Highlights of 2010

A novel scheme for enhancing electron localization in intense-field dissociation is outlined. Through manipulation of a bound vibrational wavepacket in the exemplar deuterium molecular ion, simulations demonstrate that the application of multiple phase-locked, few-cycle IR pulses can provide a powerful scheme for directing the molecular dissociation pathway. By tuning the time delay and carrier–envelope–phase for a sequence of pulse interactions, the probability of the electron being localized to a chosen nucleus can be enhanced to above 80%.

A two-centre convergent close-coupling method, which includes the positronium-formation channel, has been developed for positron scattering on helium. Convergent, pseudoresonance-free cross sections have been obtained. This is only possible if complete expansions are used on both the positronium and helium centres. The method is valid for all projectile energies and all transitions, including breakup, which is associated with excitation of positive-energy helium and positronium pseudostates. Generally, good agreement between calculated cross sections and available experimental data has been found across all incident energies.

We have fabricated and tested an atom chip that operates as a matter wave interferometer. In this communication we describe the fabrication of the chip by ion-beam milling of gold evaporated onto a silicon substrate. We present data on the quality of the wires, on the current density that can be reached in the wires and on the smoothness of the magnetic traps that are formed. We demonstrate the operation of the interferometer, showing that we can coherently split and recombine a Bose–Einstein condensate with good phase stability.

Photoionization (PI) of multiply and highly charged ions was studied using an electron beam ion trap and synchrotron radiation at the BESSY II electron storage ring. The versatile new method introduced here extends the range of ions accessible for PI investigations beyond current limitations by providing a dense target of ions in arbitrary, i.e. both low and high charge states. Data on near-threshold PI of N^{3 +} and Ar^{8 +} ions, species of astrophysical and fundamental interest, show high resolution and accuracy allowing various theoretical models to be distinguished, and highlight shortcomings of available PI calculations. We compare our experimental data with our new fully relativistic PI calculations within a multiconfiguration Dirac–Fock approach and with other advanced calculations and find generally good agreement; however, detailed examination reveals significant deviations, especially at the threshold region of Ar^{8 +}.

We study the spatial profile of high order harmonics generated by a transient grating of rotational excitation. We show that the phase modulation of the harmonic emission as a function of molecular alignment is encoded in the diffraction pattern. In molecular nitrogen, the phase difference between aligned and isotropic molecules decreases from 1.6 rad for harmonic 19 to less than 0.3 rad for harmonic 27. In CO₂ we observe a strong phase jump for the highest harmonics. The position of this phase jump in the harmonic spectrum depends on the laser intensity, reflecting the contribution from multiple molecular orbitals to the harmonic emission.

We study the effect of a many-body interaction on inter-band oscillations in a two-band Bose–Hubbard model with an external Stark force. Weak and strong inter-band oscillations are observed, where the latter arise from a resonant coupling of the bands. These oscillations collapse and revive due to a weak two-body interaction between the atoms. Effective models for oscillations in and out of resonance are introduced that provide predictions for the system's behaviour, particularly for the time scales for the collapse and revival of the resonant inter-band oscillations.

Five-fold differential cross sections (5DCS) for electron impact ionization of a diatomic molecule have been explored experimentally as a function of molecular alignment. Using H₂ as a test system, we exploited dissociative ionization by 200eV electrons to obtain the alignment of the internuclear axis. Separation of ground-state ionization from autoionization is discussed. 5DCS are investigated for the direct channel and found to be in good agreement with M3DW calculations discarding at the same time a simple two-centre interference model discussed recently in the literature.

We study the exact entanglement and entropy dynamics of two qubits interacting with a common zero-temperature non-Markovian reservoir. It is a commonly held view that entanglement loss due to environmental decoherence is accompanied by loss of purity of the state of the system. We demonstrate that such an intuitive picture does not always apply: the deterioration of entanglement and purity does not necessarily come together; i.e. revivals of entanglement can be accompanied by deterioration of purity. To complete our investigation on entanglement–mixedness interplay we consider the case of initial mixed states and study how the entanglement dynamics and its revivals are related to both the initial purity and the initial entanglement.

