Table of contents

A useful method of analysing sets of several overlapping multichannel resonances, which are extremely difficult to analyse by other means, is presented. It exploits the Lorentzian profiles of eigenvalues {q_i(E)} of the energy-dependent time-delay matrix, avoided at their crossings. The contribution to {q_i(E)} from each S-matrix pole stands out clearly and uniquely from the contributions from the other poles. Thus the resonances that can be easily missed by the conventional technique of studying the eigenphase sum δ(E) or its derivative are singled out as diabatic resonances in a visually unambiguous way. The method is illustrated with the continuum states of the helium atom and the positronium negative ion Ps⁻ below the threshold for He⁺(n = 5) or Ps(n = 5). Many new resonances are found for Ps⁻.

The absorption spectrum of a double-well state, the state of H₂, has been computed and compared with new experimental spectra in the energy range around its dissociation limit. Above the potential barrier, it is shown that the vibronic levels unexpectedly preserve a character linked to a definite well even if this character (the absorption probability) is partly shared by the neighbouring levels. The dissociation cross section presents beat-like oscillations at two frequencies as related to the classical oscillators of the two wells. The agreement with a measured high-resolution spectrum is excellent.

Experiments with Bose–Einstein condensates of dilute atomic gases require temperatures as low as hundreds of nanokelvins but obviously cannot be performed at zero absolute temperature. So the approximate theory of such a gas at nonzero temperatures is needed. In this topical review we describe a classical field approximation which satisfies this need. As modes of light, also modes of atomic field may be treated as classical waves, provided they contain sufficiently many quanta. We present a detailed description of the classical field approximation stressing the significant role of the observation process as the necessary interface between our calculations and measurements. We also discuss in detail the determination of temperature in our approach and stress its limitations. We also review several applications of the classical field approximation to dynamical processes involving atomic condensates.

Plans to employ tungsten in the divertor region of the International Thermonuclear Experimental Reactor require radiative and collisional data for modelling x-ray emissions of highly ionized stages of tungsten. In an earlier paper, we reported on the results of fully relativistic R-matrix calculations for W⁴⁶⁺ that included the effects of radiation damping on the resonance contributions. In this paper, we present the results of similar fully relativistic, radiatively damped R-matrix calculations for W⁴⁴⁺ and W⁴⁵⁺. Radiation damping is found to be small for W⁴⁵⁺, but is appreciable for many of the excitations from the ground and metastable levels of W⁴⁴⁺. Rates from the present calculations will be combined with those from the calculations for W⁴⁶⁺ and employed for collisional-radiative modelling for these ions.

The ion-sphere model for a strongly coupled dense plasma is applied to investigate the influence of plasma screening on the low-lying 2s^2 1S₀ → 2s2p¹P₁^o resonance and 2s^2 1S₀ → 2s2p³P₁^o intercombination transitions of beryllium-like ions. Detailed computations have been performed for the ions O⁴⁺, Ne⁶⁺, Si¹⁰⁺, Ar¹⁴⁺, Fe²²⁺ and Mo³⁸⁺, based on multiconfiguration Dirac–Fock wavefunctions. The plasma screening is found to increase the excitation energy along the whole isoelectronic sequence, leading to a quite sizeable blueshift of these lines. For a given plasma density, the blueshift for the ¹S₀ → ³P₁^o intercombination line is usually larger than that for the ¹S₀ → ¹P₁^o resonance line. For a given ion, moreover, the shift increases as the plasma density is enlarged, i.e. for a more and more strongly coupled plasma. In particular, the intensity ratio of the resonance-to-intercombination line appears sensitive with regard to the electron density in the plasma. The effect of a strongly coupled dense plasma on the oscillator strengths and emission rates is also reported.

