The electron-impact direct ionization cross section for ${\mathrm{Mo}}^{+}$ is calculated using both nonperturbative close-coupling and perturbative distorted-wave methods. When distorted-wave calculations for $4d^5\to 4d^4$ direct ionization are added to distorted-wave calculations for $4p\to nl$ excitation-autoionization, the experimental measurements are found to be 60% lower than the theoretical predictions. Inclusion of nonperturbative three-body Coulomb effects, present in time-dependent close-coupling calculations, are found to reduce the distorted-wave $4d^5\to 4d^4$ direct ionization cross section by 25%. This is by far the largest reduction yet seen when comparing the two methods for direct subshell ionization of an atomic positive ion in the ground state. However, when the close-coupling calculations for $4d^5\to 4d^4$ direct ionization are added to distorted-wave calculations for $4p\to nl$ excitation-autoionization, the experimental measurements are still 45% lower than the theoretical predictions. Although we further investigate correlation effects in the initial target state and term-dependent potential effects in the ejected electron state in an attempt to understand the small magnitude of the experimental measurements, the discrepancy between theory and experiment remains unexplained.

We propose a fault-tolerant loading scheme to produce an array of fermions in an optical lattice of the high fidelity required for applications in quantum-information processing and the modeling of strongly correlated systems. A cold reservoir of fermions plays a dual role as a source of atoms to be loaded into the lattice via a Raman process and as a heat bath for sympathetic cooling of lattice atoms. Atoms are initially transferred into an excited motional state in each lattice site and then decay to the motional ground state, creating particle-hole pairs in the reservoir. Atoms transferred into the ground motional level are no longer coupled back to the reservoir, and doubly occupied sites in the motional ground state are prevented by Pauli blocking. This scheme has strong conceptual connections with optical pumping and can be extended to load high-fidelity patterns of atoms.

We propose a scheme to achieve quantum computation with neutral atoms whose interactions are catalyzed by single photons. Conditional quantum gates, including an $N$-atom Toffoli gate and nonlocal gates on remote atoms, are obtained through cavity-assisted photon scattering in a manner that is robust to random variation in the atom-photon coupling rate and which does not require localization in the Lamb-Dicke regime. The dominant noise in our scheme is automatically detected for each gate operation, leading to signalled errors which do not preclude efficient quantum computation even if the error probability is close to the unity.

We study the entanglement of general (pure or mixed) two-mode Gaussian states of continuous-variable systems by comparing the two available classes of computable measures of entanglement: entropy-inspired Gaussian convex-roof measures and positive partial transposition–inspired measures (negativity and logarithmic negativity). We first review the formalism of Gaussian measures of entanglement, adopting the framework introduced in M. M. Wolf et al., Phys. Rev. A 69, 052320 (2004), where the Gaussian entanglement of formation was defined. We compute explicitly Gaussian measures of entanglement for two important families of nonsymmetric two-mode Gaussian state: namely, the states of extremal (maximal and minimal) negativities at fixed global and local purities, introduced in G. Adesso et al., Phys. Rev. Lett. 92, 087901 (2004). This analysis allows us to compare the different orderings induced on the set of entangled two-mode Gaussian states by the negativities and by the Gaussian measures of entanglement. We find that in a certain range of values of the global and local purities (characterizing the covariance matrix of the corresponding extremal states), states of minimum negativity can have more Gaussian entanglement of formation than states of maximum negativity. Consequently, Gaussian measures and negativities are definitely inequivalent measures of entanglement on nonsymmetric two-mode Gaussian states, even when restricted to a class of extremal states. On the other hand, the two families of entanglement measures are completely equivalent on symmetric states, for which the Gaussian entanglement of formation coincides with the true entanglement of formation. Finally, we show that the inequivalence between the two families of continuous-variable entanglement measures is somehow limited. Namely, we rigorously prove that, at fixed negativities, the Gaussian measures of entanglement are bounded from below. Moreover, we provide some strong evidence suggesting that they are as well bounded from above.

We consider probabilistic cloning of a state chosen from a mutually nonorthogonal set of pure states, with the help of a party holding supplementary information in the form of pure states. When the number of states is 2, we show that the best efficiency of producing $m$ copies is always achieved by a two-step protocol in which the helping party first attempts to produce $m-1$ copies from the supplementary state, and if it fails, then the original state is used to produce $m$ copies. On the other hand, when the number of states exceeds two, the best efficiency is not always achieved by such a protocol. We give examples in which the best efficiency is not achieved even if we allow any amount of one-way classical communication from the helping party.

We derive lower limits on the inefficiency and classical communication costs of dilution between two-term bipartite pure states that are partially entangled. We first calculate explicit relations between the allowable error and classical communication costs of entanglement dilution using a previously described protocol, then consider a two-stage dilution from singlets with this protocol followed by some unknown protocol for conversion between partially entangled states. Applying overall lower bounds on classical communication and inefficiency to this two-stage protocol, we derive bounds for the unknown protocol. In addition we derive analogous (but looser) bounds for general pure states.

