High-resolution Cl ${2p}^{-1}$ photoelectron (PES) and Cl ${2p}^{-1}\to 3p{\pi }^{-2}$ normal Auger electron spectra (AES) have been measured using different photon energies. The PES and AES have been fitted simultaneously taking vibrational and electronic lifetime-interference terms into account. Detailed results are presented for the Cl ${2p}^{-1}\to 3p{\pi }^{-2}$ normal Auger transitions. The equilibrium distances, vibrational energies, and anharmonicities for the ${}^1{\Sigma }^{+},$ ${}^1\Delta$, and ${}^3{\Sigma }^{-}$ final states are derived from the intensity distribution of the vibrational substates. The relative intensities for the different Cl ${2p}^{-1}\to 3p{\pi }^{-2}$ normal Auger transitions are also given. In addition to the Cl ${2p}^{-1}\to 3p{\pi }^{-2}$ transitions, some fine structures are observed that show a photon energy dependence. These structures are assigned to decays of doubly excited or shake-up/off states.

The distorted-wave Born approximation with spin averaged static exchange potential and modified semiclassical exchange potential has been used to calculate the triple-differential cross section of He ${(1s}^2)$ and Ar ${(3p}^6)$ in the coplanar to perpendicular plane geometry. Similar to helium a strong interference minima in the cross section has been observed for the argon atom for all the incident electron angles. The triple-differential cross section in the case of argon at low- and high-scattering angles is in very good agreement with the present experiment. Inclusion of postcollision interaction gives a dip at 45° (characteristic feature of p-orbital targets), which has not been observed exactly at 45° by the experiment and calculations without postcollision interaction. The agreement with experiment in case of helium is quite good, but in the case of argon, some discrepancies are still there.

We report a high-resolution coincident experiment between the Ar ${2p}_{1/2}$ and ${2p}_{3/2}$ photoelectrons and subsequently emitted $L_{23}{-M}_{23}{M}_{23}$ Auger electrons, where the $2p$ photoelectrons were ionized far above the threshold. We demonstrate the applicability of the coincidence method when separating the different ionization and decay channels even in the case of strongly overlapping peaks. The measurement was carried out with a new high-energy-resolution electrostatic analyzer that is capable of recording the single and coincidence spectra at large range of emission angles simultaneously. An anomalous value was found for the ${}^2{P}_{3/2}{-(}^3{P}_{012}{/}^1{D}_2)$ intensity ratio in the ${2p}_{3/2}$ coincidence spectrum, while all the other relative intensities are in good agreement with the previously published data. The possible reasons for the difference are discussed.

The decay channels of the $\mathrm{Ar}{2s}^{-1}$ and ${2p}^{-1}$ and $\mathrm{Kr}{3p}^{-1}$ and ${3d}^{-1}$ electronic hole states have been investigated by means of photoelectron-photoion coincidence measurements following innershell ionization using synchrotron radiation. With the method of final ion-charge resolving electron spectroscopy it has become possible to disentangle different contributions to the electron spectrum and to determine the decay probabilities ${P(nl}^{-1}\to n+)$ of the above-mentioned hole states ${(nl}^{-1})$ to the final ionic charge states $n+.\mathrm{}$ A high correlation with threefold or even fourfold charged ions has been found in all cases. Possible decay routes, via cascade or direct double Auger processes, are discussed on the basis of energy-level schemes calculated with the Hartree-Fock method. Special emphasis is laid on the examination of the $\mathrm{Kr}{3p}^{-1}$ decay process, where the two fine-structure components $\mathrm{}(j=1/2,3/2\mathrm{})\mathrm{}$ exhibit noticeably different decay probabilities to ${\mathrm{Kr}}^{3+}$ and ${\mathrm{Kr}}^{4+}$ final ionic charge states.

Superelastic scattering, a collision process in which the scattered electron gains energy, has been investigated for metastable ${\mathrm{C}}^{4+}\left(1s2s{}^{3}S\right)$ and ${\mathrm{O}}^{6+}\left(1s2s{}^{3}S\right)$ He-like ions colliding with quasifree ${\mathrm{H}}_2$-target electrons. The measured superelastic-scattering cross sections for $1s2s{}^3\overrightarrow{S}{1s}^2{}^{1}S$ deexcitation show a strong dependence on the collision energy and the atomic number of the He-like ion. R-matrix calculations for the time-reversed equivalent process of electron-impact excitation are compared with the measured cross sections through the principle of detailed balance. In these calculations, the impulse approximation is utilized in conjunction with the Compton profile of the target electrons for the collisions of ions with quasifree electrons studied here. Calculations were performed taking into account the anisotropy of the cross sections as a function of the scattering angle, which equals $180°$ in the projectile rest frame for the reported measurements. Comparison between theory and experiment was used to infer the metastable $1s2s{}^{3}S$ fraction in the incident He-like beam.

