Photon-number Fock states play important roles in quantum optics. However, the generation of the states are very difficult because of the lack of large effective nonlinearities. In this Rapid Communication, I show a scheme to produce the Fock state from an input coherent state by using only linear optics method and projection measurements. I analyze the case of five-photon number state generation from coherent states for a specific case of the scheme. It is possible to remove the vacuum state from the output state when one introduces quantum nondemolition measurement that can also be implemented with all linear optics setup.

We have measured the charge-state distributions of argon and potassium after ionization by photons with energies near the $K$-shell ionization threshold. Despite the similarity in core electron configurations, the two atoms show remarkable differences in the resulting distribution of ion charge states. The valence electron in potassium is rarely a spectator during core relaxation, and its presence enhances the loss of electrons excited into Rydberg levels or strongly reduces the recapture of slow photoelectrons during postcollision interaction.

A method is developed that treats on equal footing bound states, resonance formation, and scattering problems in one-dimensional systems. The approach allows one to deal with nonlocal, energy-dependent potentials and is conceptually analogous to the variable phase method where the role of the scattering phase and the amplitude functions is played by nonlocal reflection and transmission functions. The formal results are illustrated and analyzed by simple examples and the physical significance of these examples is pointed out.

The propagation of nuclear polaritons through a scattering system composed of two spatially separated targets was studied. The stainless-steel targets were highly enriched in resonant ${}^{57}\mathrm{Fe}$ nuclei and had different thicknesses, and, in addition, the resonance width of one target could be increased by cooling it to low temperatures. A nuclear polariton state was created in both targets almost instantly by a short flash of synchrotron radiation. The propagation of the polariton could be perturbed by a time-dependent Doppler shift of the resonance energy in one of the targets. The effect of inversion of the target sequence on the polariton propagation was in the focus of our investigations. It was shown both theoretically and experimentally that in general a two-target scattering system is not commutative.

We explore the physical mechanism to coherently transfer the quantum information of spin by connecting two spins to an isotropic antiferromagnetic spin ladder system as data bus. Due to a large spin gap existing in such a perfect medium, the effective Hamiltonian of the two connected spins can be archived as that of Heisenberg type, which possesses a ground state with maximal entanglement. We show that the effective coupling strength is inversely proportional to the distance of the two spins and thus the quantum information can be transferred between the two spins separated by a longer distance, i.e., the characteristic time of quantum-state transferring linearly depends on the distance.

We study ${\mathrm{C}}_{70}$ fullerene matter waves in a Talbot-Lau interferometer as a function of their temperature. While the ideal fringe visibility is observed at moderate molecular temperatures, we find a gradual degradation of the interference contrast if the molecules are heated before entering the interferometer. A method is developed to assess the distribution of the microcanonical temperatures of the molecules in free flight. This way the heating-dependent reduction of interference contrast can be compared with the predictions of quantum theory. We find that the observed loss of coherence agrees quantitatively with the expected decoherence rate due to the thermal radiation emitted by the hot molecules.

It is well known that measurements performed on spatially separated entangled quantum systems can give rise to correlations that are nonlocal, in the sense that a Bell inequality is violated. They cannot, however, be used for superluminal signaling. It is also known that it is possible to write down sets of “superquantum” correlations that are more nonlocal than is allowed by quantum mechanics, yet are still nonsignaling. Viewed as an information-theoretic resource, superquantum correlations are very powerful at reducing the amount of communication needed for distributed computational tasks. An intriguing question is why quantum mechanics does not allow these more powerful correlations. We aim to shed light on the range of quantum possibilities by placing them within a wider context. With this in mind, we investigate the set of correlations that are constrained only by the no-signaling principle. These correlations form a polytope, which contains the quantum correlations as a (proper) subset. We determine the vertices of the no-signaling polytope in the case that two observers each choose from two possible measurements with $d$ outcomes. We then consider how interconversions between different sorts of correlations may be achieved. Finally, we consider some multipartite examples.

It is shown that the density matrix equations for a three-level quantum system interacting with external radiation fields can lead to negative populations if arbitrary dephasing rates and shifts are included in these equations. To guarantee non-negative populations, the equations themselves impose certain restrictions on the dephasing widths and shifts. The constraints on the widths are shown to be identical to those that can be derived from a model of Markovian dephasing events, independent of any atom-field interaction.