Using a classical trajectory Monte Carlo method, we have computed the three-body recombination (two free positrons and an anti-proton scattering into one free positron and an anti-hydrogen atom: ) in the presence of electrons. By simply reversing the sign of all of the particles, these results can be applied to three-body recombination of matter in the presence of positrons. An important parameter is the fraction of light particles which are of the opposite sign; we performed calculations for several values of this parameter and find a substantial effect even for small fractions. We have also included a strong magnetic field in the calculation since this seems the most likely way to have mixed sign light particles in the same region of space. Our results will be useful for future anti-hydrogen experiments. We identify the main mechanisms controlling the recombination process.

The adiabatic approximation in open systems is formulated through the effective Hamiltonian approach. By introducing an ancilla, we embed the open system dynamics into a non-Hermitian quantum dynamics of a composite system composed of the open system and ancilla, the adiabatic evolution of the open system is then defined as the adiabatic dynamics of the composite system. Validity and invalidity conditions for this approximation as well as the relation to the other definition are established and discussed. A high-order adiabatic approximation for open systems is introduced. As an example, the adiabatic condition for an open spin- particle in time-dependent magnetic fields is analysed.

Absolute measurements of the photoionization cross sections of the singly charged ions in the sequence Ca to Ni are presented, focussing on the 3p → 3d resonance region. Major differences are found in both spectral structure and cross section as the 3d shell is filled progressively. The behaviour of the total oscillator strength is studied as well as its relation to the collapse of the 3d orbital. The 3p⁵3d ¹P term is found to have an influence on the spectra even when further 3d electrons are added and this dependence combined with the effect of Hund's rule leads to a considerable simplification in the structure of the absorption spectra before the half-filled 3d shell, while from the half-filled 3d shell Hund's rule is the main simplifying effect.

Auger–photoelectron coincidence measurements for polarized targets are theoretically considered. We present a general expression for the cross section of sequential double photoionization of polarized atoms within the two-step model. Linear magnetic and alignment dichroism in coincidence experiments is discussed in more detail. For illustration, we made ab initio calculations based on the multiconfigurational Dirac–Fock theory for the 4d photoionization of atomic Sn followed by N–OO Auger decay.

We have employed an intuitive model and 3D Monte Carlo simulations to study the effect of ballistic expansion of the atomic cloud on the collision energies and the resulting frequency shift in a fountain clock. In particular, we show that the energies relevant for collisions contributing to the clock shift correspond to temperatures which may be significantly smaller than the temperature of the cloud at launch. Both the assumptions and predictions of our model are related to realistic parameters of operating caesium fountain clocks.

We check a recent proposal (Goto and Ichimura 2004 Phys. Rev. A 70 012305) for a controlled phase gate through adiabatic passage under the influence of spontaneous emission and the cavity decay. We show a modification of the above proposal could be used to generate the necessary conditional phase gates in the two-qubit Grover search. Conditioned on no photon leakage either from the atomic excited state or from the cavity mode during the gating period, we numerically analyse the success probability and the fidelity of the two-qubit conditional phase gate by adiabatic passage. A comparison made between our proposed gating scheme and a previous one shows that Goto and Ichimura's scheme is an alternative and feasible way in the optical cavity regime for two-qubit gates and could be generalized in principle to multi-qubit gates.

The UV absorption spectrum of relatively dense Zn vapour is measured and the absolute absorption coefficient in the far blue wing of the self-broadened Zn (4¹P₁ − 4¹S₀) 213.8 nm resonance line, associated with the (¹1_u ← X¹0_g⁺) molecular transition, is quantitatively analysed for the first time. The satellite band, resulting from transitions occurring in the region of the potential barrier in the excited ¹1_u state, is observed at about 1600 cm⁻¹ from the line centre and utilized for probing available theoretical interaction data via quantum simulations of this spectrum. A comparison between the simulated band and experimental one shows that the former in any case is considerably blue shifted, indicating that the theoretical findings concerning the height of the potential barrier in the excited state involved are overestimated. The corrected height and position of this barrier, yielding very good agreement between the simulated and experimental band positions, are established to be (1470 ± 50) cm⁻¹ and (3.8 ± 0.2) Å, respectively. The error bounds are here determined predominantly by the ground-state potential uncertainty, which still remains an open question. The general behaviour of the dipole transition moment function for the considered transition is also briefly discussed.

