Showing papers in "Journal of Physics B in 2006"
TL;DR: The theoretical description and experimental methods and results for above-threshold ionization (ATI) by few-cycle pulses are reviewed in this article, where a pulse is referred to as a fewcycle pulse if its detailed shape, parametrized by its carrier-envelope phase, affects its interaction with matter.
Abstract: The theoretical description and the experimental methods and results for above-threshold ionization (ATI) by few-cycle pulses are reviewed. A pulse is referred to as a few-cycle pulse if its detailed shape, parametrized by its carrier-envelope phase, affects its interaction with matter. Angular-resolved ATI spectra are analysed with the customary strong-field approximation (SFA) as well as the numerical solution of the time-dependent Schrodinger equation (TDSE). After a general discussion of the characteristics and the description of few-cycle pulses, the behaviour of the ATI spectrum under spatial inversion is related to the shape of the laser field. The ATI spectrum both for the direct and for the rescattered electrons in the context of the SFA is evaluated by numerical integration and by the method of steepest descent (saddle-point integration), and the results are compared. The saddle-point method is modified to avoid the singularity of the dipole transition matrix element at the steepest-descent times. With the help of the saddle-point method and its classical limit, namely the simple-man model, the various features of the ATI spectrum, their behaviour under inversion, the cut-offs and the presence or absence of ATI peaks are analysed as a function of the carrier-envelope phase of the few-cycle laser field. All features observed in the spectra can be explained in terms of a few quantum orbits and their superposition. The validity of the SFA and the concept of quantum orbits are established by comparing the ATI spectra with those obtained numerically from the ab initio solution of the TDSE.
515 citations
TL;DR: A review of recent work in the field of helium nanodroplet spectroscopy with an emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent is provided in this article.
Abstract: This article provides a review of recent work in the field of helium nanodroplet spectroscopy with an emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.
378 citations
TL;DR: In this article, the authors investigate entanglement dynamics of two isolated atoms, each in its own Jaynes-Cummings cavity, and show analytically that initial entropy has interesting subsequent time evolution, including the so-called sudden death effect.
Abstract: We investigate entanglement dynamics of two isolated atoms, each in its own Jaynes–Cummings cavity. We show analytically that initial entanglement has an interesting subsequent time evolution, including the so-called sudden death effect.
316 citations
TL;DR: In this paper, the authors reported the observation of single photon emission from single SiV (silicon-vacancy) centers in diamond produced by ion implantation and discussed problems that arise from the nonradiaditve transitions which lower the brightness of the source.
Abstract: We report the observation of single photon emission from single SiV (silicon-vacancy) centres in diamond produced by ion implantation. The high photostability and the narrow emission bandwidth of about 5 nm at room temperature make SiV centres interesting as a single photon source in practical quantum cryptography. We discuss problems that arise from the nonradiaditve transitions which lower the brightness of the source.
287 citations
TL;DR: In this paper, the main experimental observables for energy transfer to a cluster and the major theoretical approaches have been devised, and compared the cluster response to standard 780 nm light pulses with the response to 100 nm pulses, already obtained at a VUV free electron laser (FEL) source, and with 3 nm light which will be available from x-ray FEL sources.
Abstract: Femtosecond laser pulses have proven to provide valuable insight into the dynamics of microscopic systems by using pump–probe techniques. Applied to atomic clusters even a single pulse of varying pulse duration can reveal how and when energy from the laser pulse is transferred effectively to the cluster. We review the main experimental observables for energy transfer to a cluster and the major theoretical approaches which have been devised. Most importantly, we compare the cluster response to standard 780 nm light pulses with the response to 100 nm pulses, already obtained at a VUV free electron laser (FEL) source, and with 3 nm light which will be available from x-ray FEL sources.
276 citations
200 citations
TL;DR: In this article, a scheme for symmetric multiparty quantum state sharing of an arbitrary m-qubit state with m Greenberger-Horne-Zeilinger states was proposed.
