We study the focusing of large neutral molecules in a molecular beam using electric fields. Since all quantum states of these molecules are high-field seeking under the practical experimental conditions, alternating gradient (AG) focusing has to be applied. The optimal ac frequency that yields the highest transmission depends on $m/\mu$, where $\mu$ is the dipole moment and $m$ denotes the mass. Therefore, an AG focuser can be used to select species with different $m/\mu$ ratios, e.g., the conformers of neutral molecules [Phys. Rev. Lett. 100, 133003 (2008)]. Here we demonstrate both theoretically and experimentally how the resolution of such an $m/\mu$ selector can be optimized.

In above-threshold ionization of rare-gas atoms, photoelectron spectra recorded in the ${10}^{13}$–${10}^{14}$ W cm${}^{-2}$ range exhibit enhancements in the rescattering plateaus that do not have a unified theoretical interpretation yet. Here an experimental search for such enhancements is reported in simple molecules with ionization potentials near those of rare-gas atoms such as ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ for argon and ${\mathrm{O}}_2$ for xenon, and in other molecules such as CO${}_2$ and ${\mathrm{N}}_2$O. Only ${\mathrm{H}}_2$ exhibits the enhancements previously observed in atoms. The ${\mathrm{H}}_2$ particularity is interpreted in terms of its simpler ion structure and associated ionization paths compared with other molecules.

We propose a scheme for generating isolated attosecond pulse (IAP) via high-order harmonic generation in gases using a chirped two-color laser field of multicycle duration. In contrast to previous techniques where the stable carrier-envelope phase (CEP) of the driving laser pulses is a prerequisite for IAP generation, the proposed scheme is robust against the large variation of CEP. We show the generation of IAP with an intensity fluctuation less than $50\%$ and an intensity contrast ratio higher than $5:1$ when the CEP shift is as large as $1.35$$\pi$.

We consider two Bose particles held in a bipartite lattice with two different separations fitting the nearest-neighbor tight-binding approximation. Applying a strong driving field of high frequency and a large interaction intensity, we analytically construct entangled Floquet states and slowly varying Rabi-oscillation states of the system. Under the interaction-independent and interaction-dependent conditions of the selective coherent destruction of tunneling, we propose a scheme for manipulating separated and co-site transports of the two particles in the same or opposite directions. The results could be useful for transporting quantum information carried by a few particles in a bipartite superlattice.

By relying on an advanced Jones calculus, we analyze the polarization properties of light upon propagation through metamaterial slabs in a comprehensive manner. Based on symmetry considerations, we show that all periodic metamaterials may be divided into five different classes only. It is shown that each class differently affects the polarization of the transmitted light and sustains different eigenmodes. We show how to deduce these five classes from symmetry considerations and provide a simple algorithm that can be applied to decide to which class a given metamaterial belongs by measuring only the transmitted intensities.

We study a coherent superposition $t\mathrm{â}+r{\mathrm{â}}^{†}$ of field annihilation and creation operators acting on continuous variable systems and propose its application for quantum state engineering. Specifically, it is investigated how the superposed operation transforms a classical state to a nonclassical one, together with emerging nonclassical effects. We also propose an experimental scheme to implement this elementary coherent operation and discuss its usefulness to produce an arbitrary superposition of number states involving up to two photons.

Polychromatic dynamic localization in tight-binding lattices with long-range interaction is theoretically proposed and experimentally demonstrated in curved-waveguide photonic lattices. Efficient suppression of discrete diffraction over the whole white-light spectral region ($450-750$ nm) has been demonstrated in femtosecond-laser-written triangular-waveguide lattices with first- and second-order coupling.

By amplifying photonic qubits it is possible to produce states that contain enough photons to be seen with the human eye, potentially bringing quantum effects to macroscopic scales [P. Sekatski, N. Brunner, C. Branciard, N. Gisin, and C. Simon, Phys. Rev. Lett. 103, 113601 (2009)]. In this paper we theoretically study quantum states obtained by amplifying one side of an entangled photon pair with different types of optical cloning machines for photonic qubits. We propose a detection scheme that involves lossy threshold detectors (such as the human eye) on the amplified side and conventional photon detectors on the other side. We show that correlations obtained with such coarse-grained measurements prove the entanglement of the initial photon pair and do not prove the entanglement of the amplified state. We emphasize the importance of the detection loophole in Bell violation experiments by giving a simple preparation technique for separable states that violate a Bell inequality without closing this loophole. Finally, we analyze the genuine entanglement of the amplified states and its robustness to losses before, during, and after amplification.

