We present accurate cross sections for two-photon double ionization of helium at photon energies above and below the threshold for sequential double ionization. Above this threshold $\left(54.4\mathrm{eV}\right)$, sequential ionization competes with nonsequential ionization. Remarkably, even below $54.4\mathrm{eV}$, sequential ionization leaves a clear signature in the magnitude and shape of both the total and energy-sharing cross sections—even though at these energies it is only a virtual process.

The polarization of high-order harmonic radiation emitted from ${\mathrm{N}}_2$ molecules interacting with a linearly polarized laser pulse is investigated theoretically. We find that the exchange effect between the recombining electron and the bound core electrons imprints a clear signature onto the high-order harmonic polarization and its dependence on the alignment angle between the molecular axis and driving laser electric field. Our analysis reveals an observable for the experimental investigation of many-electron dynamics in intense laser fields.

We present a strategy for optimal control of the ground-state dynamics in multidimensional systems based on a combination of the semiclassical Wigner distribution approach with direct quantum chemical molecular dynamics (MD) “on the fly.” This allows one to treat all degrees of freedom without the need for precalculation of global potential energy surfaces. We demonstrate the scope of our theoretical procedure on two prototype systems representing rigid symmetrical molecules $\left({\mathrm{Na}}_3\mathrm{F}\right)$ and flexible biomolecules with low-frequency modes (glycine). We show that the ground-state isomerization process can be selectively driven by ultrashort laser pulses with different shapes which are characteristic of the prototype systems. Thus, our method opens perspectives for control of the functionality of biomolecules. Moreover, assignment of the underlying processes to pulse shapes based on MD allows one to use optimal control as a tool for analysis.

We introduce a technique to detect separable states using semidefinite programs. This approach provides a sufficient condition for separability of a state that is based on the existence of a certain local linear map applied to a known separable state. When a state is shown to be separable, a proof of this fact is provided in the form of an explicit convex decomposition of the state in terms of product states. All states in the interior of the set of separable states can be detected in this way, except maybe for a set of measure zero. Even though this technique is more suited for a numerical approach, an analytical criterion for separability can also be derived.

We consider protecting a Hadamard operation from independent dephasing, bit flipping, and dissipation. These environment-induced errors are represented by three uncorrelated reservoirs of thermalized bosons and we show that the protection is achievable through continuous dynamical decoupling. We find that, to decouple the Hadamard evolution from the environmental influence, we need a control field of higher frequency if the boson spectral density is superohmic rather than if it is ohmic. We also study the relevance of bit flipping and dissipation to the gate fidelity when it is protected from dephasing, showing how robust this partial protection is against these other perturbations. Finally, we calculate an efficient field arrangement capable of protecting simultaneously the gate operation from these three error classes.

An analysis of experimental fully differential data for single ionization in $100\mathrm{MeV}∕\mathrm{amu}$ ${\mathrm{C}}^{6+}+\mathrm{He}$ collisions is reported. We present a convolution of the first Born approximation with elastic scattering by using an event generator technique. Furthermore, the calculation is convoluted with all known experimental resolutions. Our analysis shows that elastic scattering is a viable explanation for surprising structures observed in the fully differential cross sections outside the scattering plane. Furthermore, it may even explain discrepancies in the “recoil peak” frequently observed for both ion and electron impact.

We have realized a two-dimensional permanent magnetic lattice of Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a single $300\text{−}\mathrm{nm}$ magnetized layer of FePt, patterned using optical lithography. Our magnetic lattice consists of more than 15 000 tightly confining microtraps with a density of $1250\text{traps}∕{\mathrm{mm}}^2$. Simple analytical approximations for the magnetic fields produced by the lattice are used to derive relevant trap parameters. We load ultracold atoms into at least 30 lattice sites at a distance of approximately $10\mu \mathrm{m}$ from the film surface. The present result is an important first step toward quantum information processing with neutral atoms in magnetic lattice potentials.