Absolute measurements are presented for the positron-impact cross sections for direct ionization and positronium formation of noble gas atoms in the range of energies from threshold to 90 eV. The experiment uses a cold, trap-based positron beam and the technique of studying positron scattering in a strong magnetic field. The current data show generally good, quantitative agreement with previous measurements taken using a qualitatively different method. However, significant differences in the cross sections for both direct ionization and positronium formation are also observed. An analysis is presented that yields another, independent measurement of the direct ionization and positronium formation cross sections that is in agreement with the present, direct measurements to within $\pm 10\%$ for argon, krypton, and xenon. Comparison with available theoretical predictions yields good quantitative agreement for direct ionization cross sections, and qualitative agreement in the case of positronium formation.

We report branching ratios between the ground and excited states of iodine atoms in the photodissociation of sodium iodide. We employ wave packet propagation techniques to study the optimal production of polarized iodine atoms and find experimentally realizable laser parameters to control the outcome. Application of a learning algorithm shows that the product branching can be controlled by suitably varying the time delay, the chirp, and the relative phase of the pump and control laser pulses. Periodic modulation of the polarized iodine atom branching ratio as a function of the delay between the firing of the two ultrashort laser pulses provides interesting insights into the photodissociation process.

Based on the phase-conjugate polarization interference between two-pathway excitations, we obtained an analytic closed form for the second-order or fourth-order Markovian stochastic correlation of the V three-level sum-frequency polarization beat (SFPB) in attosecond scale. Novel interferometric oscillatory behavior is exposed in terms of radiation-radiation, radiation-matter, and matter-matter polarization beats. The phase-coherent control of the light beams in the SFPB is subtle. When the laser has broadband linewidth, the homodyne detected SFPB signal shows resonant-nonresonant cross correlation, a drastic difference for three Markovian stochastic fields, and the autocorrelation of the SFPB exhibits hybrid radiation-matter detuning terahertz damping oscillation. As an attosecond ultrafast modulation process, it can be extended intrinsically to any sum frequency of energy levels. It has been also found that the asymmetric behaviors of the polarization beat signals due to the unbalanced controllable dispersion effects between the two arms of interferometer do not affect the overall accuracy in case using the SFPB to measure the Doppler-free energy-level sum of two excited states.

We study the giant Kerr nonlinear interaction between two ultraweak optical fields in which the cross-phase-modulation is not accompanied by spectral broadening of the interacting pulses. This regime is realizable in atomic vapors, when a weak probe pulse, upon propagating through the electromagnetically induced transparency (EIT) medium, interacts with a signal pulse that is dynamically trapped in a photonic band gap created by spatially periodic modulation of its EIT resonance. We find that large conditional phase shifts and entanglement between the signal and probe fields can be obtained with this scheme. The attainable $\pi$ phase shift, accompanied by negligible absorption and quantum noise, is shown to allow a high-fidelity realization of the controlled-phase universal logic gate between two single-photon pulses.

In this Comment on the paper by ], it is pointed out that (i) a group-theoretical argument advanced in the first part of their derivation is both flawed and not needed in what follows, (ii) they then assert a formula that leads to a physically incorrect phase (as distinguished from group) delay time in the motion of a free particle, and (iii) their subsequent derivation contains a mathematical error, such that the outcome is a new formula for the tunneling time that avoids the second problem, but such that the result is unfounded and is therefore of unknown physical content.

We propose to use Bragg spectroscopy to measure the excitation spectrum of the Mott-insulator state of an atomic Bose gas in an optical lattice. We calculate the structure factor of the Mott insulator taking into account both the self-energy corrections of the atoms and the corresponding dressing of the atom-photon interaction. We determine the scattering rate of photons in the stimulated Raman transition and show that by measuring this scattering rate in an experiment, in particular, the excitation gap of the Mott insulator can be determined.

