The question of the existence of a nonzero minimum dipole moment $D_0$ that can sustain an electron bound state for an electric dipole in two dimensions is examined both classically and quantum mechanically. The results suggest that in the latter case, $D_0⩽0.209$ compared to the Fermi-Teller value 0.904 for three dimensions (in atomic units).

The inseparability of photon-added Gaussian states which are generated from two-mode Gaussian states by adding photons is investigated. According to the established inseparability conditions [New J. Phys. 7, 211 (2005); Phys. Rev. Lett. 96, 050503 (2006)], we find that even if a two-mode Gaussian state is separable, the photon-added Gaussian state becomes entangled when the purity of the Gaussian state is larger than a certain value. The lower bound of entanglement of symmetric photon-added Gaussian states is derived. The result shows that entanglement of the photon-added Gaussian states is involved with high-order moment correlations. We find that fidelity of teleporting coherent states cannot be raised by employing the photon-added Gaussian states as a quantum channel of teleportation.

In this paper, we discuss the security of the differential-phase-shift quantum-key-distribution (DPSQKD) protocol by introducing an improved version of the so-called sequential attack, which was originally discussed by Waks et al. [Phys. Rev. A 73, 012344 (2006)]. Our attack differs from the original form of the sequential attack in that the attacker Eve modulates not only the phases but also the amplitude in the superposition of the single-photon states which she sends to the receiver. Concentrating especially on the “discretized Gaussian” intensity modulation, we show that our attack is more effective than the individual attack, which had been the best attack up to present. As a result of this, the recent experiment with communication distance of $100\mathrm{km}$ reported by Diamanti et al. [Opt. Express 14, 13073 (2006)] turns out to be insecure. Moreover, it can be shown that in a practical experimental setup which is commonly used today, the communication distance achievable by the DPSQKD protocol is less than $95\mathrm{km}$.

We consider a system consisting of a $\Lambda$-type atom and a $V$-type atom, which are individually trapped in two spatially separated cavities that are connected by an optical fiber. We show that an extremely entangled state of the two atoms can be deterministically generated through both photon emission of the $\Lambda$-type atom and photon absorption of the $V$-type atom in an ideal situation. The influence of various decoherence processes such as spontaneous emission and photon loss on the fidelity of the entangled state is also investigated. We find that the effect of photon leakage out of the fiber on the fidelity can be greatly diminished in some special cases. In regard to the effect of spontaneous emission and photon loss from the cavities, we find that the present scheme with high fidelity may be realized under current experiment conditions. We also show that either the $\Lambda$- or $V$-type level configuration can be realized on the same atom by initializing the atomic state, and we show the present scheme is scalable.

A relativistic $R$-matrix close-coupling method is developed and implemented based on effective many-body Hamiltonians for accurate representation of the target and collisional states in multielectron ions. The effective Hamiltonian in relativistic multireference many-body perturbation theory accurately accounts for short-range many-body interactions unaccounted for in the extant nonrelativistic and Breit-Pauli $R$-matrix methods. The method is successfully applied to the near-threshold electron impact excitation of the $3s^2$ ${}^{1}S\to 3s3p$ ${}^{3}P$ transition in Mg-like argon $\left({\mathrm{Ar}}^{6+}\right)$ ion where the observed disagreement between the experimental absolute total cross sections and those predicted by using the Breit-Pauli $R$-matrix method reveals an inadequacy of the $R$-matrix methods based on the configuration-interaction representation of the basis states.

Relativistic above-threshold ionization (ATI) and high-harmonic generation in the long pulse regime are investigated via the Klein-Gordon equation employing the strong-field approximation. The energy and angular spectra of the rescattering electrons for ATI in the relativistic low frequency tunneling regime are examined. While the relativistic drift significantly reduces the rescattering efficiency in conventional sinusoidal laser waves, the employment of laser pulses specially tailored as a strong attosecond pulse train (APT) allows to counteract the drift. The optimal conditions for high-energy rescattering are found in relation to the duration of the attosecond pulse, to the time delay between pulses in the APT, as well as to the spectral content of the APT. The stability of the harmonic yield vs deformations of the tailored pulse shape is shown. The proposed method renders energies of more than $1\mathrm{MeV}$ possible for the rescattering atomic electron.

We use second- and fourth-order correlation functions accessible in time-of-flight images to investigate the effects of rotation on one-dimensional ultracold bosons confined to a ring lattice. There exists a critical rotation frequency at which the ground state of a weakly interacting and integer-filled atomic gas is fragmented into a macroscopic superposition of two states with different circulation. The formation of such a quantum superposition (“cat”) state is accompanied by the opening of a gap in the spectrum, and by a sudden rearrangement of the momentum distribution which lowers the threshold of the Mott insulator transition. We show that both the entangled character of the ground state and the enhancement of quantum correlations can be detected in the density-density correlations of the expanding cloud. Our studies demonstrate the usefulness of these correlations for identifying physics in cold atomic systems.