Abstract: We present a scheme for symmetric multiparty quantum state sharing of an arbitrary m-qubit state with m Greenberger–Horne–Zeilinger states following some ideas from the controlled teleportation (2005 Phys Rev A 72 02338) The sender Alice performs m Bell-state measurements on her 2m particles and the controllers need only take some single-photon product measurements on their photons independently, not multipartite entanglement measurements, which makes this scheme more convenient than the latter Also it does not require the parties to perform a controlled-NOT gate on the photons for reconstructing the unknown m-qubit state and it is an optimal one as its efficiency for qubits approaches 100% in principle
191 citations
TL;DR: In this article, an atom localization scheme based on coherent population trapping is presented. But the authors consider atomic transitions in a Lambda configuration where the control field is a standing-wave field and the probe field produces coherence between the two ground states.
Abstract: We present an atom localization scheme based on coherent population trapping. We consider atomic transitions in a Lambda configuration where the control field is a standing-wave field. The probe field and the control field produce coherence between the two ground states and prepare the atom in a pure state. We show that the population in one of the ground states has the same fringe pattern as produced by a Fabry-Perot interferometer and thus measurement of this population would localize the atom. Interestingly enough the role of the cavity finesse is played by the ratio of the intensities of the pump and probe. This is in fact the reason for obtaining extreme subwavelength localization.
170 citations
TL;DR: In this paper, the complete valence shell photoelectron spectra of cytosine, thymine and adenine have been investigated experimentally and theoretically using the many-body Green's function method.
Abstract: The complete valence shell photoelectron spectra of cytosine, thymine and adenine have been investigated experimentally and theoretically. Vertical ionization energies and spectral intensities have been evaluated using the many-body Green's function method, thereby enabling theoretical photoelectron spectra to be derived. In cytosine, the influence of tautomers and rotational conformers has been investigated. The calculated spectra display a satisfactory agreement with the experimental data and this has allowed most of the photoelectron bands to be assigned. Photoelectron asymmetry parameters have been determined from angle resolved spectra recorded with synchrotron radiation. The experimental data show that the electronic configuration of the five outer orbitals in cytosine, thymine and adenine is π, σ, π, σ, π. Vertical ionization energies have been measured for all the outer-valence orbitals even though some of the associated bands overlap significantly.
117 citations
TL;DR: In this article, it was shown that in the majority of problems the force densities lead to identical results, where the theories do differ, and the authors attempt to determine which of the two descriptions is the more reliable.
Abstract: The study of the mechanical effects of light on a dielectric medium has led to two quite distinct forms for the force density: one based on the microscopic distribution of charges and the other on the distribution of atomic dipoles. Both approaches are based directly on the Lorentz force, but it has been suggested that they lead, in a number of cases, to significantly different predictions. In this paper we address this paradoxical situation and show that in the majority of problems the force densities lead to identical results. Where the theories do differ we attempt to determine which of the two descriptions is the more reliable.
113 citations
TL;DR: This work presents a three-party quantum secure direct communication protocol by using Greenberg–Horne–Zeilinger (GHZ) states and entanglement swapping and shows that the scheme is secure against eavesdropper's commonly used attacks.
Abstract: We present a three-party quantum secure direct communication protocol by using Greenberg–Horne–Zeilinger (GHZ) states and entanglement swapping. The proposed scheme realizes authorized parties' secure exchange of their respective secret messages simultaneously and directly in a set of devices. We show that the scheme is secure against eavesdropper's commonly used attacks. We also generalize the protocol to the N-party case by using N-partite GHZ states.
TL;DR: The angular correlation of Lyman-alpha photons detected in coincidence with the 10.2 eV energy-loss electrons have been measured for 54.4 eV incident electrons exciting the n=2 levels of atomic hydrogen as mentioned in this paper.