We consider a system composed of a trapped atom and a trapped ion. The ion charge induces in the atom an electric dipole moment, which attracts it with an $r^{-4}$ dependence at large distances. In the regime considered here, the characteristic range of the atom-ion interaction is comparable or larger than the characteristic size of the trapping potential, which excludes the application of the contact pseudopotential. The short-range part of the interaction is described in the framework of quantum-defect theory, by introducing some short-range parameters, which can be related to the $s$-wave scattering length. When the separation between traps is changed we observe trap-induced shape resonances between molecular bound states and vibrational states of the external trapping potential. Our analysis is extended to quasi-one-dimensional geometries, when the scattering exhibit confinement-induced resonances, similar to the ones studied before for short-range interactions. For quasi-one-dimensional systems we investigate the effects of coupling between the center of mass and relative motion, which occurs for different trapping frequencies of atom and ion traps. Finally, we show how the two types of resonances can be employed for quantum state control and spectroscopy of atom-ion molecules.

We describe a technique for mapping the spatial variation of static electric, static magnetic, and rf magnetic fields using a pulsed atomic or molecular beam. The method is demonstrated using a beam designed to measure the electric dipole moment of the electron. We present maps of the interaction region, showing sensitivity to (i) electric field variation of $1.5\mathrm{V}∕\mathrm{cm}$ at $3.3\mathrm{kV}∕\mathrm{cm}$ with a spatial resolution of $15\mathrm{mm}$; (ii) magnetic field variation of $5\mathrm{nT}$ with $25\mathrm{mm}$ resolution; (iii) radio-frequency magnetic field amplitude with $15\mathrm{mm}$ resolution. This diagnostic technique is very powerful in the context of high-precision atomic and molecular physics experiments, where pulsed beams have not hitherto found widespread application.

We have measured motional heating rates of trapped atomic ions, a factor that can influence multi-ion quantum logic gate fidelities. Two simplified techniques were developed for this purpose: one relies on Raman sideband detection implemented with a single laser source, while the second is even simpler and is based on time-resolved fluorescence detection during Doppler recooling. We applied these methods to determine heating rates in a microfrabricated surface-electrode trap made of gold on fused quartz, which traps ions $40\mathrm{\mu }\mathrm{m}$ above its surface. Heating rates obtained from the two techniques were found to be in reasonable agreement. In addition, the trap gives rise to a heating rate of $300\pm 30{\mathrm{s}}^{-1}$ for a motional frequency of $5.25\mathrm{MHz}$, substantially below the trend observed in other traps.

We present a solution of the three-fermion problem in a harmonic potential across a Feshbach resonance. We compare the spectrum with that of the two-body problem and show that it is energetically unfavorable for the three fermions to occupy one lattice site rather than two. We also demonstrate the existence of an energy level crossing in the ground state with a symmetry change of its wave function, suggesting the possibility of a phase transition for the corresponding many-body case.

Bosons with nonzero spin exhibit a rich variety of superfluid and insulating phases. Most phases support coherent spin oscillations, which have been the focus of numerous recent experiments. These spin oscillations are Rabi oscillations between discrete levels deep in the insulator, while deep in the superfluid they can be oscillations in the orientation of a spinful condensate. We describe the evolution of spin oscillations across the superfluid-insulator quantum phase transition. For transitions with an order parameter carrying spin, the damping of such oscillations is determined by the scaling dimension of the composite spin operator. For transitions with a spinless order parameter and gapped spin excitations, we demonstrate that the damping is determined by an associated quantum impurity problem of a localized spin excitation interacting with the bulk critical modes. We present a renormalization group analysis of the quantum impurity problem and discuss the relationship of our results to experiments on ultracold atoms in optical lattices.

We apply Bayesian logic to optimally estimate the differential phase in a discrete-time, dual-interferometer measurement. This method is particularly relevant to the case of a gravity gradiometer, where the gravity gradient between cold-atom fountain interferometers can be estimated from the differential phase, despite the presence of large common phase (acceleration) fluctuations. Given an accurate model, the bias-free algorithm we present is optimal and leverages experimental knowledge of the system noise, classical or quantum, to outperform other typical estimators, including ellipse-fitting techniques.

