A two-dimensional model atom is employed to study the ionization behavior of helium subjected to strong laser fields in the high-frequency regime. The evolution of the system is studied by means of numerical integration of the Schrödinger equation beyond the dipole approximation. Ionization probabilities of the two-electron atom in highly intense laser fields have been calculated for different pulse shapes. It is confirmed that the mutual repulsion between the two electrons as well as the length of the laser pulses significantly alter the ionization probabilities of the system. Nondipole effects are shown to lead to a considerable increase of the ionization probabilities. For certain laser pulse shapes, a regime of ionization suppression is investigated which exists in addition to two-electron stabilization. The applicability of our model scheme to the case of heliumlike systems is discussed, and it is shown that stabilization may also occur in these systems, and how it is altered in dependence of the nuclear charge.

We propose a scheme utilizing quantum interference to control the transport of atoms in a one-dimensional optical lattice by a single impurity atom. The two internal state of the impurity represent a spin-$1∕2$ (qubit), which in one spin state is perfectly transparent to the lattice gas, and in the other spin state acts as a single atom mirror, confining the lattice gas. This allows one to “amplify” the state of the qubit, and provides a single-shot quantum nondemolition measurement of the state of the qubit. We derive exact analytical expression for the scattering of a single atom by the impurity, and give approximate expressions for the dynamics of a gas of many interacting bosonic or fermionic atoms.

The main goal of this paper is to provide a connection between the generalized robustness of entanglement $\left(R_g\right)$ and the geometric measure of entanglement $\left(E_{GME}\right)$. First, we show that the generalized robustness is always higher than or equal to the geometric measure. Then we find a tighter lower bound to $R_g\left(\rho \right)$ based only on the purity of $\rho$ and its maximal overlap to a separable state. As we will see it is also possible to express this lower bound in terms of $E_{GME}$.

Using the known perturbation series for the idempotent Dirac density matrix in powers of a given one-body potential $V\left(\mathbf{r}\right)$, Stoddart and March (SM) generated a corresponding series for the kinetic energy density. While the general term of the SM series is known, a summation has not been achieved to date. A contribution to solve this problem is made here by exhibiting an explicit form of the above kinetic energy density for Fermions filling an arbitrary number of closed shells, when the confinement is harmonic. This example is of considerable current interest because of ongoing experiments on ultracold atomic gases of Fermions.

We investigate multipartite entanglement dynamics in disordered spin-$1∕2$ lattice models exhibiting a transition from integrability to quantum chaos. Borrowing from the generalized entanglement framework, we construct measures for correlations relative to arbitrary local and bilocal spin observables, and show how they naturally signal the crossover between distinct dynamical regimes. Analytical estimates are obtained in the short- and long-time limits. Our results are in qualitative agreement with predictions from the random matrix theory and are relevant to both condensed-matter physics and to the stability of quantum information in disordered quantum computing hardware.

The quantum cloner machine maps an unknown arbitrary input qubit into two optimal clones and one optimal flipped qubit. By combining linear and nonlinear optical methods we experimentally implement a scheme that, after the cloning transformation, restores the original input qubit in one of the output channels, by using local measurements, classical communication, and feedforward. This nonlocal method demonstrates how the information on the input qubit can be restored after the cloning process. The realization of the reversion process is expected to find useful applications in the field of modern multipartite quantum cryptography.

We derive an expression for the total photodissociation cross section of a molecule incorporating both direct and indirect processes that proceed through excited resonances, and show that it exhibits generalized Beutler-Fano line shapes. Assuming that the closed system can be modeled by random-matrix theory, we derive the statistical properties of the photodissociation cross section and find that they are significantly affected by the direct processes. In the limit of isolated resonances, we find that direct processes suppress the correlation hole of the cross-section autocorrelation function and lead to a maximum in the cross-section distribution.

