We present a discretely modulated continuous-variable quantum key distribution system in free space by using strong coherent states. The amplitude noise in the laser source is suppressed to the shot-noise limit by using a mode cleaner combined with a frequency shift technique. Also, it is proven that the phase noise in the source has no impact on the final secret key rate. In order to increase the encoding rate, we use broadband homodyne detectors and the no-switching protocol. In a realistic model, we establish a secret key rate of $46.8$ kbits/s against collective attacks at an encoding rate of $10$ MHz for a $90\%$ channel loss when the modulation variance is optimal.

Spin chains have long been considered as candidates for quantum channels to facilitate quantum communication. We consider the transfer of a single excitation along a spin-1/2 chain governed by Heisenberg-type interactions. We build on the work of Balachandran and Gong [V. Balachandran and J. Gong, Phys. Rev. A 77, 012303 (2008)] and show that by applying optimal control to an external parabolic magnetic field, one can drastically increase the propagation rate by two orders of magnitude. In particular, we show that the theoretical maximum propagation rate can be reached, where the propagation of the excitation takes the form of a dispersed wave. We conclude that optimal control is not only a useful tool for experimental application, but also for theoretical inquiry into the physical limits and dynamics of many-body quantum systems.

At impact energies $\gtrsim$$1$ GeV/u the projectile-electron excitation and loss occurring in collisions between highly charged ions and neutral atoms is already strongly influenced by the presence of atomic electrons. To treat these processes in collisions with heavy atoms we generalize the symmetric eikonal model, used earlier for considerations of electron transitions in ion-atom collisions within the scope of a three-body Coulomb problem. We show that at asymptotically high collision energies this model leads to an exact transition amplitude and is very well suited to describe the projectile-electron excitation and loss at energies above a few GeV/u. In particular, by considering a number of examples we demonstrate advantages of this model over the first Born approximation at impact energies of $~$$1-30$ GeV/u, which are of special interest for atomic physics experiments at the future GSI facilities.

The $J$-matrix approach to electron-atom scattering is applied to ionization processes. We consider the Temkin-Poet model of $e$-H ionization. Convergence issues are studied with greater detail than previously possible using other close-coupling methods. The numerical strengths of the technique are emphasized with the long-term goal of application to ionization-plus-excitation processes.

We numerically solve the Boltzmann equation for trapped fermions in the normal phase by using the test-particle method. After discussing a couple of tests in order to estimate the reliability of the method, we apply it to the description of collective modes in a spherical harmonic trap. The numerical results are compared with those obtained previously by taking moments of the Boltzmann equation. We find that the general shape of the response function is very similar in both methods, but the relaxation time obtained from the simulation is significantly longer than that predicted by the method of moments. It is shown that the result of the method of moments can be corrected by including fourth-order moments in addition to the usual second-order ones and that this method agrees very well with our numerical simulations.

The variational method of coupled Gaussian functions is applied to Bose-Einstein condensates with long-range interactions. The time dependence of the condensate is described by dynamical equations for the variational parameters. We present the method and analytically derive the dynamical equations from the time-dependent Gross-Pitaevskii equation. The stability of the solutions is investigated using methods of nonlinear dynamics. The concept presented in this article will be applied to Bose-Einstein condensates with monopolar $1/r$ and dipolar $1/r^3$ interaction in the subsequent article [S. Rau et al., Phys. Rev. A 82, 023611 (2010)], where we will present a wealth of phenomena obtained using the ansatz with coupled Gaussian functions.

Bose-Einstein condensates with an attractive $1/r$ interaction and with dipole-dipole interaction are investigated in the framework of the Gaussian variational ansatz introduced by S. Rau, J. Main, and G. Wunner [Phys. Rev. A 82, 023610 (2010)]. We demonstrate that the method of coupled Gaussian wave packets is a full-fledged alternative to direct numerical solutions of the Gross-Pitaevskii equation, or even superior in that coupled Gaussians are capable of producing both stable and unstable states of the Gross-Pitaevskii equation and thus of giving access to yet unexplored regions of the space of solutions of the Gross-Pitaevskii equation. As an alternative to numerical solutions of the Bogoliubov–de Gennes equations, the stability of the stationary condensate wave functions is investigated by analyzing the stability properties of the dynamical equations of motion for the Gaussian variational parameters in the local vicinity of the stationary fixed points. For blood-cell-shaped dipolar condensates it is shown that on the route to collapse the condensate passes through a pitchfork bifurcation, where the ground state itself turns unstable, before it finally vanishes in a tangent bifurcation.