We construct systematic expansions around four and two spatial dimensions for a Fermi gas near the unitarity limit. Near four spatial dimensions such a Fermi gas can be understood as a weakly interacting system of fermionic and bosonic degrees of freedom. To the leading and next-to-leading orders in the expansion over $ϵ=4-d$, with $d$ being the dimensionality of space, we calculate the thermodynamic quantities and the fermion quasiparticle spectrum, both at unitarity and around the unitarity point. Then the phase structure of the polarized Fermi gas in the unitary regime is studied in the $ϵ$ expansion. At unitarity the unpolarized superfluid state and a fully polarized normal state are separated by a first order phase transition. However, on the BEC side of the unitarity point, in a certain range of the two-body binding energy, these two phases are separated in the phase diagram by more exotic phases, including gapless superfluid phases with one or two Fermi surfaces and a superfluid phase with a spatially varying condensate. We also show that the unitary Fermi gas near two spatial dimensions is weakly interacting, and calculate the thermodynamic quantities and the fermion quasiparticle spectrum in the expansion over $\overline{ϵ}=d-2$.

Thermodynamics of the unitary Fermi gas at finite temperature is investigated from the perspective of the expansion over $ϵ=4-d$ with $d$ being the dimensionality of space. We show that the thermodynamics is dominated by bosonic excitations in the low temperature region $T\ll T_c$. Analytic formulas for the thermodynamic functions as functions of the temperature are derived to the lowest order in $ϵ$ in this region. In the high temperature region where $T\sim T_c$, bosonic and fermionic quasiparticles are excited. We determine the critical temperature $T_c$ of the superfluid phase transition and the thermodynamic functions around $T_c$ to the leading and next-to-leading orders in $ϵ$.

An optical spanner is a light beam that can exert a torque on an object. It is demonstrated in this Rapid Communication that, with the aid of applied electric and magnetic fields, a light beam with initially linear polarization and initially zero total spin angular momentum can interact with an optically active medium, resulting in a change of the ratio of left-handed circularly polarized photons to right-handed ones. Thus the total spin angular momentum of the light is changed, which leads to a torque, creating an electrically and magnetically controlled optical spanner on the medium. For a linearly polarized $632.8\mathrm{nm}$ laser beam incident on a $100-\mu \mathrm{m}$-long $\mathrm{Ce}:{\mathrm{Bi}}_{12}\mathrm{Ti}{\mathrm{O}}_{20}$ whisker crystal with $5\mu \mathrm{m}$ radius, if the magnetic field is fixed at $-1.8\mathrm{T}$, both the left- (right-)handed circularly polarized photon number and the total spin angular momentum vary with the applied electric field in a sinusoidal way, which means the torque exerted by the optical spanner on the crystal also varies sinusoidally with the electric field. It is found that at 50 $(\text{or}-50)\mathrm{kV}∕\mathrm{cm}$, 56% right- (left-)handed circularly polarized photons are translated into left- (right-)handed ones, which corresponds to a transfer of $0.56\hslash$ spin angular momentum contributed by each photon.

We introduce a tunnel lock that can be exploited to divide, delay, and alter the direction of traveling clouds of cold atoms. This versatile free-space element is implemented by crossing two atom tunnels formed by low-intensity, blue-detuned dark-hollow (Bessel mode) laser beams. We show that clouds of cold Rb atoms initially moving within one tunnel can be transferred to the other without heating by gating the intensities of the two tunnels—a tunnel lock—with an efficiency limited by the overlap volume. The element also can be used to divide a single cloud into smaller clouds, each having a distinct momentum.

In order to meet experimental conditions, the generation, evolution, and self-stabilization of optical dissipative light bullets from non-spherically-symmetric input pulses is studied. Steady-state solutions of the $(3+1)$-dimensional complex cubic-quintic Ginzburg-Landau equation are computed using the variational approach with a trial function asymmetric with respect to three transverse coordinates. The analytical stability criterion is extended to systems without spherical symmetry, allowing determination of the domain of dissipative parameters for stable solitonic solutions. The analytical predictions are confirmed by numerical evolution of the asymmetric input pulses toward stable dissipative light bullets. Once established, the dissipative light bullet remains surprisingly robust.

One and two photon wave functions are derived by projecting the quantum state vector onto simultaneous eigenvectors of the number operator and a recently constructed photon position operator [Phys. Rev. A 59, 954 (1999)] that couples spin and orbital angular momentum. While only the Landau-Peierls wave function defines a positive definite photon density, a similarity transformation to a biorthogonal field-potential pair of positive frequency solutions of Maxwell’s equations preserves eigenvalues and expectation values. We show that this real space description of photons is compatible with all of the usual rules of quantum mechanics and provides a framework for understanding the relationships amongst different forms of the photon wave function in the literature. It also gives a quantum picture of the optical angular momentum of beams that applies to both one photon and coherent states. According to the rules of quantum mechanics, this wave function gives the probability to count a photon at any position in space.