We study the spontaneous emission of a cesium atom in the vicinity of a subwavelength-diameter fiber. We show that the confinement of the guided modes and the degeneracy of the excited and ground states substantially affect the spontaneous emission process. We demonstrate that different magnetic sublevels have different decay rates. When the fiber radius is about $200\mathrm{nm}$, a significant fraction (up to 28%) of spontaneous emission by the atom can be channeled into guided modes. Our results may find applications for developing nanoprobes for atoms and efficient couplers for subwavelength-diameter fibers.

We consider the three-boson problem with $\delta$-function interactions in one spatial dimension. Three different approaches are used to calculate the phase shifts, which we interpret in the context of the effective range expansion, for the scattering of one free particle off a bound pair. We first follow a procedure outlined by McGuire in order to obtain an analytic expression for the desired $S$-matrix element. This result is then compared to a variational calculation in the adiabatic hyperspherical representation, and to a numerical solution to the momentum-space Faddeev equations. We find excellent agreement with the exact phase shifts, and comment on some of the important features in the scattering and bound-state sectors. In particular, we find that the $1+2$ scattering length is divergent, marking the presence of a zero-energy resonance which appears as a feature when the pairwise interactions are short range. Finally, we consider the introduction of a three-body interaction, and comment on the cutoff dependence of the coupling.

We propose a theoretical method for investigation of ionization of atoms and molecules in intense laser fields that copes with the effects of excited state dynamics (or intramolecular electronic dynamics). The time-evolving wave packet composed of only bound electronic states, $\mid {\Phi }_i\left(t\right)⟩$, is introduced into a framework of the intense-field $S$-matrix theory. Then, the effects of both Coulomb field and radiation field on the bound electron(s) are well described by $\mid {\Phi }_i\left(t\right)⟩$, while the effects of a radiation field on a freed electron are also treated in a nonperturbative way. We have applied the theory to ionization of H and $\mathrm{H}_2{}^{+}$ in ultrashort intense laser pulses. Although only a small number of Gaussian functions are used in the expansion of $\mid {\Phi }_i\left(t\right)⟩$, the present method can quantitatively reproduce the features of enhanced ionization of $\mathrm{H}_2{}^{+}$ obtained by an accurate grid propagation method. This agreement supports the view that field-induced population transfer between the lowest two electronic states triggers the enhancement of ionization at large internuclear distances. We also applied the method to calculate the photoelectron momentum distribution of H in an intense near-infrared field. A broad low intensity component due to “rescattering” appears in the distribution of the momentum perpendicular to the polarization direction of an applied laser field, as observed in the experiments of single ionization of noble gas atoms. The present method provides a practical way of properly describing the nonperturbative nature of field-induced dynamics of an electron (or electrons) in the presence of both Coulomb and radiation fields.

While the one-dimensional (1D) Bose gas appears to exhibit superfluid response under certain conditions, it fails the Landau criterion according to the elementary excitation spectrum calculated by Lieb. The apparent riddle is solved by calculating the dynamic structure factor of the Lieb-Liniger 1D Bose gas. In a dual representation, strongly interacting bosons are mapped into weakly interacting fermions, which are treated in the Hartree-Fock and generalized random-phase approximations. The dynamic structure factor is calculated analytically in this approximation, which is valid to first order in $1∕\gamma$, where $\gamma$ is the dimensionless interaction strength of the model. The results clearly indicate a suppression of superfluidity-breaking umklapp excitations near the Tonks-gas limit, which should be observable by Bragg scattering in current experiments.

We extend our recent treatment of a well-known model of a nonrelativistic hydrogen-like atom confined within an impenetrable spherical “box” of radius . Analytical closed-form solutions are presented for the first-order perturbation corrections appropriate to the dipole polarization of states. solutions are expressed conveniently in terms of Kummer (confluent hypergeometric) functions, exactly as for the field-free atom, by exploiting several well-known recurrence and differential relations for these functions. The functions and all necessary integrals are evaluated using standard routines, which exploit analytical properties of these functions. The accuracy of our procedures has been checked by a similar calculation of the dipole shielding factors. Our procedures are easily extended to states of nonzero angular momentum, as well as to higher-multipole perturbations. Our exact values may be of interest in assessing the utility of this simple model of atomic confinement.

Coherent population trapping (CPT) signals can be optimized with amplitude-modulated pumping light. As shown theoretically and experimentally in this paper, the optimum wave forms produce strong CPT signals at both low buffer-gas pressures, where the hyperfine structure (HFS) splitting of the optical absorption lines is well resolved, and also at high buffer-gas pressures, where the HFS is no longer optically resolved due to pressure broadening. On the other hand, CPT resonances from frequency-modulated waves are severely degraded for these high-pressure conditions. High buffer-gas pressures may offer advantages for miniature frequency standards and magnetometers, including suppression of diffusion losses in miniature cells, suppression of light shifts, and less stringent requirements for frequency stability of the pumping light sources.