We present the effects of frequency chirping of laser pulses on (1+1)-photon resonance-enhanced dissociation dynamics of ${\mathrm{H}}_2.$ The dissociation occurs via two closely spaced nonadiabatically coupled intermediate levels which are in one-photon resonance or near resonance with the initial level. Predissociating levels embedded into continua are considered. When the first laser field is sufficiently intense and suitably chirped, the dissociation probability is enhanced by adiabatic rapid passage through the avoided crossing arising from the frequency swept radiative interaction. The whole population of the ground level can be effectively transferred to the intermediate levels by this technique facilitating the dissociation process by the second field. We also report the effect of frequency detuning and chirp width on the dissociation probability. Widths of the two peaks of the dissociation line shape increase with an increase in chirp width, resulting in the possibility of control in the dissociation yield. When the first field is a laser pulse of low intensity and constant frequency and the second laser frequency is chirped, predissociating levels take important parts in the dissociation dynamics and we obtain a signature of the nonadiabatic effect of the first step on the second step of photodissociation dynamics. This feature is due to the presence of the predissociating levels and the nonadiabatic mixing of two intermediate levels. All these results can be explained in terms of the adiabatic dressed levels.

Formation of atomic pairs by the dissociation of a molecular condensate or by inelastic collisions in an atomic condensate due to a time-dependent curve crossing process is studied beyond the mean-field approximation. The number of atoms formed by the spontaneous process is described by a Landau-Zener formula multiplied by an exponential amplification factor due to quantum many-body effects. Correlated atomic pairs are formed in squeezed states. The rate of stimulated processes depends on the relative phase of the two fields.

Passive mode-locking (ML) operation in semiconductor quantum-well (QW) lasers with Fabry-Perot (FP) cavities is investigated theoretically and experimentally. Structural and operational conditions for a semiconductor QW laser to exhibit “amplitude modulation (AM)” or “frequency modulation” ML operations are investigated by numerical analysis based on a semiclassical laser model. It is shown experimentally that clear AM ML pulses self-build and persist under continuous current injection in ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}/{\mathrm{In}}_{1-x}{\mathrm{Ga}}_x{\mathrm{As}}_y{\mathrm{P}}_{1-y}/\mathrm{InP}$-based FP QW lasers with cavity lengths of $300-600\hspace{0.5em}\mu \mathrm{m}.$ The observed ML features are explained in terms of the laser model.

We present a formalism for the description of fluorescence from optically active materials embedded in a photonic crystal structure possessing a photonic band gap or pseudogap. An electromagnetic field expansion in terms of Bloch modes of the crystal is used to develop the equations for fluorescence in terms of the local density of photon modes available to the emitting atoms in either the high or low dielectric regions of the crystal. We then obtain expressions for fluorescence spectra and emission dynamics for luminescent materials in photonic crystals. The validity of our formalism is demonstrated through the calculation of relevant quantities for model photon densities of states. The connection of our calculations to the description of realistic systems is discussed. We also describe the consequences of these analyses on the accurate description of the interaction between radiative systems and the electromagnetic reservoir within photonic crystals.

A theory for the photon statistics of a random laser is presented. Noise is described by Langevin terms, where fluctuations of both the electromagnetic field and of the medium are included. The theory is valid for all lasers with small outcoupling when the laser cavity is large compared to the wavelength of the radiation. The theory is applied to a chaotic laser cavity with a small opening. It is known that a large number of modes can be above threshold simultaneously in such a cavity. It is shown that the amount of fluctuations is increased above the Poissonian value by an amount that depends on the number of modes above threshold.

We rephrase the Wootters-Fields construction [W. K. Wootters and B. C. Fields, Ann. Phys. 191, 363 (1989)] of a full set of mutually unbiased bases in a complex vector space of dimensions ${N=p}^r,$ where p is an odd prime, in terms of the character vectors of the cyclic group G of order p. This form may be useful in explicitly writing down mutually unbiased bases for ${N=p}^r.$

We consider the problem to single out a particular state among $2^n$ orthogonal pure states. As it turns out, in general the optimal strategy is not to measure the particles separately, but to consider joint properties of the n-particle system. The required number of propositions is n. There exist $2^n!$ equivalent operational procedures to do so. We enumerate some configurations for three particles, in particular the Greenberger-Horne-Zeilinger (GHZ) and W states, which are specific cases of a unitary transformation. For the GHZ case, an explicit physical meaning of the projection operators is discussed.