Abstract: The angular correlation of Lyman-alpha photons detected in coincidence with the 10.2 eV energy-loss electrons have been measured for 54.4 eV incident electrons exciting the n=2 levels of atomic hydrogen. For electron scattering angles in the range from 10 to 140 degrees the in-plane angular correlations were measured for photon angles from 10 to 140 degrees . These data were interpreted in terms of lambda , the ratio of the differential cross sections mod aml mod 2 for magnetic sublevel ml excitation, and R, the real part of the product (a0a1) in which the angle brackets indicate summation over the spin states. The error bars on these values are significantly smaller than on previous measurements of Williams (1975), Hood et al (1979) and Weigold et al. (1980). No existing theory predicts values in agreement with the present data. An angular correlation in which the photon detector axis is perpendicular to the scattering plane is shown to measure directly the 2p state angular differential cross section as well as the ratio of that cross section to the total 2p cross section. Also, by using an appropriate quenching pulse, the 2s state angular differential cross section, sigma (2s), is measured.
TL;DR: In this article, a relativistic 115-level R-matrix analysis of Ni-like W46+ was performed for the International Thermonuclear Experimental Reactor (ITER) and the effects of radiation damping.
Abstract: The current design plans for the International Thermonuclear Experimental Reactor (ITER) call for tungsten to be employed for certain plasma facing components in the divertor region. Thus, accurate atomic collision data are needed for emission modelling of tungsten. Electron-impact excitation and radiative rates are of particular importance for Ni-like W, since this ion emits some of the most intense spectral lines of all ionization stages. We report on a fully relativistic 115-level R-matrix calculations of W46+, which includes the effects of radiation damping. Although radiation damping is very important in most highly ionized species, its effects are reduced in this case because of the closed-shell Ni-like ground state. The rates from these relativistic atomic calculations will be employed for collisional-radiative modelling of this ion.
TL;DR: In this article, a servo algorithm is presented that stabilizes a laser frequency to the single-ion signal and eliminates errors due to laser frequency drift in order to obtain the highest possible stability under realistic experimental conditions.
Abstract: A fundamental limit to the stability of a single-ion optical frequency standard is set by quantum noise in the measurement of the internal state of the ion. We discuss how the interrogation sequence and the processing of the atomic resonance signal can be optimized in order to obtain the highest possible stability under realistic experimental conditions. A servo algorithm is presented that stabilizes a laser frequency to the single-ion signal and that eliminates errors due to laser frequency drift. Numerical simulations of the servo characteristics are compared to experimental data from a frequency comparison of two single-ion standards based on a transition at 688 THz in 171Yb+. Experimentally, an instability σy(100 s) = 9 × 10−16 is obtained in the frequency difference between both standards.
TL;DR: The recent development of experimental photoionization cross sections of atomic ions has been reviewed in this article, where it has been possible during the last ten years to perform a large number of absolute cross-section measurements using the merged-beam method, with implications for the modelling of astrophysical and laboratory plasmas for which fundamental data of this kind are required.
Abstract: The recent development of experimental photoionization cross sections of atomic ions has been reviewed. Owing to the construction of intense undulator-based photon sources it has been possible during the last ten years to perform a large number of absolute cross-section measurements using the merged-beam method. Photoionization cross sections provide a critical test of theoretical calculations, with implications for the modelling of astrophysical and laboratory plasmas for which fundamental data of this kind are required.
TL;DR: A semi-empirical relation which can be used to determine the total attenuation cross sections of samples containing H, C, N and O in the energy range 145-1332 keV has been derived based on the total cross sections as discussed by the authors.
Abstract: A semi-empirical relation which can be used to determine the total attenuation cross sections of samples containing H, C, N and O in the energy range 145–1332 keV has been derived based on the total attenuation cross sections of several sugars, amino acids and fatty acids. The cross sections have been measured by performing transmission experiments in a narrow beam good geometry set-up by employing a high-resolution hyperpure germanium detector at seven energies of biological importance such as 145.4 keV, 279.2 keV, 514 keV, 661.6 keV, 1115.5 keV, 1173.2 keV and 1332.1 keV. The semi-empirical relation can reproduce the experimental values within 1–2%. The total attenuation cross sections of five elements carbon, aluminium, titanium, copper and zirconium measured in the same experimental set-up at the energies mentioned above have been used in a new matrix method to evaluate the effective atomic numbers and the effective electron densities of samples such as cholesterol, fatty acids, sugars and amino acids containing H, C, N and O atoms from their effective atomic cross sections. The effective atomic cross sections are the total attenuation cross sections divided by the total number of atoms of all types in a particular sample. Further, a quantity called the effective atomic weight was defined as the ratio of the molecular weight of a sample to the total number of atoms of all types in it. The variation of the effective atomic number was systematically studied with respect to the effective atomic weight and a new semi-empirical relation for Zeff has been evolved. It is felt that this relation can be very useful to determine the effective atomic number of any sample having H, C, N and O atoms in the energy range 145–1332 keV irrespective of its chemical structure.