The zero temperature phase diagram of binary boson–fermion mixtures in two-colour superlattices is investigated. The eigenvalue problem associated with the Bose–Fermi–Hubbard Hamiltonian is solved using an exact numerical diagonalization technique, supplemented by an adaptive basis truncation scheme. The physically motivated basis truncation allows us to access larger systems in a fully controlled and very flexible framework. Several experimentally relevant observables, such as the matter–wave interference pattern and the condensate fraction, are investigated in order to explore the rich phase diagram. At symmetric half filling a phase similar to the Mott-insulating phase in a commensurate purely bosonic system is identified and an analogy to recent experiments is pointed out. Furthermore, a phase of complete localization of the bosonic species generated by the repulsive boson–fermion interaction is identified. These localized condensates are of a different nature than the genuine Bose–Einstein condensates in optical lattices.

The superfluorescence emission in a pumped four-level multiparticle atomic sample is investigated. The characteristics of the superfluorescence radiation are obtained. We show that the decay interference among the involved atomic transitions leads to shorter delay times and faster decay rates.

The hyperfine structure of the 2³P state of ³He with and without an external magnetic field is precisely calculated. All the linear terms, diamagnetic terms and the α² relativistic correction terms are included in the Zeeman Hamiltonian. The values of the fields for 32 crossings and 5 anticrossings of the magnetic sublevels are theoretically predicted for magnetic field strengths up to 10 000 gauss. The results are compared with experimental data and other theoretical works. All related matrix elements are calculated with high accuracy by the use of double basis set Hylleraas-type variational wavefunctions.

The electron dynamics and radiation characteristics of nonlinear Thomson scattering in a linearly polarized intense few-cycle laser pulse are investigated. In the few-cycle laser pulse, the electron dynamics depends sensitively on the carrier-envelope (CE) phase of the driving pulse. The Thomson scattering radiation also shows many remarkable phase-sensitive characteristics. For most CE phases, the left–right symmetry of radiation angular distributions is broken down and the well-known fourfold symmetry is reduced to a twofold symmetry. The radiation spectra also depend on the CE phase and show a left–right asymmetry for most CE phases. These phenomena can be attributed to the properties of the few-cycle pulse and become less evident with increasing duration of the driving pulse. Moreover, possible ways to observe these phenomena are suggested. In addition, we propose that phase-sensitive characteristics of nonlinear Thomson scattering can be utilized to determine the CE phase of the intense few-cycle pulse.

We propose to use a large cloud of cold trapped ions as a medium for quantum optics and quantum information experiments. Contrary to most recent realizations of qubit manipulation based on a small number of trapped and cooled ions, we study the case of traps containing a macroscopic number of ions. We first introduce the generic criteria involved with the operation of quantum memories that store quantum information associated with continuous variables. Then we study the impact of relevant physical parameters on the expected performances of a memory implemented with cold trapped ions.

Ab-initio and density functional theory (DFT) calculations have been carried out for zinc clusters Zn_n (n = 2–32, n is the number of atoms to form a cluster) to elucidate the structure and electronic charge states of the clusters and the mechanism of clustering. The binding energies of Zn atoms were negligibly small at n = 2–3, whereas the energy increased significantly at n = 4 (the first transition). The second transition occurred at n = 8–16. In the larger clusters (n = 16–32), the binding energy increased slightly with increasing cluster size (n). The cluster size dependence of the binding energy and bond length between zinc atoms agreed well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in the surface region of the cluster have a positive charge, whereas those in the interior region have a negative charge with a large population in the 4p orbital. The formation mechanism of zinc clusters was discussed on the basis of the theoretical results.

We report the generation of polarization-entangled photon pairs at a telecom band using two cascade periodically poled lithium niobate waveguides in a fibre loop. The phase of an entangled photon state was stabilized automatically in our setup. The average visibility was about 93.5 ± 2.6% for the entangled photon pairs after subtracting accidental coincidences in our measurement.