We describe a measurement-based state preparation scheme for the efficient build-up of cluster states in atom-cavity systems. As in a recent proposal for the generation of maximally entangled atom pairs [Metz et al., Phys. Rev. Lett. 97, 040503 (2006)], we use an electron shelving technique to avoid the necessity for the detection of single photons. Instead, the successful fusion of smaller into larger clusters is heralded by an easy-to-detect macroscopic fluorescence signal. High fidelities are achieved even in the vicinity of the bad cavity limit and are essentially independent of the concrete size of the system parameters.

We construct a theory of continuous-variable entanglement-assisted quantum error correction. We present an example of a continuous-variable entanglement-assisted code that corrects for an arbitrary single-mode error. We also show how to implement encoding circuits using passive optical devices, homodyne measurements, feedforward classical communication, conditional displacements, and off-line squeezers.

We present experimental evidence for pronounced electron transfer from C to ${\mathrm{F}}^{+}$ happening during the breakup of $\mathrm{C}{\mathrm{H}}_3{\mathrm{F}}^{+}$ ions in gas phase produced by resonant photoemission following F $1s\to 6a_1^{*}$ core excitation of $\mathrm{C}{\mathrm{H}}_3\mathrm{F}$. We measured the momentum of the ionic fragments in coincidence with the F $KVV$ Auger electrons that show a Doppler shift reflecting the motion of the F nucleus. The correlation between Doppler shift and ion momentum is opposite for the ${\mathrm{F}}^{+}$ and the $\mathrm{C}\mathrm{H}_2{}^{+}$ fragments, indicating that $\mathrm{C}\mathrm{H}_2{}^{+}$ is produced by electron transfer from C to ${\mathrm{F}}^{+}$, after the Auger electron emission from excited moving F. This finding is rationalized by calculations of the potential energy curves of the main states involved in the excitation and decay processes.

Within the framework of the relativistic quantum theory the multiphoton above-threshold ionization (ATI) of hydrogenlike atoms in the field of super-strong electromagnetic radiation of linear polarization, taking into account the photoelectron stimulated bremsstrahlung (SB) on the residual ion, is investigated. The latter is considered in the Born approximation by the Coulomb field which allows us to simplify the previous results in the generalized eikonal approximation upon the scattering potential and obtain an analytic formula for the relativistic ATI rate summed over the intermediate transitions of a photoelectron through the virtual states of the atomic continuum. The energetic spectra and angular distributions of photoelectrons are obtained with the secondary SB of photoelectrons. Due to the latter the partial ATI rates are 4 times larger than the ATI rates of the direct process. In contrast to the nonrelativistic case, the main peaks in the angular distributions of photoelectrons are shifted toward the wave propagation direction and have a tendency to be aligned about this direction with the increase of the field intensity.

Theoretical analysis of the Autler-Townes splitting pattern in a three-level cascade excitation of a Doppler-broadened sample reveals that the observed Autler-Townes splitting is not only a function of the coupling-laser Rabi frequency, as in the homogeneously broadened case, but can also strongly depend on the wavenumber ratio of the coupling and probe lasers. Utilizing a steady-state density matrix formalism, we derive exact expressions for the populations of the intermediate and upper levels for a homogeneously broadened system. In the limit of a weak probe field, analytical expressions for Doppler-broadened excitation spectra from the intermediate and upper levels are derived. Using these expressions, we investigate the critical role of Doppler broadening on the observation of Autler-Townes splitting and its asymptotic behavior as a function of the coupling Rabi frequency for counterpropagating and copropagating configurations for different wavenumber ratios of the lasers.

We examine fully coherent two-pulse propagation in a $\Lambda$-type medium, under two-photon resonance conditions and including inhomogeneous broadening. We examine the effects of both short pulse preparation and medium preparation. We contrast the cases in which the two pulses have or have not matched envelopes, and media with and without ground state coherence. We find that an extended interpretation of the area theorem for single-pulse self-induced transparency is able to unify two-pulse propagation scenarios, including some aspects of electromagnetically induced transparency and stimulated Raman scattering. We present numerical solutions of both three-level and adiabatically reduced two-level density matrix equations and Maxwell’s equations, and show that many features of the solutions are quickly interpreted with the aid of analytical solutions that we also provide for restricted cases of pulse shapes and preparation of the medium. In the limit of large one-photon detuning, we show that the adiabatic two-level equations commonly used to study stimulated Raman scattering are not reliable for pulse areas in the $2\pi$ range, which allows puzzling features of previous numerical work to be understood.