TL;DR: In this paper, the authors performed calculations for electron collisions with tetrahydrofuran (THF) using the UK molecular R-matrix codes and reported ab initio integral cross section for incident energies up to 10 eV.
Abstract: We have performed calculations for electron collisions with tetrahydrofuran (THF) using the UK molecular R-matrix codes. This is the largest molecule ever treated with the R-matrix method, and the only biologically relevant molecule of this size studied theoretically in the inelastic regime. We report ab initio integral cross section for incident energies up to 10 eV. No shape resonances have been found for this system, but a few core-excited resonances are present.
TL;DR: In this article, the authors studied the possibility for monitoring electron motion in a molecule using two ultrashort laser pulses, where the first prepares a coherent superposition of two electronic molecular states whereas the second photoionizes the molecule.
Abstract: We study analytically the possibility for monitoring electron motion in a molecule using two ultrashort laser pulses. The first prepares a coherent superposition of two electronic molecular states whereas the second (attosecond pulse) photoionizes the molecule. We show that interesting information about electron dynamics can be obtained from measurement of the photoelectron spectra as a function of the time delay between two pulses. In particular, asymmetries in photoelectron angular distribution provide a simple signature of the electron motion within the initial time-dependent coherently coupled two molecular states. Both asymmetries and electron spectra show very strong two-centre interference patterns. We illustrate these effects using as an example a dissociating hydrogen molecular ion probed by the attosecond pulses.
TL;DR: In this paper, the outer core level photoionization of the free 3D metal atoms from Sc to Cu is discussed and the experimental 3p, 3s and 2p photoemission and photo absorption spectra are discussed.
Abstract: This review covers the outer core level photoionization of the free 3d metal atoms from Sc to Cu. The experimental 3p, 3s and 2p photoemission and photoabsorption spectra are discussed. A comparison emphasizes common features and distinct differences. The interpretation of the data based on ab initio calculations reveals the influence of multi-electron interactions in the 3d metal atoms. We focus on the fundamental effects and main interactions which govern the electronic structure of these open shell atoms. (Some figures in this article are in colour only in the electronic version)
TL;DR: In this paper, the sub-cycle dynamics of strong-field ionization by an intense few-cycle laser pulse are discussed. But the dynamics are strongly dependent both on the presence or absence of excited states in the field-free potential and on the structure of the continuum.
Abstract: We discuss the sub-cycle dynamics of strong-field ionization by an intense few-cycle laser pulse. Even at high intensities when all the excited states are embedded into the continuum, the dynamics are found to be strongly dependent both on the presence or absence of excited states in the field-free potential and on the structure of the continuum. Both Coulomb effects in the continuum and polarization of the initial state are important even for qualitative description of sub-cycle ionization. We show how to consistently include effects of the binding (Coulomb) potential on the continuum part of the electron wavefunction while retaining the effects of the laser field on the bound part. We demonstrate quantitative accuracy of our approach in the barrier-suppression region of laser intensities.
TL;DR: In this paper, the use of periodic arrays of permanent magnetic films for producing magnetic lattices of microtraps for confining, manipulating and controlling small clouds of ultracold atoms and quantum degenerate gases was proposed.