Semiconductor lasers are particularly well suited for the implementation of pump-noise suppression, yielding a reduction of the intensity noise in the laser. In this simple picture, the maximal amount of squeezing is equal to the quantum efficiency. However, experimental results on intensity noise reduction by pump-noise suppression are usually above this limit. This discrepancy suggests that additional noise sources must be involved. Here we successfuly interpret the full noise behavior of a single-mode laser diode far above threshold by considering two excess noise sources: the leakage current fluctuations across the laser and the Petermann excess noise. We have estimated the contribution of each noise source using the results of the correlations between the laser output intensity noise and the voltage fluctuations across the laser diode (light-voltage correlations) and obtained good agreement between our theory and experimental results.

We present a detailed experimental investigation into the off-resonant preparation of a vibrational coherence for multiorder Raman generation. In particular, the importance of the balance between the conflicting requirements for generating a strong coherence and for adiabatic evolution of the system are examined. For the parameters accessible in our experiment, we find that the strongest frequency modulation occurs under conditions for which there is some nonadiabatic behavior, and the coherence is less than the peak value obtainable. We investigate the modulation of a narrowband probe pulse, and an ultrashort probe pulse, in the coherently prepared medium, and demonstrate efficient modulation of both probe pulses resulting in the generation of $360\mathrm{THz}$ bandwidth fields. This system therefore has an application in the field of subfemtosecond pulse generation.

We present a small network for the testing of the entanglement of two ballistic electron waveguide qubits. The network produces different output conditional on the presence or absence of entanglement. The structure of the network allows for the determination of successful entanglement operations through the measurement of the output of a single qubit. We also present a simple model of a dynamic Coulomb-like interaction and use it to describe some characteristics of a proposed scheme for the entanglement of qubits in ballistic electron waveguides.

We propose a scheme for broadcasting entanglement at a distance based on linear optics. We show that an initial polarization entangled state can be simultaneously split and transmitted to a pair of observers situated at different locations with the help of two conditional Bell-state analyzers based on two beam splitters characterized by the same reflectivity $R$. In particular for $R=\frac{1}{3}$ the final states coincide with the output states obtained by the broadcasting protocol proposed by Buzek et al. [Phys. Rev. A 55, 3327 (1997)]. Further we present a different protocol called telecloning of entanglement, which combines the many-to-many teleportation and nonlocal optimal asymmetric cloning of an arbitrary entangled state. This scheme allows the optimal transmission of the two nonlocal optimal clones of an entangled state to two pairs of spatially separated receivers.

We recently developed a theoretical framework to treat low-energy electron scattering from helical macromolecules. In this article, we use this framework to extend our previous model of simple base-pair scatterers, organized into the DNA structure, to include the backbone. The internal diffraction pattern due to base pairs is still present, but addition of the backbone screens the base pairs by a factor of 2. More interestingly, the effect of constructive interference on the phosphate groups within the backbone itself is seen to be strong at lower energies. We perform a calculation for electrons incident perpendicular and parallel to the axis of a fragment and find comparable electron patterns on the phosphate groups at the surface of films consisting of vertically or horizontally arranged segments relative to the substrate.

We have investigated the near-threshold behavior of the dipole and nondipole angular anisotropy parameters in photodetachment of the negative ion ${\mathrm{I}}^{-}$ by performing both analytical and numerical calculations. The numerical calculations of the angular asymmetry parameters were carried out both in the one-electron Hartree-Fock (HF) approximation and with account of the many-electron correlations within the framework of the random phase approximation with exchange (RPAE). The calculated results are presented for the outer and intermediate subshells of the iodine negative ion ${\mathrm{I}}^{-}$ and contrasted with those of the neutral neighbor atom Xe. Essential differences in the HF and RPAE near-threshold behavior of the angular distribution parameters between the negative ion and the neutral atom are demonstrated.

Following the proposal by [F. Yamaguchi et al. Phys. Rev. A 66, 010302(R) (2002)], we present an alternative way to implement the two-qubit Grover search algorithm in cavity QED. Compared with Yamaguchi et al.’s proposal, with a strong resonant classical field added, our method is insensitive to both the cavity decay and thermal field, and does not require that the cavity remain in the vacuum state throughout the procedure. Moreover, the qubit definitions are the same for both atoms, which makes the experiment easier. The strictly numerical simulation shows that our proposal is good enough to demonstrate a two-qubit Grover’s search with high fidelity.

We propose a multiband Fermi-Bose Hubbard model with on-site fermion-boson conversion and general filling factor in three dimensions. Such a Hamiltonian models an atomic Fermi gas trapped in a lattice potential and subject to a Feshbach resonance. We solve this model in the two-state approximation for paired fermions at zero temperature. The problem then maps onto a coupled Heisenberg spin model. In the limit of large positive and negative detuning, the quantum phase transitions in the Bose Hubbard and paired-Fermi Hubbard models are correctly reproduced. Near resonance, the Mott states are given by a superposition of the paired-fermion and boson fields and the Mott-superfluid borders go through an avoided crossing in the phase diagram.