A laser, be it an optical laser or an atom laser, is an open quantum system that produces a coherent beam of bosons (photons or atoms, respectively). Far above threshold, the stationary state ${\rho }_{\mathrm{ss}}$ of the laser mode is a mixture of coherent-field states with random phase, or, equivalently, a Poissonian mixture of number states. This paper answers the question: can descriptions such as these, of ${\rho }_{\mathrm{ss}}$ as a stationary ensemble of pure states, be physically realized? Here physical realization is as defined previously by us [H. M. Wiseman and J. A. Vaccaro, Phys. Lett. A 250, 241 (1998)]: an ensemble of pure states for a particular system can be physically realized if, without changing the dynamics of the system, an experimenter can (in principle) know at any time that the system is in one of the pure-state members of the ensemble. Such knowledge can be obtained by monitoring the baths to which the system is coupled, provided that coupling is describable by a Markovian master equation. Using a family of master equations for the (atom) laser, we solve for the physically realizable (PR) ensembles. We find that for any finite self-energy $\chi$ of the bosons in the laser mode, the coherent-state ensemble is not PR; the closest one can come to it is an ensemble of squeezed states. This is particularly relevant for atom lasers, where the self-energy arising from elastic collisions is expected to be large. By contrast, the number-state ensemble is always PR. As the self-energy $\chi$ increases, the states in the PR ensemble closest to the coherent-state ensemble become increasingly squeezed. Nevertheless, there are values of $\chi$ for which states with well-defined coherent amplitudes are PR, even though the atom laser is not coherent (in the sense of having a Bose-degenerate output). We discuss the physical significance of this anomaly in terms of conditional coherence (and hence conditional Bose degeneracy).

As discussed in the preceding paper [Wiseman and Vaccaro, preceding paper, Phys. Rev. A 65, 043605 (2002)], the stationary state of an optical or atom laser far above threshold is a mixture of coherent field states with random phase, or, equivalently, a Poissonian mixture of number states. We are interested in which, if either, of these descriptions of ${\rho }_{\mathrm{ss}}$ as a stationary ensemble of pure states, is more natural. In the preceding paper we concentrated upon the question of whether descriptions such as these are physically realizable (PR). In this paper we investigate another relevant aspect of these ensembles, their robustness. A robust ensemble is one for which the pure states that comprise it survive relatively unchanged for a long time under the system evolution. We determine numerically the most robust ensembles as a function of the parameters in the laser model: the self-energy $\chi$ of the bosons in the laser mode, and the excess phase noise $\nu .$ We find that these most robust ensembles are PR ensembles, or similar to PR ensembles, for all values of these parameters. In the ideal laser limit $(\nu =\chi =0),$ the most robust states are coherent states. As the phase noise or phase dispersion is increased through $\nu$ or the self-interaction of the bosons $\chi ,$ respectively, the most robust states become more and more amplitude squeezed. We find scaling laws for these states, and give analytical derivations for them. As the phase diffusion or dispersion becomes so large that the laser output is no longer quantum coherent, the most robust states become so squeezed that they cease to have a well-defined coherent amplitude. That is, the quantum coherence of the laser output is manifest in the most robust PR ensemble being an ensemble of states with a well-defined coherent amplitude. This lends support to our approach of regarding robust PR ensembles as the most natural description of the state of the laser mode. It also has interesting implications for atom lasers in particular, for which phase dispersion due to self-interactions is expected to be large.

Laser-assisted electron-ion recombination is investigated with an emphasis on the spectrum of the emitted high-energy photons and its modification due to the recollision of the incident electron and the ion. Numerical results for the soft-x-ray power spectra, added up over all intermediate laser photon channels, are presented as a function of the incident electron energy for different laser field intensities. For strong laser fields, maxima, and additional structures are found in these spectra for incident electron energies of the order of magnitude of the ponderomotive energy. We show that the laser-assisted electron-ion recombination, that includes the rescattering of the electron at the ion before the recombination, is a process complementary to the well-known processes of high-order harmonic generation and high-order above-threshold ionization. All these processes can be explained, using the three-step scenario. A semiclassical analysis is presented which shows that for the laser-assisted electron-ion recombination real solutions of the saddle-point equations exist, contrary to what is found with high-order harmonic generation and high-order above-threshold ionization when only complex solutions are permitted. For low incoming electron energies, the cutoff of the emitted soft-x-ray photon energies, including the process of rescattering, is higher than in the case of the direct recombination process. The height of the rescattering plateau is 6–7 orders of magnitude lower than that of the direct process. However, for higher incident electron energies we obtain the unexpected result that the difference between the height of the rescattering plateau and the height of the direct plateau can be less than one order of magnitude.