Abstract: We propose the use of periodic arrays of permanent magnetic films for producing magnetic lattices of microtraps for confining, manipulating and controlling small clouds of ultracold atoms and quantum degenerate gases. Using analytical expressions and numerical calculations we show that periodic arrays of magnetic films can produce one-dimensional (1D) and two-dimensional (2D) magnetic lattices with non-zero potential minima, allowing ultracold atoms to be trapped without losses due to spin flips. In particular, we show that two crossed layers of periodic arrays of parallel rectangular magnets plus bias fields, or a single layer of periodic arrays of square-shaped magnets with three different thicknesses plus bias fields, can produce 2D magnetic lattices of microtraps having non-zero potential minima and controllable trap depth. For arrays with micron-scale periodicity, the magnetic microtraps can have very large trap depths (~0.5 mK for the realistic parameters chosen for the 2D lattice) and very tight confinement.
TL;DR: In this paper, the properties of an electromagnetically induced grating formed in a Ξ-type three-level atomic system in the presence of a standing wave pump and travelling wave probe fields at resonant and off-resonant two-photon absorption were analyzed.
Abstract: We have analysed the properties of an electromagnetically induced grating formed in a Ξ-type three-level atomic system in the presence of a standing wave pump and travelling wave probe fields at resonant and off-resonant two-photon absorption. We have also shown the validity of this grating in an inhomogeneously broadened regime.
TL;DR: In this article, the first observation of ultracold LiC molecules was made in a two-species magneto-optical trap and detected by two-photon ionization and time-of-flight mass spectrometry.
Abstract: We present the first observation of ultracold LiCs molecules. The molecules are formed in a two-species magneto-optical trap and detected by two-photon ionization and time-of-flight mass spectrometry. The production rate coefficient is found to be in the range 10−18 cm3 s−1 to 10−16 cm3 s−1, at least an order of magnitude smaller than for other heteronuclear diatomic molecules directly formed in a magneto-optical trap.
TL;DR: In this article, momentum-space images of low-energy electrons generated by the interaction of short intense laser pulses with argon atoms at high intensities were measured over a wavelength range from 400 to 800 nm.
Abstract: We have measured momentum-space images of low-energy electrons generated by the interaction of short intense laser pulses with argon atoms at high intensities. We have done this over a wavelength range from 400 to 800 nm. The spectra show considerable structure in both the energy and angular distributions of the electrons. Some, but not all, energy features can be identified as multi-photon resonances. The angular structure shows a regularity which transcends the resonant structure and may be due instead to diffraction. The complexity of the results defies easy model-dependent interpretations and invites full solutions to Schrodinger's equation for these systems.
TL;DR: In this paper, the authors theoretically investigate optical bistability (OB) behavior in a four-level atomic system confined in a unidirectional ring cavity and show that the threshold intensity of OB can be reduced without decreasing the range of the hysteresis loop via adjusting the effect of spontaneously generated coherence or the relative phase between two applied fields.
Abstract: We theoretically investigate optical bistability (OB) behaviour in a four-level atomic system confined in a unidirectional ring cavity. It is shown that OB can be controlled by adjusting the intensity of the coupling field and the value of the cooperativity parameter. More importantly, our numerical results show that the threshold intensity of OB can be reduced without decreasing the range of the hysteresis loop via adjusting the effect of spontaneously generated coherence (SGC) or the relative phase between two applied fields. The influence of the frequency detuning of the coupling field on the OB behaviour is also discussed.
TL;DR: In this paper, a new formula for the interference modulation is derived within the framework of the multiple scattering technique, which differs from the classical Cohen-Fano formula by the addition of twice the scattering phase of the photoelectron by the neighbouring atom.