Chemical reactions between ultracold ${}^9\mathrm{Be}^{+}$ ions and room-temperature molecular hydrogen isotopomers and between ultracold ${\mathrm{H}}_3^{+}$ ions and room-temperature ${\mathrm{O}}_2$ have been studied in a laser-cooling ion trap apparatus. For small Coulomb crystals of beryllium ions, reactions can be followed at the single-ion level. We demonstrate characterization of a chemical reaction in which neither one of the reactants nor the product is directly detectable. In this case molecular dynamics simulations were used for the determination of ion numbers from images of the ${}^9\mathrm{Be}^{+}$ ion ensemble. The observed reaction rates are in agreement with the Langevin ion-neutral reaction theory.

We have investigated the process of high-order harmonic generation in light alkanes by using femtosecond laser pulses. We show the experimental results cannot be matched by a model that assumes a single active electron only in a hydrogenic $s$ orbital. Clear evidences are shown of the important role played by the $p$-like character originating from the covalent $\mathrm{C}\text{−}\mathrm{H}$ bond. By constructing a suitable mixture of $s$-type and $p$-type atomic wave functions, an excellent agreement between measurements in methane and simulations is found, thus confirming the validity of the developed method as a general tool for the analysis of high-order harmonic generation in complex molecules.

Based on the dielectric formalism in the generalized random phase approximation, we generalize the description of a Bose-Einstein condensed gas to allow for a relative velocity between the superfluid and normal fluid. In this model, we determine the critical velocity dynamically as the transition point between stable and unstable dynamics. Unlike the zero temperature case, at finite temperature the relative critical velocity of a dilute Bose-Einstein gas is lower than the sound velocity. The requirement of Galilean invariance essential to obtain an invariant result necessitates the use of a gapless and conserving approximation.

We study the number of coincidences in a Hong-Ou-Mandel interferometer exit whose arms have been supplemented with the addition of one or two optical cavities. The fourth-order correlation function at the beam splitter exit is calculated. In the regime where the cavities lengths are larger than the one-photon coherence length, photon coalescence and anticoalescence interference is observed. Feynman’s path diagrams for the indistinguishable processes that lead to quantum interference are presented. The construction of an optical XOR gate is discussed as an application for the Hong-Ou-Mandel interferometer with two cavities.

We develop a multimode theory to design the frequency mode matching in the photon-subtracting operation by an on/off-type photon detector on a wideband squeezed light. The output state thus generated is known to be the Schrödinger-cat-like state. Its Wigner function constructed by the homodyne tomography must have a negative dip around the phase space origin. In order to observe this nonclassical effect, however, one must achieve good mode matching between the fields observed by the on/off and homodyne detectors, those are quite different types from each other. The events are defined for finite measurement time duration $T$, and are measured by the homodyne detector with finite bandwidth $B$. We provide an explicit formulus to calculate the Wigner function of the state, taking into account multimode aspects of the squeezed light and the time and band limitation to the quantum states. A unified picture of the mode matching in terms of the single parameter $BT$ is presented, covering the cw and pulsed schemes. The nonclassical effect can be realized and measured most remarkably when the modes excited in the detectors could be restricted to a single mode by taking $BT$ as small as possible, and could reduce dark counts in the on/off detector and the vacuum noise invasion in the homodyne detector. Practical design of experimental setup is described based on presently available technologies.

Sharp resonances in optical excitation of ruby by laser pulses are observed as Zeeman sublevels anticross in an external magnetic field. For the $R_1$ optical line of ruby $\left(694.3\mathrm{nm}\right)$ excitation drops when the external magnetic field strength approaches $4.14\mathrm{kG}$ while for the $R_2$ line $\left(692.9\mathrm{nm}\right)$ it increases in $4.14$ and $2.07\mathrm{kG}$ fields. A simple theoretical description of the observed effects on the basis of level mixing explains both situations.

We calculate the characteristics of ultraslow light in an inhomogeneously broadened medium. We present analytical and numerical results for the group delay as a function of power of the propagating pulse. We apply these results to explain the recently reported saturation behavior [Baldit et al., Phys. Rev. Lett. 95, 143601 (2005)] of ultraslow light in rare-earth-ion-doped crystal.