We experimentally investigate a scheme for detecting single atoms magnetically trapped on an atom chip. The detector is based on the photoionization of atoms and the subsequent detection of the generated ions. We describe the characterization of the ion detector with emphasis on its calibration via the correlation of ions with simultaneously generated electrons. A detection efficiency of $47.8\pm 2.6\%$ is measured, which is useful for single-atom detection, and close to the limit allowing atom counting with sub-Poissonian uncertainty.

In this paper we study the role of surface plasmon modes in the Casimir effect. The Casimir energy can be written as a sum over the modes of a real cavity and one may identify two sorts of modes, two evanescent surface plasmon modes and propagative modes. As one of the surface plasmon modes becomes propagative for some choice of parameters we adopt an adiabatic mode definition where we follow this mode into the propagative sector and count it together with the surface plasmon contribution, calling this contribution “plasmonic.” We evaluate analytically the contribution of the plasmonic modes to the Casimir energy. Surprisingly we find that this becomes repulsive for intermediate and large mirror separations. The contribution of surface plasmons to the Casimir energy plays a fundamental role not only at short but also at large distances. This suggests possibilities to tailor the Casimir force via a manipulation of the surface plasmons properties.

Coincidence structures in the transverse plane of type-II spontaneous parametric down-conversion carrying orbital angular momentum are obtained. Azimuthal symmetry breaking around the pump beam direction reveals itself on these quantum images. Analytical expressions for the amplitude probability of the down-conversion process are shown including the nonlinear polarizability components.

Coherent x-ray diffraction microscopy (CXDM) using an x-ray free electron laser (XFEL) is expected to open up a new frontier of structural studies in materials science and biology, while radiation damage of samples under the extremely intense x rays is a matter of considerable concern. Two superresolution methods for CXDM proposed in this paper offer solutions of the problems by numerical data analysis. Promising results for future applications of CXDM with XFEL were obtained in a numerical simulation.

From the noncommutative nature of quantum mechanics, estimation of canonical observables $\widehat{q}$ and $\widehat{p}$ is essentially restricted in its performance by the Heisenberg uncertainty relation, $⟨\Delta {\widehat{q}}^2⟩⟨\Delta {\widehat{p}}^2⟩⩾{\hslash }^2∕4$. This fundamental lower bound may become bigger when taking the structure and quality of a specific measurement apparatus into account. In this paper, we consider a particle subjected to a linear dynamics that is continuously monitored with efficiency $\eta ∊(0,1]$. It is then clarified that the above Heisenberg uncertainty relation is replaced by $⟨\Delta {\widehat{q}}^2⟩⟨\Delta {\widehat{p}}^2⟩⩾{\hslash }^2∕4\eta$ if the monitored system is unstable, while there exists a stable quantum system for which the Heisenberg limit is reached.

We witness experimentally the presence of macroscopic coherence in Gaussian quantum states using a recently proposed criterion [E. G. Cavalcanti and M. D. Reid, Phys. Rev. Lett. 97 170405 (2006)]. The macroscopic coherence stems from interference between macroscopically distinct states in phase space, and we prove experimentally that a coherent state contains these features with a distance in phase space of $0.51\pm 0.02$ shot noise units. This is surprising because coherent states are generally considered being at the border between classical and quantum states, not yet displaying any nonclassical effect. For squeezed and entangled states the effect may be larger but depends critically on the state purity.

The determination of the state fidelity and the detection of entanglement are fundamental problems in quantum information experiments. We investigate how these goals can be achieved with a minimal effort. We show that the fidelity of Greenberger-Horne-Zeilinger and $W$ states can be determined with an effort increasing only linearly with the number of qubits. We also present simple and robust methods for other states, such as cluster states and states in decoherence-free subspaces.