The Coulomb explosion of OCS in laser pulses of 55 fs and peak intensity of $2\times {10}^{15}\hspace{0.5em}{\mathrm{W}\mathrm{}\mathrm{cm}}^{-2}$ has been investigated using an ion-momentum imaging technique to determine the shape of the exploding molecule. The results, which are interpreted using a generalized, classical, enhanced ionization model show a range of geometries consistent with earlier measurements on ${\mathrm{CO}}_2,$ exhibiting an enhancement of the bond angle, but no appreciable straightening. Consideration of the pre-Coulombic motion improves the agreement of our results with the enhanced ionization model, whilst revealing the presence of increased molecular bending.

We design a universal quantum homogenizer, which is a quantum machine that takes as an input a system qubit initially in the state $\rho$ and a set of N reservoir qubits initially prepared in the same state $\xi .$ In the homogenizer the system qubit sequentially interacts with the reservoir qubits via the partial swap transformation. The homogenizer realizes, in the limit sense, the transformation such that at the output each qubit is in an arbitrarily small neighborhood of the state $\xi$ irrespective of the initial states of the system and the reservoir qubits. This means that the system qubit undergoes an evolution that has a fixed point, which is the reservoir state $\xi .$ We also study approximate homogenization when the reservoir is composed of a finite set of identically prepared qubits. The homogenizer allows us to understand various aspects of the dynamics of open systems interacting with environments in nonequilibrium states. In particular, the reversibility vs irreversibility of the dynamics of the open system is directly linked to specific (classical) information about the order in which the reservoir qubits interacted with the system qubit. This aspect of the homogenizer leads to a model of a quantum safe with a classical combination. We analyze in detail how entanglement between the reservoir and the system is created during the process of quantum homogenization. We show that the information about the initial state of the system qubit is stored in the entanglement between the homogenized qubits.

This paper shows that a classical system containing a conventional yes/no decision-making component can behave like a quantum system of spin measurements in many ways (although it lacks a wave function) when, in principle, there are no deterministic decision procedures to govern the decision making, and when probabilistic decision procedures consistent with the system are introduced. Most notably, the system violates Bell’s inequalities. Moreover, since the system is simple and macroscopic, its similarities to quantum systems arguably provide an insight into quantum mechanics and, in particular, EPR experiments. Thus, from the qualitative correspondences, $\mathrm{decisions}\leftrightarrow \mathrm{quantum}$ measurements and the impossibility of deterministic decision $\mathrm{procedures}\leftrightarrow \mathrm{quantum}$ noncommutativity, we conclude that the violation of Bell’s inequalities in quantum mechanics does not require the existence of an unknown nonclassical nonlocality. It can merely be a result of local noncommutativity combined with nonlocalities of the classical type. The proposed classical decision-making system is a nonquantum theoretical construct possessing complementarity features in Bohr’s sense.

We model the behavior of an ion trap with all ions driven simultaneously and coupled collectively to a heat bath. The equations for this system are similar to the irreversible dynamics of a collective angular momentum system known as the Dicke model. We show how the steady state of the ion trap as a dissipative many-body system driven far from equilibrium can exhibit quantum entanglement. We calculate the entanglement of this steady state for two ions in the trap and in the case of more than two ions we calculate the entanglement between two ions by tracing over all the other ions. The entanglement in the steady state is a maximum for the parameter values corresponding roughly to a bifurcation of a fixed point in the corresponding semiclassical dynamics. We conjecture that this is a general mechanism for entanglement creation in driven dissipative quantum systems.

Although universal continuous-variable quantum computation cannot be achieved via linear optics (including squeezing), homodyne detection, and feed-forward, inclusion of ideal photon-counting measurements overcomes this obstacle. These measurements are sometimes described by arrays of beam splitters to distribute the photons across several modes. We show that such a scheme cannot be used to implement ideal photon counting and that such measurements necessarily involve nonlinear evolution. However, this requirement of nonlinearity can be moved “off-line,” thereby permitting universal continuous-variable quantum computation with linear optics.