Following our previous brief report [Phys. Rev. Lett. 88, 173003 (2002)], we report here a detailed study of electromagnetically induced transparency (EIT) and dark fluorescence in a cascade three-level diatomic lithium system using optical-optical double resonance (OODR) spectroscopy for both resonance and off resonance coupling. When a strong coupling laser couples the intermediate state $A{}^1\Sigma _u^{+}(v=13,J=14)$ to the upper state $G{}^1\Pi _g(v=11,J=14)$ of ${}^7\mathrm{Li}_2$, the fluorescence from both $A{}^1\Sigma _u^{+}$ and $G{}^1\Pi _g$ states was drastically reduced as the weak probe laser was tuned through the resonance transition between the ground state $X{}^1\Sigma _g^{+}(v=4,J=15)$ and the excited state $A{}^1\Sigma _u^{+}(v=13,J=14)$. The strong coupling laser makes an optically thick medium transparent for the probe transition. In addition, the fact that fluorescence from the upper state $G{}^1\Pi _g(v=11,J=14)$ was also dark when both lasers were tuned at resonance implies that the molecules were trapped in the ground state. We used density matrix methods to simulate the response of an open molecular three-level system to the action of a strong coupling field and a weak probe field. The analytical solutions were obtained under the steady-state condition. We have incorporated the magnetic sublevel $\left(M\right)$ degeneracy of the rotational levels in the line shape analysis and report $\mid M\mid$ dependent line shape splitting. Our theoretical calculations are in excellent agreement with the observed fluorescence spectra. We show that the coherence is remarkably preserved even when the coupling field was detuned far from the resonance.

We demonstrate experimentally that it is possible to prepare and detect photon pairs created by spontaneous parametric downconversion which exhibit simultaneous position-momentum and polarization correlations. We discuss the use of these correlations in several four-dimensional key distribution protocols.

Cold atomic clouds and Bose-Einstein condensates have been stored in a $10\mathrm{cm}$ diameter vertically oriented magnetic ring. An azimuthal magnetic field enables low-loss propagation of atomic clouds over a total distance of $2\mathrm{m}$, with a heating rate of less than $50\mathrm{nK}∕\mathrm{s}$. The vertical geometry was used to split an atomic cloud into two counter-rotating clouds which were recombined after one revolution. The system will be ideal for studying condensate collisions and ultimately Sagnac interferometry.

The von Neumann interaction between a particle and an apparatus has been considered in the measurement of the position of a particle when the interaction lasts for a finite amount of time. When the measurement has finite duration, both the motion of the pointer and the particle influences the result of the measurement. Provided that the particle is in an eigenstate of its position at the start of the measurement, the pointer will indicate the arithmetic average between the initial and final position of the particle. Furthermore, the probability that the pointer will indicate a given average value is equal to the transition probability for the undisturbed particle to experience the change in position. If the initial state of the pointer is a narrow wave packet, then for any initial state of the particle, the measurement yields, approximately, the undisturbed probability distribution for the position of the particle at the end of the measurement.

We present a hashing protocol for distilling multipartite Calderbank-Shor-Steane states by means of local Clifford operations, Pauli measurements, and classical communication. It is shown that this hashing protocol outperforms previous versions by exploiting information theory to a full extent and not only applying controlled-NOT gates as local Clifford operations. Using the information-theoretical notion of a strongly typical set, we calculate the asymptotic yield of the protocol as the solution of a linear programming problem.

We observed the $3snp$ autoionizing Rydberg series of the Be atom in order to investigate the double-photoexcitation processes in two-$s$-electron systems. We employed synchrotron radiation to photoexcite the Be atoms and measured the generated ${\mathrm{Be}}^{+}$ photoions by the time-of-flight method. The $3snp$ $(n=3\text{–9})$ photoexcitation resonance peaks with interloper state of $3p4s$ that converges to ${\mathrm{Be}}^{+}\left(3p\right)$ threshold were observed. We derived the resonance parameters of $3snp$ series from a fitting procedure and obtained the Fano parameter $q$, energy position $E_0$, and resonance width $\Gamma$. These parameters are in good agreement with theoretical values. In the vicinity of the $3s5p$ state these experimental results clearly revealed the influence of the interloper $3p4s$ state, and the comparison with the numerical calculations indicates that more detailed calculations might be required to fully explain this phenomenon.