous-variable entanglement, which was proposed by van Loock and Furusawa.

We discuss a toy model for adiabatic quantum computation which displays some phenomenological properties expected in more realistic implementations. This model has two free parameters: the adiabatic evolution parameter $s$ and the $\alpha$ parameter, which emulates many-variable constraints in the classical computational problem. The proposed model presents, in the $s\text{−}\alpha$ plane, a line of first-order quantum phase transition that ends at a second-order point. The relation between computation complexity and the occurrence of quantum phase transitions is discussed. We analyze the behavior of the ground and first excited states near the quantum phase transition, the gap, and the entanglement content of the ground state.

Techniques to calculate one-loop radiative corrections to hyperfine splitting including binding corrections to all orders have been developed in the last decade for $s$ states of atoms and ions. In this paper these methods are extended to $p_{1∕2}$ states for three cases. In the first case, the point-Coulomb $2p_{1∕2}$ hyperfine splitting is treated for the hydrogen isoelectonic sequence, and the lowest order result, $\frac{\alpha }{4\pi }{E}_F$, is shown to have large binding corrections at high $Z$. In the second case, neutral alkali-metal atoms are considered. In the third case, hyperfine splitting of the $2p_{1∕2}$ state of lithiumlike bismuth is treated. In the latter two cases, correlation corrections are included and, in addition, the point is stressed that uncertainties associated with nuclear structure, which complicate comparison with experiment for $s$ states, are considerably reduced because of the smaller overlap with the nucleus.

We present detailed collisional-radiative modeling for a benchmark x-ray spectrum of highly charged tungsten ions in the range between 3 and $10\mathrm{\AA }$ produced in an electron beam ion trap (EBIT) with a beam energy of $4.08\mathrm{keV}$. Remarkably good agreement between calculated and measured spectra was obtained without adjustable parameters, highlighting the well-controlled experimental conditions and the sophistication of the kinetic simulation of the non-Maxwellian tungsten plasma. This agreement permitted the identification of spectral lines from Cu-like ${\mathrm{W}}^{45+}$ and Ni-like ${\mathrm{W}}^{46+}$ ions, led to the reinterpretation of a previously known line in Ni-like ion as an overlap of electric-quadrupole and magnetic-octupole lines, and revealed subtle features in the x-ray spectrum arising from the dominance of forbidden transitions between excited states. The importance of level population mechanisms specific to the EBIT plasma is discussed as well.

The experimental photoabsorption spectra of singly charged cationic mercury clusters ($\mathrm{Hg}_N{}^{+}$) show a sharp change in behavior at cluster size $N=6$. Both relativistic density functional theory (DFT) and wave function based methods reveal that this corresponds to a structural change from linear to three-dimensional isomers. The simulated electronic excitation spectra obtained from time-dependent relativistic DFT agree well with the experimental results. Our quantum theoretical treatment confirms the change from single electron-hole excitations in small linear clusters to plasmonlike collective transitions for the larger three-dimensional clusters.

The interaction of atomic clusters with short, intense pulses of laser light to form extremely hot, dense plasmas has attracted extensive experimental and theoretical interest. The high density of atoms within the cluster greatly enhances the atom-laser interaction, while the finite size of the cluster prevents energy from escaping the interaction region. Recent technological advances have allowed experiments to probe the laser-cluster interaction at very high photon energies, with interactions much stronger than suggested by theories for lower photon energies. We present a model of the laser-cluster interaction that uses nonperturbative $R$-matrix techniques to calculate inverse bremsstrahlung and photoionization cross sections for Herman-Skillman atomic potentials. We describe the evolution of the cluster under the influence of the processes of inverse bremsstrahlung heating, photoionization, collisional ionization and recombination, and expansion of the cluster. We compare total energy absorbed with the Hamburg experiment of Wabnitz et al. [Nature 420, 482 (2002)] and ejected electron spectra with Laarmann et al. [Phys. Rev. Lett. 95, 063402 (2005)].

We consider the quantum reflection of vortices in Bose-Einstein condensates incident on a solid surface. Compared with the condensate without angular momentum, the rotation of the vortex gives unique opportunities to consider the interaction process between a matter wave and a solid surface. Due to the atom-surface interaction and dynamic excitations induced by interatomic interaction, our numerical simulations show that the reflected vortex would become unstable for sufficiently low velocity and large interaction strength. The stability diagram for the reflected vortex is given, and physical mechanism for this stability diagram is discussed. It is found that the expansion ratio between the reflected and incident vortices gives the boundary between stable and unstable regions for the reflected vortex.