Abstract: The core-level photoelectron spectra of N2 molecules are observed at high energy resolution, resolving the 1σg and 1σu components as well as the vibrational components in the extended energy region from the threshold up to 1 keV. The σg/σu cross section ratios display modulation as a function of photoelectron momentum due to the two-centre interference, analogous to the classical Young’s double-slit experiment, as predicted by Cohen and Fano a long time ago. The Cohen–Fano interference modulations display different phases depending on the vibrational excitations in the core-ionized state. Extensive ab initio calculations have been performed within the Hartree–Fock and random phase approximations in prolate spheroidal coordinates. The dependence of photoionization amplitudes on the vibrational states was taken into account using the Born–Oppenheimer approximation. The ab initio results are in reasonable agreement with the experimental data. The theoretical analysis allows the modulation to be connected with the onset of transitions to the states of increasing orbital angular momentum which occurs at increasing photon energies. Deviation from the Cohen–Fano formula is found for both the experimental and the ab initio results and is attributed to electron scattering by the neighbouring atom. A new formula for the interference modulation is derived within the framework of the multiple scattering technique. It differs from the classical Cohen–Fano formula by the addition of twice the scattering phase of the photoelectron by the neighbouring atom. We demonstrate that
TL;DR: In this paper, the authors present complete collisional-radiative modelling results for the soft x-ray emission lines of Fe16+ in the 15 A-17 A range.
Abstract: We present complete collisional-radiative modelling results for the soft x-ray emission lines of Fe16+ in the 15 A–17 A range. These lines have been the subject of much controversy in the astrophysical and laboratory plasma community. Radiative transition rates are generated from fully relativistic atomic structure calculations. Electron-impact excitation cross sections are determined using a fully relativistic R-matrix method employing 139 coupled atomic levels through n = 5. We find that, in all cases, using a simple ratio of the collisional rate coefficient times a radiative branching factor is not sufficient to model the widely used diagnostic line ratios. One has to include the effects of collisional-radiative cascades in a population model to achieve accurate line ratios. Our line ratio results agree well with several previous calculations and reasonably well with tokamak experimental measurements, assuming a Maxwellian electron-energy distribution. Our modelling results for four EBIT line ratios, assuming a narrow Gaussian electron-energy distribution, are in generally poor agreement with all four NIST measurements but are in better agreement with the two LLNL measurements. These results suggest the need for an investigation of the theoretical polarization calculations that are required to interpret the EBIT line ratio measurements.
TL;DR: In this paper, the authors describe the principles of alternating gradient (AG) deceleration and set out criteria to be followed in decelerator design, construction and operation, and exemplify this by 2D-imaging studies of a decelerated beam of metastable CO molecules.
Abstract: Beams of polar molecules can be focused using an array of electrostatic lenses in alternating gradient (AG) configuration. They can also be accelerated or decelerated by applying an appropriate high-voltage switching sequence to the lenses. AG focusing is applicable to molecules in both low-field- and high-field-seeking states and is particularly well suited to the problem of decelerating heavy molecules and those in their ground rotational state. We describe the principles of AG deceleration and set out criteria to be followed in decelerator design, construction and operation. We calculate the longitudinal and transverse focusing properties of a decelerator, and exemplify this by 2D-imaging studies of a decelerated beam of metastable CO molecules.
TL;DR: In this article, the effects of the incoherent pumping field and spontaneously generated coherence (SGC) on the phase control of group velocity were studied and the effect of a relative phase between probe and coupling fields on the absorption and the dispersion were discussed.
Abstract: This paper studies the effects of the incoherent pumping field and the spontaneously generated coherence (SGC) on the phase control of group velocity. The effects of a relative phase between probe and coupling fields on the absorption and the dispersion are then discussed. It is shown that the phase dependence of the group velocity not only depends on the existence of the SGC, but also depends on the existence of the incoherent pump field. We show that for the weak probe field, and in the presence of SGC, the existence of the incoherent pump field is a necessary condition for the phase control of the dispersion, the absorption and the group index.
TL;DR: In this paper, the authors performed molecular strong-field approximation calculations in the length and velocity gauges on N2 to produce photoelectron angular distributions, total ionization rates and ion signals as a function of the orientation between the laser field and the molecule.
Abstract: Molecular strong-field approximation calculations in the length and velocity gauges are performed on N2 to produce photoelectron angular distributions, total ionization rates and ion signals as a function of the orientation between the laser field and the molecule. Our study is supplemented by molecular tunnelling calculations, the results of which are in qualitative agreement with the experimental findings and the length gauge molecular strong-field approximation results.