We discuss the stability of homonuclear and heteronuclear mixtures of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ atoms in the metastable $2\hspace{0.5em}{}^3{S}_1$ state (${\mathrm{He}}^{*}$) and predict positions and widths of Feshbach resonances by using the asymptotic-bound-state model. All calculations are performed without fit parameters, using ab initio calculations of molecular potentials. One promising very broad Feshbach resonance ($\Delta B={72.9}_{-19.3}^{+18.3}$ mT) is found that allows for tuning of the interisotope scattering length.

Bremsstrahlung including the polarization correlations is revisited by presenting a theoretical model which is suited for the short-wavelength limit. The fast incoming electron is described by a Sommerfeld-Maue function, whereas for the outgoing electron an exact Dirac function is used. The cross section for electrons radiatively scattered from gold is calculated in the impact energy range 0.2–10 MeV and is compared to results from the Elwert-Haug theory and to available experimental data and relativistic partial-wave calculations. As an ultrarelativistic benchmark, the polarization correlations for 10 MeV electrons and 3–10 MeV photons are also given.

We analyze the photoelectron emission spectrum in atomic above-threshold ionization by a linearly polarized short-laser pulse. Direct electrons can be characterized by both intracycle and intercycle interferences. The former results from the coherent superposition of two different electron trajectories released in the same optical cycle, whereas the latter is the consequence of the superposition of multiple trajectories released in different cycles. In the present article, a semiclassical analytical expression for the complete (both intracycle and intercycle) interference pattern is derived. We show that the recently proposed semiclassical description in terms of a diffraction process at a time grating remains qualitatively unchanged in the presence of the long-range Coulomb potential. The latter causes only a phase shift of the intracycle interference pattern. We verify the predictions of the semiclassical model by comparison with full three-dimensional (3D) time-dependent Schrödinger equation (TDSE) solutions. One key finding is that the subcycle interference structures originating from trajectories launched within a time interval of less than 1 femtosecond should be experimentally observable also in low-resolution spectra for longer multicycle pulses.

We investigate fragmentation processes induced by core-level photoexcitation in CF${}_4$ at both the carbon and fluorine $K$ edges by means of partial-ion-yield spectroscopy. The molecule CF${}_4$ is a textbook example of systems in which Jahn-Teller coupling strongly manifests itself in the photoabsorption spectrum. Spectral features related to Jahn-Teller and quasi-Jahn-Teller splitting are observed, and important differences in the fragmentation pathways are revealed depending on the symmetries of the core-excited states. We interpret these experimental observations on the grounds of symmetry lowering from the $T_d$ to the $C_{3v}$ point group as well as preferential orientation with respect to the polarization vector of the incident radiation.

The existence of multidimensional matter-wave solitons in a crossed optical lattice (OL) with a linear optical lattice (LOL) in the $x$ direction and a nonlinear optical lattice (NOL) in the $y$ direction, where the NOL can be generated by a periodic spatial modulation of the scattering length using an optically induced Feshbach resonance is demonstrated. In particular, we show that such crossed LOLs and NOLs allow for stabilizing two-dimensional solitons against decay or collapse for both attractive and repulsive interactions. The solutions for the soliton stability are investigated analytically, by using a multi-Gaussian variational approach, with the Vakhitov-Kolokolov necessary criterion for stability; and numerically, by using the relaxation method and direct numerical time integrations of the Gross-Pitaevskii equation. Very good agreement of the results corresponding to both treatments is observed.

We study few-boson tunneling in a one-dimensional double well with a spatially modulated interaction. The dynamics changes from Rabi oscillations in the noninteracting case to a highly suppressed tunneling for intermediate coupling strengths followed by a reappearance near the fermionization limit. With extreme interaction inhomogeneity in the regime of strong correlations, we observe tunneling between the higher bands. The dynamics is explained on the basis of the few-body spectrum and stationary eigenstates. For a higher number of particles $N⩾3$, it is shown that the inhomogeneity of the interaction can be tuned to generate tunneling resonances. Finally, a tilted double well and its interplay with the interaction asymmetry are discussed.