Energy distributions are measured for ions emitted upon Coulomb explosion of ${\mathrm{Ar}}_n$ clusters ($n=400-900$) upon irradiation by intense three-cycle pulses ($10$ fs) of $800$-nm laser light of peak intensity $5\times {10}^{14}\hspace{0.5em}\mathrm{W}\mathrm{\hspace{0.5em}}{\mathrm{cm}}^{-2}$. With few-cycle pulses, there is insufficient time for the cluster to undergo expansion; this results in overall dynamics that are significantly different from those in the many-cycle regime. The peak ion energies are much lower than those obtained when $100$-fs pulses of the same intensity are used; they are almost independent of the size of the cluster (over the range $400-900$ atoms). Ion yields are measured to be larger in the direction that is perpendicular to the laser-polarization vector than along it. Model molecular dynamics calculations are used to qualitatively rationalize this unexpected anisotropy in terms of shielding by a spatially asymmetric electronic-charge cloud within the cluster.

We assess quantum nonlocality of multiparty entangled thermal states by studying, quantitatively, both tripartite and quadripartite states belonging to the Greenberger-Horne-Zeilinger, W, and linear cluster-state classes and showing violation of relevant Bell-like inequalities. We discuss the conditions for maximizing the degree of violation against the local thermal character of the states and the inefficiency of the detection apparatuses. We demonstrate that such classes of multipartite entangled states can be made to last quite significantly, notwithstanding adverse operating conditions. This opens up the possibility for coherent exploitation of multipartite quantum channels made out of entangled thermal states. Our study is accompanied by a detailed description of possible generation schemes for the states analyzed.

We present an upper bound of the entanglement evolution for two-mode Gaussian pure states under a one-sided Gaussian map. Based on this, the optimization of entanglement evolution is studied. Even if complete information about the one-sided Gaussian noisy channel does not exist, one can still maximize the entanglement distribution by testing the channel with only two specific states.

We study forces between two neutral atoms, modeled as three-dimensional harmonic oscillators, arising from mutual influences mediated by an electromagnetic field but not from their direct interactions. We allow as dynamical variables the center-of-mass motion of the atom, its internal degrees of freedom, and the quantum field treated relativistically. We adopt the method of nonequilibrium quantum field theory which can provide a first-principles, systematic, and unified description including the intrinsic and induced dipole fluctuations. The inclusion of self-consistent back-actions makes possible a fully dynamical description of these forces valid for general atom motion. In thermal equilibrium we recover the known forces—London, van der Waals, and Casimir-Polder—between neutral atoms in the long-time limit. We also reproduce a recently reported force between atoms when the system is out of thermal equilibrium at late times. More noteworthy is the discovery of the existence of a type of (or identification of the source of some known) interatomic force which we call the “entanglement force,” originating from the quantum correlations of the internal degrees of freedom of entangled atoms.

Equations of the optimized-effective-potential method in a basis set representation are solved with the use of the incomplete Cholesky decomposition technique. The resulting local potential is expanded in terms of the products of occupied and virtual Kohn-Sham orbitals thus avoiding the use of auxiliary basis sets. It is demonstrated that, for a sufficiently large orbital basis set satisfying the condition of linear dependence of these products, stable and numerically accurate solutions of the OEP method can be obtained with the use of the suggested computational approach.

We address the few-body problem using the adiabatic hyperspherical representation. A general form for the hyperangular Green’s function in $d$ dimensions is derived. The resulting Lippmann-Schwinger equation is solved for the case of three particles with $s$-wave zero-range interactions. Identical particle symmetry is incorporated in a general and intuitive way. Complete semianalytic expressions for the nonadiabatic channel couplings are derived. Finally, a model to describe the atom loss due to three-body recombination for a three-component Fermi gas of ${}^6\mathrm{Li}$ atoms is presented.