The theory of optical dispersive shocks generated in the propagation of light beams through photorefractive media is developed. A full one-dimensional analytical theory based on the Whitham modulation approach is given for the simplest case of a sharp steplike initial discontinuity in a beam with one-dimensional striplike geometry. This approach is confirmed by numerical simulations, which are extended also to beams with cylindrical symmetry. The theory explains recent experiments where such dispersive shock waves have been observed.

We present a calculation of the $4d\text{−}4f$ energy separation in Li I using two advanced techniques in atomic structure theory, namely, the relativistic all-order method and the multiconfiguration Hartree-Fock (MCHF) method. The accuracy of our calculations was investigated by conducting a third-order many-body perturbation theory calculation that allowed us to evaluate the importance of fourth- and higher-order corrections. A large-scale MCHF calculation was performed using the active space method and the core-polarization approximation. The obtained results provide an important test of these methods against each other and are shown to agree with the most accurate available experimental data.

In a recent work [Phys. Rev. Lett. 98, 023004 (2007)], we investigated the influence of the attractive van der Waals interaction on the pair distribution and Penning ionization dynamics of ultracold Rydberg gases. Here we extend this description to atoms initially prepared in Rydberg states exhibiting repulsive interaction. We present calculations based on a Monte Carlo algorithm to simulate the dynamics of many atoms under the influence of both repulsive and attractive long-range interatomic forces. Redistribution to nearby states induced by blackbody radiation is taken into account, changing the effective interaction potentials. The model agrees with experimental observations, where the ionization rate is found to increase when the excitation laser is blue detuned from the atomic resonance.

In two recent articles [D. Meiser and P. Meystre, Phys. Rev. A 73, 033417 (2006); 74, 065801 (2006)], Meiser and Meystre describe the coupled dynamics of a dense gas of atoms and an optical cavity pumped by a laser field. They make two important simplifying assumptions: (i) The gas of atoms forms a regular lattice and can be replaced by a fictitious mirror and (ii) the atoms strive to minimize the dipole potential. We show that the two assumptions are inconsistent: The configuration of atoms minimizing the dipole potential is not a perfect lattice. Assumption (ii) is erroneous, as in the strong coupling regime the dipole force does not arise from the dipole potential. The real steady state, where the dipole forces vanish, is indeed a regular lattice. Furthermore, the bistability predicted by Meiser and Meystre does not occur in this system.

In their Comment, Asbóth and Domokos question the consistency of two approximations introduced in two recent articles [D. Meiser and P. Meystre, Phys. Rev. A 73, 033417 (2006); 74, 065801 (2006)]. They show that the minimization of the atomic dipole potential does not in general give rise to a regular lattice of ultracold atoms and that the steady-state positions of atoms in a cavity are not normally characterized by a minimum of the dipole potential. Instead, the steady-state positions correspond to a vanishing force on each individual atom, the relevant force being proportional to the derivative of the optical field mode function. We agree with Asbóth and Domokos that the atomic lattice should be irregular. Using a microscopic model we show however that in the regime considered in our works and also in the preceding Comment the concept of force is generally not well defined, and a semiclassical theory may be inadequate for the description of the atomic motion.

We determine the temperature $\left(T\right)$ dependence of first- and second-sound-like mode frequencies for trapped Fermi gases undergoing the BCS to Bose–Einstein condensation (BEC) crossover. Our results are based on numerical solution of the two-fluid equations in conjunction with a microscopic calculation of thermodynamical variables. As in experiment and at unitarity, we show that the lowest radial breathing mode is $T$ independent. At finite $T$, higher-order breathing modes strongly mix with second sound. Their complex $T$ dependence should provide an alternative way of measuring the transition temperature $T_c$.