Based on the analysis of the virial theorem and stress tensor, it is shown that the average energy of the Coulomb system under a static external field in a given volume is completely defined by a one-particle density matrix. In this case, a one-particle density matrix satisfies a closed equation expressing the equilibrium condition in a given static external field. An expression for the pressure of a heterogeneous system of interacting particles is found.

We report on the attainment of Bose-Einstein condensation of ${}^{86}\mathrm{Sr}$. This isotope has a scattering length of about $+800a_0$ and thus suffers from fast three-body losses. To avoid detrimental atom loss, evaporative cooling is performed at low densities around $3\times {10}^{12}$ ${\mathrm{cm}}^{-3}$ in a large volume optical dipole trap. We obtain almost pure condensates of $5\times {10}^3$ atoms.

We propose an expansion of the unitary evolution operator, associated with a given Schrödinger equation, in terms of a finite product of explicit unitary operators. In this manner, this unitary expansion can be truncated at the desired level of approximation, as shown in the given examples.

A digital signature is a mathematical scheme for demonstrating the authenticity of a digital message or document. For signing quantum messages, some arbitrated quantum signature (AQS) schemes have been proposed. It was claimed that these AQS schemes could guarantee unconditional security. However, we show that they can be repudiated by the receiver Bob. To conquer this shortcoming, we construct an AQS scheme using a public board. The AQS scheme not only avoids being disavowed by the receiver but also preserves all merits in the existing schemes. Furthermore, we discover that entanglement is not necessary while all these existing AQS schemes depend on entanglement. Therefore, we present another AQS scheme without utilizing entangled states in the signing phase and the verifying phase. This scheme has three advantages: it does not utilize entangled states and it preserves all merits in the existing schemes; the signature can avoid being disavowed by the receiver; and it provides a higher efficiency in transmission and reduces the complexity of implementation.

We consider two-dimensional networks composed of nodes initially linked by two-qubit mixed states. In these networks we develop a global error correction scheme that can generate distance-independent entanglement from arbitrary network geometries using rank-2 states. By using this method and combining it with the concept of percolation, we also show that the generation of long-distance entanglement is possible with rank-3 states. Entanglement percolation and global error correction have different advantages depending on the given situation. To reveal the trade-off between them we consider their application to networks containing pure states. In doing so we find a range of pure-state schemes, each of which has applications in particular circumstances: For instance, we can identify a protocol for creating perfect entanglement between two distant nodes. However, this protocol cannot generate a singlet between any two nodes. In contrast, we can also construct schemes for creating entanglement between any nodes, but the corresponding entanglement fidelity is lower.

We show that the coherent coupling of atomic qubits at distant nodes of a quantum network, composed of several cavities linked by optical fibers, can be arbitrarily controlled via the selective pairing of Raman transitions. The adiabatic elimination of the atomic excited states and photonic states leads to selective qubit-qubit interactions, which would have important applications in quantum-information processing. Quantum gates between any pair of distant qubits and parallel two-qubit operations on selected qubit pairs can be implemented through suitable choices of the parameters of the external fields. Selective pairing of Raman transitions also allows the generation of spin chains and cluster states without the requirement that the cavity-fiber coupling be smaller than the detunings of the Raman transitions.

We experimentally investigate the dynamics of classical and quantum correlations of a Bell diagonal state in a non-Markovian dephasing environment. The sudden transition from a classical to a quantum decoherence regime is observed during the dynamics of a Bell diagonal state. Due to the refocusing effect of the overall relative phase, the quantum correlation revives from near zero and then decays again in the subsequent evolution. However, the non-Markovian effect is too weak to revive the classical correlation, which remains constant in the same evolution range. With the implementation of an optical ${\sigma }_x$ operation, the sudden transition from a quantum to a classical revival regime is obtained, and correlation echoes are formed. Our method can be used to control the revival time of correlations, which would be important in quantum memory.