We consider theoretically ionization of an atomic target by fast electron impact at large energy and momentum transfer and in the presence of laser radiation. The laser electric-field amplitude is weak compared to the typical field in the target. Two frequency regimes are investigated according to whether the laser frequency is (i) much smaller than or (ii) resonant to the frequency of the transition from the ground to the first excited target state. Fast incident, scattered, and ejected electrons are described using Volkov solutions. The dressing of the bound-electron state by the laser field is accounted for within time-dependent perturbation theory in the case of the low-frequency regime and within the rotating wave approximation in the case of a resonant one. The interaction of the incident electron with the target is treated in the first Born approximation. For atomic hydrogen embedded in a linearly or circularly polarized laser field, we discuss how the polarization-vector orientation influences the momentum-dependent $(e,2e)$ differential cross sections assisted by exchange of few photons between the colliding system and the field. In addition, we inspect the dependence of the cross sections on the dressing of the hydrogen state.

We analyze the behavior of a four-state, two-path atomic interferometer driven by shaped ultrafast laser pulses. The laser pulses interact with atomic rubidium, exciting the atoms to the $5D$ state via two intermediate resonances (the $5P_{1/2}$ and $5P_{3/2}$). The relative phase of the two paths can be modified by applying a varying spectral phase at the wavelength corresponding to one of the resonant transitions for each pathway. We trace out the behavior of the system from the simplest case of weak-field excitation with resonant fields to strong-field excitation with a broadband source. Our measurements and analysis reveal that while interference is observed for all field strengths and spectral widths, the character of the interference changes substantially.

We present a theory of high-order harmonic generation by diatomic molecules exposed to an elliptically polarized laser field. This theory is based on the molecular strong-field approximation with the laser-field-dressed initial bound state and the undressed final state. The interference minima, observed for linear polarization, are blurred with the increase of the laser-field ellipticity. The $n$th harmonic emission rate has contributions of the components of the $T$-matrix element in the direction of the laser-field polarization and in the direction perpendicular to it. We analyze the destructive interference condition for this perpendicular component. Taking into account that the aligned molecules are an anisotropic medium for high-harmonic generation, we introduce elliptic dichroism as a measure of this anisotropy and discuss possibilities of its use for determining the molecular structure.

We introduce an optimal control approach that is capable of yielding the best achievable field-free three-dimensional (3-D) alignment in an arbitrary molecule. Our analysis provides the optimal route to the best aligned state along with interesting insights into the 3-D alignment dynamics of polyatomic systems.

Electron-impact ionization of atomic hydrogen in the presence of a linearly polarized laser field is studied in the second Born approximation. The state of fast electron in the laser field is described by the Volkov state, while the dressed state of the ejected slow electron and atomic hydrogen is treated perturbatively to the first order. We calculate the triple differential cross section in the Ehrhardt asymmetric coplanar geometry. Numerical results show that the triple differential cross sections of the first Born approximation are significantly improved when the second-order corrections are taken into account.

We derive an expression for the collective Casimir-Polder interaction of a trapped gas of condensed bosons with a plane surface through coupling of the condensate atoms with the electromagnetic field. A systematic perturbation theory is developed based on a diagrammatic expansion of the electromagnetic self-energy. In the leading order, the result for the interaction energy is proportional to the number of atoms and involves the one-body density matrix of the trapped gas. At this order, atom-atom interactions and recoil effects lead to corrections compared to the single-atom theory, through shifts of the atomic transition energies. We also discuss the impact of spatial delocalization of the condensate mode.

We use Gaudin’s Fermi-Bose mapping operator to calculate exact solutions for the Lieb-Liniger model in a linear (constant-force) potential (the constructed exact stationary solutions are referred to as the Lieb-Liniger-Airy wave functions). The ground-state properties of the gas in the wedgelike trapping potential are calculated in the strongly interacting regime by using Girardeau’s Fermi-Bose mapping and the pseudopotential approach in the $1/c$ approximation ($c$ denotes the strength of the interaction). We point out that quantum dynamics of Lieb-Liniger wave packets in the linear potential can be calculated by employing an $N$-dimensional Fourier transform as in the case of free expansion.