We report on measurements of cross-species thermalization inside a magnetically trapped spin-polarized mixture of ${}^{87}\mathrm{Rb}$ and ${}^7\mathrm{Li}$ atoms with both atoms in their respective low-field-seeking magnetic substates $\mid F=2,m_F=2⟩$. Measurement of the thermalization velocity in the ultracold regime below $10\mathrm{\mu }\mathrm{K}$ allows for the derivation of the absolute value of the pure triplet $s$-wave scattering length governing the interaction. We find $\mid a_{7,87}\mid =(59\pm 19)a_{\mathrm{B}}$. We propose to study both species in the condensed regime to derive the sign of $a_{7,87}$. In this context, we present numerical solutions to the coupled Gross-Pitaevskii equation based on the hypothesis of a positive sign. According to the simulations, phase separation of the Li and Rb $\mid 2,2⟩$ clouds occurs along with a mean-field stabilization allowing for larger atom numbers of condensed ${}^7\mathrm{Li}$ atoms before collapse sets in. Observation of this mean-field stabilization would directly fix the sign of $a_{7,87}$. We discuss our results in the light of this proposal.

The photoionization spectrum of an atom in a magnetic field is calculated by combining $R$-matrix propagation with local adiabatic basis expansions. This approach considerably increases the speed and the energy range over which calculations can be performed compared to previous methods, allowing one to obtain accurate partial and total cross sections over an extended energy range for an arbitrary magnetic field strength. In addition, the cross sections for all atoms of interest can be calculated simultaneously in a single calculation. Multichannel quantum defect theory allows for a detailed analysis of the resonance structure in the continuum. Calculated cross sections for a range of atoms at both laboratory and astrophysical field strengths are presented.

The phase of the mean-field wave function of a Bose-Einstein condensate can be recovered from a time series of images. We adapt an iterative retrieval method that has been successful in linear electron and optical imaging systems to solve the nonlinear Gross-Pitaevskii equation. We address a number of issues related to the successful application of this method to the nonlinear system, including the retrieval of wave functions with nonzero net topological charge (i.e., containing vortices) and the effects of repulsive and attractive interactions in the condensate on the convergence properties of the method. Recovering the phase from the continuity-of-density equation is also investigated. An understanding of these issues is of importance for the practical implementation of phase retrieval to Bose-Einstein condensates.

We consider a duplicated two-level system where each two-level subsystem is driven by a strong resonant femtosecond pulse and a weak resonant femtosecond pulse connects cross transitions. Strong interference effects are induced in such a configuration leading to a modulation of the medium gain as a function of the relative phase between the pulses []. We study here the influence of the temporal overlap between the pulses on the dynamics of the system and the medium gain. We show that the dynamics is dominated by two competing phenomena: Light shifts (LS) induced by the driving pulse that prevails for small delays and free induction decay (FID) that prevails for large delays. LS enhance the medium gain for and reduce it for , whereas FID always leads to a decrease of the medium gain.

Thermal noise in mirror optical coatings may not only limit the sensitivity of future gravitational-wave detectors in their most sensitive frequency band but is also a major impediment for experiments that aim to reach the standard quantum limit or cool mechanical systems to their quantum ground state. We present the design and experimental characterization of a highly reflecting mirror without any optical coating. This coating-free mirror is based on total internal reflection and Brewster-angle coupling. In order to characterize its performance, the coating-free mirror was incorporated into a triangular ring cavity together with a high quality conventional mirror. The finesse of this cavity was measured using an amplitude transfer function to be about $\mathcal{F}\simeq 4000$. This finesse corresponds to a reflectivity of the coating-free mirror of about $R\simeq 99.89\%$. In addition, the dependence of the reflectivity on rotation was mapped out.

Ionization-induced change in the refractive index of a gas is shown to give rise to a substantial spectral blueshift of megawatt light pulses transmitted through a gas-filled hollow photonic-crystal fiber (PCF). This effect suggests the ways of controlling not only the rate, but also the sign of the soliton frequency shift for high-peak-power ultrashort light pulses guided in hollow PCFs filled with Raman-active ionizing gases.

We show that the negativity of a general two-mode Gaussian state can be explicitly expressed in terms of an optimal uncertainty product in position-momentum space. Such an uncertainty product is shown to have the greatest violation of a separability criterion based on positive partial transposition. Our analytic formula indicates the observables determining the negativity. For asymmetric Gaussian states, we show that the negativity is controlled by an asymmetric parameter which sets an upper bound for the negativity.