We report results from studies of the Autler-Townes (AT) effect observed in sodium molecules from a molecular beam. A relatively weak laser field $P$ couples an initially populated rovibronic level $g$ in the electronic ground state (here $X{}^1\Sigma _g^{+},v^{″}=0$, $J^{″}=7$) to a selected excited rovibronic level $e$ (here $A{}^1\Sigma _u^{+},v^{\prime }=10$, $J^{\prime }=8$), which in turn is coupled by a relatively strong laser field $S$ to a more highly excited level $f$ (here $5{}^1\Sigma _g^{+},v=10$, $J=9$), a scheme we idealize as a three-state ladder. The AT effect is seen by scanning the frequency of the $P$ field while recording fluorescence from both the $e$ and $f$ levels in separate detection channels. We present qualitative theoretical considerations showing that, when the $P$ field is weak, the ratio of doublet component areas in the excitation spectrum from level $f$ can be used to determine the lifetime of this level. We obtain a value of $17\pm 3\mathrm{ns}$. When the $P$ field is stronger, such that its Rabi frequency is larger than the decay rate of level $e$, the fraction of $f$-level population that decays to the intermediate electronic state $A{}^1\Sigma _u^{+}$ can be deduced from the AT spectrum. When supplemented with values of Franck-Condon and Hönl-London factors, our measurements give a value for the branching ratio (the fraction returning to level $e$) of $r_e=0.145$ with a statistical error of $\pm 0.004$. The use of a strong $P$ field on the $g\text{−}e$ transition and a weak $S$ field as a probe on the $e\text{−}f$ transition results in complex line shapes in the excitation spectrum of level $f$, not showing the familiar Autler-Townes doublet structure.

We consider the question of whether there are any fundamental limits to the maximum time delay that can be achieved for a pulse propagating through a slow-light medium. We include in our analysis what we consider to be the dominant competing effects, and we show that in principle they do not lead to a limitation on the maximum achievable time delay. From this result we conclude that, through optimization, one should be able to delay a pulse by very many pulse lengths; the ability to do so can have important implications for the use of slow-light methods for applications in photonics.

We investigate how decoherence affects the short-time separation between quantum and classical dynamics for classically chaotic systems, within the framework of a specific model. For a wide range of parameters, the distance between the corresponding phase-space distributions depends on a single parameter $\chi$ that relates an effective Planck constant ${\hslash }_{\mathrm{eff}}$, the Lyapunov coefficient, and the diffusion constant. This distance peaks at a time that depends logarithmically on ${\hslash }_{\mathrm{eff}}$, in agreement with previous estimations of the separation time for Hamiltonian systems. However, for $\chi \lesssim 1$, the separation remains small, going down with ${\hslash }_{\mathrm{eff}}^2$, so the concept of separation time loses its meaning.

The fragmentation dynamics of $\mathrm{H}_2{}^{+}$ molecular ions in intense laser fields is investigated by means of a high-precision ab initio method beyond the Born-Oppenheimer approximation. Special attention is paid to the detailed Coulomb explosion (CE) mechanisms and dynamics through excited vibrational states of $\mathrm{H}_2{}^{+}$. A kinetic-energy release (KER) spectrum and CE phenomenon is predicted, in which the kinetic-energy distribution of ${\mathrm{H}}^{+}$ ions exhibits a series of peaks separated by one photon energy. A proposed scheme for observation of the KER spectrum and CE dynamics is presented.

The present paper suggests the continuum theory of Tkachenko modes in a rotating two-dimensional Bose-Einstein condensate taking into account density inhomogeneity and compressibility of the condensate. The theory is based on the solution of coupled hydrodynamic equations for vortex and liquid motion with proper boundary conditions, which were derived for the condensate described by the Thomas-Fermi approximation. Compressibility becomes essential at rapid rotation with angular velocity close to the trap frequency. The theory is in reasonable agreement with experimental observation of Tkachenko modes.

The possibility of negative optical path length in left-handed media (LHM) is shown to lead to complete suppression of spontaneous emission of an atom in front of a mirror with a layer of LHM. For the same reason two atoms put at the foci of a perfect lens formed by a parallel LHM slab [J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000)] exhibit perfect subradiance and superradiance. It is shown that these effects occur over distances that can be orders of magnitude larger than the transition wavelength and are only limited by the propagation length within the free-space decay time of the atoms. Single- and two-atom decay rates are calculated from the Greens function of the electric field in the presence of a LHM and limitations as well as potential applications are discussed.