We study the temporal evolution of the properties of a two-level atom coupled to a single-mode cavity field without dissipation with its center-of-mass motion quantized in one dimension. It is shown that, starting with a separable state, genuine tripartite entangled states can be generated under resonance conditions of the light frequency and atom transition frequency in the cold regime. The onset of Rabi oscillations is analyzed and explicit predictions for properties like emission probability and dispersions for the center-of-mass position and momenta are given for resonance and detuned conditions. Transmission-resonance effects on entanglement and other properties are also analyzed. Comparisons with the semiclassical adiabatic approximation predictions are also made.

Using the eigenmodes of the scalar-photon kernel of interacting $N$ two-level atoms in a cylindrical configuration, I obtain the time dependence of the superradiant emission for this geometry in two cases of interest. In particular, I consider first the case when one or two eigenmodes dominate the system’s dynamics for an inverted system which has been initially prepared in an isotropic state and show that for certain values of the radius of the cylinder ($r=m\lambda ,m\hspace{0.5em}\mathrm{integer}$) one obtains temporal modulation in the state function. In the second case of interest, I show that the angular distribution of the state function of a system initially prepared in a phased state appears to be chaotic; however, I show that this behavior is due to the high multiplicity of modes having close values for their amplification coefficients.

It is revealed that the generation of a coherent wave by frequency conversion of incoherent waves is a characteristic feature of three-wave interaction in a nonlinear medium when angular dispersion of input waves is properly chosen. In this case the combining action of the pairs of spectral components of incoherent waves may result in the cumulative driving of a single plane monochromatic wave in up-conversion and down-conversion processes. As a fundamental result we point out an enhancement of the spectral radiance of the generated wave in comparison with incoherent waves.

We propose a practical physical system for creation of stationary entanglement by dissipation without employing environmental engineering techniques. The system proposed is composed of two perfectly distinguishable atoms, through their significantly different transition frequencies, with only one atom addressed by an external laser field. We show that the arrangement would easily be realized in practice by trapping the atoms at a distance equal to the quarter-wavelength of a standing-wave laser field and locating one of the atoms at a node and the other at the successive antinode of the wave. The undesirable dipole-dipole interaction between the atoms, which could be large at this small distance, is adjusted to zero by a specific initial preparation of the atoms or by a specific polarization of the atomic dipole moments. Following this arrangement, we show that the dissipative relaxation can create a stationary entanglement on demand by tuning the Rabi frequency of the laser field to the difference between the atomic transition frequencies. The laser field dresses the atom and we identify that the entangled state occurs when the frequency of one of the Rabi sidebands of the driven atom tunes to the frequency of the undriven atom. It is also found that this system behaves as a cascade open system where the fluorescence from the dressed atom drives the other atom with no feedback.

It is well known that ortho-positronium (ortho-Ps) atoms are emitted with high efficiency from various porous materials following the implantation of positrons. Since the ortho-Ps lifetime in a mesoporous material may be a substantial fraction of the ortho-Ps vacuum lifetime ($142$ ns), the time dependence of Ps emission may have to be considered when conducting certain types of experiments, such as time of flight measurements or pulsed ortho-Ps-laser interactions, when using this kind of target as a positronium source. By taking into account the positron implantation profile and subsequent Ps diffusion and decay in a mesoporous film we calculate the time dependent ortho-Ps emission rate $\Gamma$($t$), which in turn allows us to establish the total annihilation rate, arising from the decay of ortho-Ps both inside and outside the sample. Using time-delayed laser spectroscopy and single-shot lifetime measurements we have directly probed the rate at which Ps is emitted into vacuum from a target with $~$3-nm diameter pores and have observed delayed ortho-Ps emission that is consistent with our model. From the ortho-Ps decay spectrum we find that, whereas a simple two-component lifetime fit gives a short lifetime of $25.3$$\pm$$0.3$ ns, an analysis that properly takes into account the emission rate yields an ortho-Ps lifetime inside the porous material of $32.3$$\pm$$1.2$ ns, demonstrating that the ortho-Ps escape rate into vacuum can significantly modify the apparent lifetime of ortho-Ps inside a mesoporous material. Our measurements yield a Ps diffusion coefficient $D$ $=$ $0.07$ $\pm$ $0.01$ cm${}^2$ ${\mathrm{s}}^{-1}$, which is consistent with a tunneling limited diffusion process.