The role of the intershell correlation effect is theoretically investigated using the example of the Ne atom in nonresonance Compton high-energy x-ray photon scattering by a free atom. The calculation results qualitatively reproduce the same results in the formalism of the generalized oscillator strength and the random phase approximation with exchange for the Compton photon and electron scattering by an atom; when the incident photon energy is 11 keV and the scattering angle is ${90}^{°}$, they correspond well with the results of the synchrotron experiment presented in the work by Jung et al. [Phys. Rev. Lett. 81, 1596 (1998)].

The general formalism of the multipolar expansion of electrostatic interactions is applied to the calculation of the potential energy between an excited atom (without fine structure) and a ground-state diatomic molecule at large mutual separations. Both partners exhibit a permanent quadrupole moment so that their mutual long-range interaction is dominated by a quadrupole-quadrupole term, which is attractive enough to bind trimers. Numerical results are given for an excited Cs($6{}^{2}P$) atom and a ground-state ${\mathrm{Cs}}_2$ molecule. The prospects for achieving photoassociation of a cold-atom–dimer pair are thus discussed and found promising. The formalism can be generalized to the long-range interaction between molecules to investigate the formation of cold tetramers.

When a train of optical pulses interacts with a medium, the time between pulses and the pulse-to-pulse phase jump have a profound effect on the outcome of the interaction. For a near-infinite train of regularly separated pulses having a constant pulse-to-pulse phase jump, as in the output of a frequency comb laser interacting with a ladder system, excitation of that system has been previously calculated and compared with experiment. In that case, the number of pulses in the train is very large, and the energy per pulse is very small. In the present work, the other extreme is experimentally examined: A small number of regularly spaced optical pulses are made to interact with a three-level ladder system in atomic rubidium. The pulses have been amplified, possibly placing the interactions into a nonperturbative regime. The experimental results are compared to a simple intuitive model and found to be in good general agreement. However, some aspects of the experimental results seem to be at odds with the simple model.

Magneto-optical traps (MOTs) of highly magnetic lanthanides open the door to explorations of novel phases of strongly correlated matter such as lattice supersolids and quantum liquid crystals. We recently reported the first MOTs of the five high-abundance isotopes of the most magnetic atom, dysprosium. Described here are details of the experimental technique employed for repumper-free Dy MOTs containing up to half a billion atoms. Extensive characterization of the MOTs’ properties—population, temperature, loading, metastable decay dynamics, and trap dynamics—is provided.

We present an analysis of Bose-Fermi mixtures in optical lattices for the case where the lattice potential of the fermions is tilted and the bosons (in the superfluid phase) are described by Bogoliubov phonons. It is shown that the Bogoliubov phonons enable hopping transitions between fermionic Wannier-Stark states; these transitions are accompanied by energy dissipation into the superfluid and result in a net atomic current along the lattice. We derive a general expression for the drift velocity of the fermions and find that the dependence of the atomic current on the lattice tilt exhibits negative differential conductance and phonon resonances. Numerical simulations of the full dynamics of the system based on the time-evolving block decimation algorithm reveal that the phonon resonances should be observable under the conditions of a realistic measuring procedure.

We establish analytically a master equation of a figure-eight all-fiber passively mode-locked laser. The nonlinear amplifying loop mirror (NALM) is used as an effective saturable absorber in order to generate short pulses. The master equation is of the cubic complex Ginzburg-Landau type, in which the coefficients explicitly depend on the characteristics of the cavity, in particular on the orientation of the polarizer, the coupling coefficient, and the length of the NALM. Single-pulse and continuous-wave (cw) solutions in both normal and anomalous dispersion are discussed analytically. In the anomalous dispersion situation, the equation governing the evolution of the system admits stable analytic pulse solutions. The pulse duration and energy are studied. The analysis provides domains in the space of the cavity parameters where energetic soliton and ultrashort pulses are obtained.

We study the entanglement of formation for arbitrary-dimensional bipartite mixed unknown states. Experimentally measurable lower and upper bounds for entanglement of formation are derived.