We present angle- and energy-resolved measurements of photoelectrons produced in strong-field ionization of Xe using a tunable femtosecond laser. An occurrence of highly oscillatory patterns in the angular distribution at low photoelectron kinetic energy is observed that correlates with channel closing/opening over a wide range of laser parameters. The correlation is investigated both experimentally and by means of systematic analysis of numerical solutions of the time-dependent Schrödinger equation. Our experimental and numerical results are in quantitative agreement with the semi-classical model introduced by Arbó et al (2008 Phys. Rev. A 78 013406), which relates the oscillatory patterns to interference between photoelectrons produced during different cycles of the laser pulse in the course of non-resonant ionization of the atom. We observe that an increase of the laser intensity eventually leads to qualitative invariance of the pattern, defining a limit on the applicability of the semi-classical model.

We study the behaviour of weakly bound clusters and their relation to the well-known three-body Efimov states. We adopt a model to describe the universal behaviour of strongly interacting bosonic systems, and we test its validity by reproducing predictions of three- and four-body universal states. Then, we extend our study to larger systems and identify a series of universal cluster states that can be qualitatively interpreted as adding one particle at a time to an Efimov trimer. The properties of these cluster states and their experimental signatures are discussed.

The electronic entanglement between two free atoms initially at rest is obtained including the effects of photon recoil, for the case when quantum dispersion can be neglected during the atomic excited-state lifetime. Unlike previous treatments using common or statistically independent reservoirs, a continuous transition between these limits is observed, which depends on the inter-atomic distance and degree of localization. The occurrence of entanglement sudden death and birth as predicted here deviates from the case where the inter-atomic distance is treated classically by a static value. Moreover, the creation of a dark state is predicted, which manifests itself by a stationary entanglement that even may be created from an initially separable state.

Relative electron-impact-induced emission cross sections for the a¹Π_g (v' = 3)–X¹Σ⁺_g (v'' = 0) and a¹Π_g (v' = 2)–X¹Σ⁺_g (v'' = 0) transitions are presented. Critical comparison is made with existing cross sections showing significant discrepancy with the widely accepted excitation function of Ajello and Shemansky (1985 J. Geophys. Res. 90 9845–61) at energies below ∼80 eV. A series of extensive measurements are presented that were performed to rule out any possible systematic or random errors in the present experimental apparatus and methodology. These efforts lead to the conclusion that the current measurements are robustly reproducible and, thus, should supplant the LBH cross-section shape of Ajello and Shemansky (1985 J. Geophys. Res. 90 9845–61).

Polar molecules such as CO are interesting target systems for high-order harmonic generation (HHG) as they can be oriented with current laser techniques, thus allowing the study of systems without inversion symmetry. However, the asymmetry of the molecule also means that the molecular orbitals are Stark shifted in energy due to their interaction with the driving laser. We extend the strong-field approximation of HHG by incorporating the Stark shift into the Lewenstein model and discuss its impact on two different gating mechanisms in CO. In system-induced gating an oriented target molecule serves as a gate by selecting every other half-cycle due to an increased (decreased) ionization rate. In field-induced gating the waveform of the driving laser is tailored such that the harmonic emission from an aligned molecule is damped (enhanced) every other half-cycle. We show that the Stark shift weakens the strength of system-induced gating and also determines the relative contribution from opposite orientations in field-induced gating. Finally, we propose a novel scheme for extending the high-order harmonic cutoff by letting the two gating mechanisms counteract each other, thus allowing for a higher laser intensity without increased ionization of the target gas.

Absolute triple-differential cross sections for intermediate energy electron impact ionization of neon and argon are reported. The data highlight ongoing challenges for theory in the description of this ionization problem and the immense value of absolute measurements over relative data in guiding the ongoing development of theory.

We have experimentally realized a hybrid trap for ultracold paramagnetic rubidium and diamagnetic ytterbium atoms by combining a bichromatic optical dipole trap for ytterbium with a Ioffe–Pritchard-type magnetic trap for rubidium. In this hybrid trap, we have investigated sympathetic cooling for five different ytterbium isotopes through elastic collisions with rubidium. A strong dependence of the thermalization rate on the mass of the specific ytterbium isotope was observed.

The photoionization of the ground state and 6p laser-excited caesium atoms was studied above their 4d ionization thresholds. The 4d photoelectron spectrum of 6p laser-excited atoms shows a stronger excitation of satellites upon ionization than its ground state counterpart. The relative intensities of satellite and main photolines show a slow variation with the incoming photon energy for both the ground state and the 6p laser-excited states. An assignment of the excited state spectra, supported by recently published ground state photoionization spectra and calculations, is given and a preliminary analysis of the 4d Auger spectrum of laser-excited atoms is also presented.