We consider a unitary transfer of an arbitrary state of a two-level atomic qubit in a cavity to the finite amplitude coherent state cavity field. Such a transfer can be used to either provide an effective readout measurement on the atom by a subsequent measurement on the light field or as a method for initializing a fixed atomic state—a so-called “attractor state,” studied previously for the case of an infinitely strong cavity field. We show that, with a suitable adjustment of the coherent amplitude and evolution time, the qubit transfers all its information to the field, attaining a selected state of high purity irrespective of the initial state.

We present measurements of the isotope shift for the $(3s3p){}^{1}P-(3s3d){}^{1}D$ transitions at $881$ nm in Mg i. The three stable magnesium isotopes yielded a shift ${}^{24}\mathrm{Mg}$–${}^{25}\mathrm{Mg}$ of $(1342\pm 20)$ MHz and ${}^{25}\mathrm{Mg}$–${}^{26}\mathrm{Mg}$ of $(1175\pm 23)$ MHz. Measured shifts are consistent with recent relativistic many-body calculations of the mass effect.

Collision-induced fragmentation of ${\mathrm{N}}_2$ was investigated for 2-MeV ${\mathrm{C}}^{+}$ ions under charge-changing conditions of ${\text{C}}^{+}\to {\text{C}}^{q+}$ ($q=0,2,3$). Coincidence measurement of fragment ions was performed by means of a momentum three-dimensional imaging technique at scattering angles of $\theta =0$ and $1.0$ mrad. Kinetic-energy release (KER) obtained for a fragmentation channel of ${\text{N}}_2^{2+}\to {\text{N}}^{+}+{\text{N}}^{+}$ was found to differ significantly in electron loss and capture collisions. In two-electron-loss collisions (${\text{C}}^{+}\to {\text{C}}^{3+}$), KER spectra were essentially identical for $\theta =0$ and $1.0$ mrad. It is concluded that the energy level of dissociating excited states of ${\mathrm{N}}_2^{2+}$ may be saturated when the interaction strength, defined as $\frac{q}{\mathit{vb}}$, exceeds $0.65$, where $q$ and $v$ are the charge and the velocity, respectively, of an incident ion, and $b$ is the impact parameter.

The standard mean-field theory for the Mott-insulator–superfluid phase transition is not sufficient to describe the Mott-insulator–paired-superfluid phase transition. Therefore, by restricting the two-species Bose-Hubbard Hamiltonian to the subspace of paired particles, and using perturbation theory, here we derive an analytic mean-field expression for the Mott-insulator–paired-superfluid transition boundary.

The robustness of multipartite entanglement of systems undergoing decoherence is of central importance to the area of quantum information. Its characterization depends, however, on the measure used to quantify entanglement and on how one partitions the system. Here we show that the unambiguous assessment of the robustness of multipartite entanglement is obtained by considering the loss of functionality in terms of two communication tasks, namely the splitting of information between many parties and the teleportation of states.

We theoretically investigate the dynamical Casimir effect (DCE) in electrical circuits based on superconducting microfabricated waveguides with tunable boundary conditions. We propose implementing a rapid modulation of the boundary conditions by tuning the applied magnetic flux through superconducting quantum-interference devices that are embedded in the waveguide circuits. We consider two circuits: (i) An open waveguide circuit that corresponds to a single mirror in free space, and (ii) a resonator coupled to a microfabricated waveguide, which corresponds to a single-sided cavity in free space. We analyze the properties of the DCE in these two setups by calculating the generated photon-flux densities, output-field correlation functions, and the quadrature squeezing spectra. We show that these properties of the output field exhibit signatures unique to the radiation due to the DCE, and could, therefore, be used for distinguishing the DCE from other types of radiation in these circuits. We also discuss the similarities and differences between the DCE, in the resonator setup, and the down-conversion of pump photons in parametric oscillators.

According to a recent paper by G. Vignale [Phys. Rev. A 77, 062511 (2008)], the problems arising in the original derivation of time-dependent density-functional theory based on the Runge-Gross (RG) action-integral functional (AIF) are because of an incorrect variational principle (VP). This argument and the proposed modification of the VP are critically analyzed. The more fundamental problem, though, is the indefiniteness of the RG AIF. In contrast to a widely held belief the indefiniteness is not eliminated in the variational procedure, which unwittingly is corroborated by Vignale’s initial point.