We describe solitary wave excitations in a Bose-Fermi mixture loaded in a one-dimensional and strongly elongated lattice. We focus on the mean-field theory under the condition that the fermion number significantly exceeds the boson number, and limit our consideration to lattice amplitudes corresponding to the order of a few recoil energies or less. In such a case, the fermionic atoms display “metallic” behavior and are well-described by the effective mass approximation. After classifying the relevant cases, we concentrate on gap solitons and coupled gap solitons in the two limiting cases of large and small fermion density, respectively. In the former, the fermionic atoms are distributed almost homogeneously and thus can move freely along the lattice. In the latter, the fermionic density becomes negligible in the potential maxima, and this leads to negligible fermionic current in the linear regime.

We propose a linear optical scheme for direct realization of a nondestructive $N$-qubit controlled phase gate without resorting to a sequence of single- and two-qubit gates. The proposed setup involves linear optical elements, nonmaximally entangled pairs of photons, and multiphoton coincidence detection. To implement the scheme, physical qubits represented by the polarization states of single photons are probabilistically encoded into the two-photon states. Based on such encoding operation, a nondestructive $N$-qubit phase gate can be directly achieved by $N$-photon interference on an array of polarization beam splitters and $N$-photon coincidence detection.

The experimental realization of optimal symmetric phase-covariant $1\to 2$ cloning of qubit states is presented. The qubits are represented by polarization states of photons generated by spontaneous parametric down-conversion. The experiment is based on the interference of two photons on a custom-made beam splitter with different splitting ratios for vertical and horizontal polarization components. From the measured data we have estimated the implemented cloning transformation using the maximum-likelihood method. The result shows that the realized transformation is very close to the ideal one and the map fidelity reaches 94%.

We study the relation between energy and entanglement in an entanglement transfer problem. We first analyze the general setup of two entangled qubits (“$a$” and “$b$”) exchanging this entanglement with two other independent qubits (“$A$” and “$B$”). Qubit “$a$” (“$b$”) interacts with qubit “$A$” (“$B$”) via a spin-exchange-like unitary evolution. A physical realization of this scenario could be the problem of two-level atoms transferring entanglement to resonant cavities via independent Jaynes-Cummings interactions. We study the dynamics of entanglement and energy for the second pair of qubits (tracing out the originally entangled ones) and show that these quantities are closely related. For example, the allowed quantum states occupy a restricted area in a phase diagram entanglement vs energy. Moreover, the curve which bounds this area is exactly the one followed if both interactions are equal and the entire four qubit system is isolated. We also consider the case when the target pair of qubits is subjected to losses and can spontaneously decay.

We show that every correctable subsystem for an arbitrary noise operation can be recovered by a unitary operation, where the notion of recovery is more relaxed than the notion of correction insofar as it does not protect the subsystem from subsequent iterations of the noise. We also demonstrate that in the case of unital noise operations one can identify a subset of all correctable subsystems—those that can be corrected by a single unitary operation—as the noiseless subsystems for the composition of the noise operation with its dual. Using the recently developed structure theory for noiseless subsystems, the identification of such unitarily correctable subsystems is reduced to an algebraic exercise.

Experimental and theoretical studies of the energy spectra of electrons ejected in ${\mathrm{H}}^{-}\text{−}\mathrm{He}$ collisions have been performed. Comparison of calculations using the Sturmian and advanced adiabatic theories with experimental data reveals the existence of a correlated electron detachment mechanism, which yields the main contribution to formation of the high-energy part of the ejected electron energy distribution. This mechanism is associated with dynamical energy transfer to the loosely bound $1s^{\prime }$ electron of ${\mathrm{H}}^{-}$ in the course of superpromotion of the inner $1s$ electron.

We have measured the relative photoionization cross section for the formation of ${\mathrm{Li}}^{2+}$ ions between 148 and $161\mathrm{eV}$ photon energy with higher photon-energy resolution than in previous ${\mathrm{Li}}^{2+}$ studies. This energy region is characterized by double and triple excitations that lead to strong enhancements in the ${\mathrm{Li}}^{2+}$ cross section. As a result, the double-to-single photoionization ratio shows a dramatic resonance structure not seen before. We have determined the resonance positions and widths using Fano-profile fits to the ${\mathrm{Li}}^{2+}$ data and compare them to previously published values and a calculated ${\mathrm{Li}}^{2+}$ cross-section curve.

The multiphoton ionization of hydrogen atoms in a strong elliptically polarized microwave field exhibits complex features that are not observed for ionization in circular and linear polarized fields. Experimental data reveal high sensitivity of ionization dynamics to the small changes of the field polarization. The multidimensional nature of the problem makes widely used diagnostics of dynamics, such as Poincaré surfaces of section, impractical. We analyze the phase-space dynamics using the finite time stability analysis rendered by the fast Lyapunov indicators technique. The concept of zero-velocity surface is used to initialize the calculations and visualize the dynamics. Our analysis provides stability maps calculated for the initial energy at the maximum and below the saddle of the zero-velocity surface. We estimate qualitatively the dependence of ionization thresholds on the parameters of the applied field, such as polarization and scaled amplitude.