The lateral Casimir force between two corrugated metallic plates makes possible a study of the nontrivial interplay of geometry and Casimir effect appearing beyond the regime of validity of the proximity-force approximation. Quantitative evaluations can be obtained by using scattering theory in a perturbative expansion valid when the corrugation amplitudes are smaller than the three other length scales: the mean separation distance $L$ of the plates, the corrugation period ${\lambda }_C$, and the plasma wavelength ${\lambda }_P$. Within this perturbative expansion, evaluations are obtained for arbitrary relative values of $L$, ${\lambda }_C$, and ${\lambda }_P$ while limiting cases, some of them already known, are recovered when these values obey some specific orderings. The consequence of these results for comparison with existing experiments is discussed at the end of the paper.

The tunnel ionization rate under a static intense electric field is calculated for some hydrocarbon molecules, acetylene, ethylene, and benzene, employing the ab initio density-functional theory. The ionization rate is found to reflect the properties of the highest occupied molecular orbital, in agreement with the description of the molecular Ammosov-Delone-Krainov theory. Many-electron effects beyond a single-electron approximation are discussed. The screening effect on the ionization rate is quantitatively evaluated.

Elastic and inelastic collision-rate constants of ${}^{52}\mathrm{Cr}$ in the temperature range of $20\mathrm{mK}\text{to}1\mathrm{K}$ are inferred from the evaporative cooling of buffer gas loaded atomic chromium. Using a model that describes the dynamics of the trapped chromium cloud during evaporation, we find $g_{\mathrm{el}}=2.15(+2.5,-1.2)\times {10}^{-10}{\mathrm{cm}}^3∕\mathrm{s}$ and $g_{\mathrm{in}}=1.36(+1.2,-0.7)\times {10}^{-12}{\mathrm{cm}}^3∕\mathrm{s}$, consistent with theory but in disagreement with previously reported measurements.

A study is made of the differential cross sections arising from the photoionization of $\mathrm{H}_2{}^{+}$. Previous studies indicated surprising differences in the shapes of the angular distributions calculated from exterior complex scaling and 2C methods. To further explore these differences, we have calculated the angular distributions from the photoionization of $\mathrm{H}_2{}^{+}$ using an independent two-body Coulomb function (2C) method and a distorted wave approach. As a final test, we also present calculations using a time-dependent technique. Our results confirm the discrepancies found previously and we present possible reasons for these differences.

Recent multiply differential experimental data taken with reaction microscopes severely challenge predictions of quantum mechanical few-body models. Here, we report on a thorough analysis of all known possible experimental resolution effects and their influence on the extracted cross sections. Using a Monte Carlo event generator to simulate true events on the basis of quantum calculations allows us to consistently incorporate all aspects of the experimental resolution of the reaction microscope. We study the effect of the instrumental function in single ionization of helium by $3.6\mathrm{MeV}∕u$ ${\mathrm{Au}}^{53+}$ and $100\mathrm{MeV}∕u$ ${\mathrm{C}}^{6+}$ ions and find it to significantly modify the simulated theoretical predictions. Nevertheless strong discrepancies between simulated and experimental data persist. Structures in the measured cross sections reported earlier, which could not be reproduced by theory, are thus not solely due to the experimental resolution. Our study suggests that the method using event generators, as routinely done in high-energy physics, provides the ultimate pathway of benchmarking calculations against experimental data on few-particle reactions studied with modern imaging techniques.

Multifragmentation of ${\mathrm{C}}_{60}$ was investigated for $2\mathrm{MeV}$ ${\mathrm{C}}^{+}$ projectile ions by means of a time-of-flight triple coincidence technique. Fragment ions and secondary electrons were measured simultaneously in coincident with charge-selected outgoing projectile particles scattered into forward angles smaller than $1.0\mathrm{mrad}$. Fragment ion distributions were found to change remarkably with respect to the scattering angle $\theta$. Relationships between $\theta$ and the impact parameter $b$ measured from the center of ${\mathrm{C}}_{60}$ were calculated classically. We found evidently that small-size fragment ions are produced only from cage-penetration collisions, while intact parent ions and their daughter ions are created in collisions nearby but outside a ${\mathrm{C}}_{60}$. Furthermore, charge state distributions of prefragmented $\mathrm{C}_{60}{}^{r+}$ ions and correlated fragment ion pairs were both found to be essentially independent of $\theta$ when plotted for individual $\mathrm{C}_m{}^{+}$ ions of fixed size. These results indicate that the production of a size-fixed specific ion may be restricted to a certain impact parameter region in which equivalent electronic energies are deposited irrespectively of $\theta$. This was confirmed also from our calculation of the energy deposition as a function of $b$ carried out using the local density approximation.