Sources of entangled photon pairs are a key component in both fundamental tests of quantum theory and practical applications such as quantum key distribution and quantum computing. In this work, we describe and characterize a source of polarization entangled photon pairs based on two spontaneous parametric down-conversion crystals in a Sagnac interferometer. Our source is compact and produces high-quality entangled states in a very flexible manner. The wavelengths of the photon pairs (around 810 and 1550 nm), the phase between orthogonal components of the entangled state, and the tangle of the state are all independently adjustable. In addition to presenting basic characterization data, we present experimental violations of Clauser-Horne-Shimony-Holt and Leggett inequalities, as well as an instance of the “beautiful” Bell inequality, which has not previously been tested experimentally.

Hyperentanglement, defined as the entanglement in several degrees of freedom (DOFs) of a quantum system, has attracted much attention recently. Here we investigate the possibility of concentrating the two-photon four-qubit systems in partially hyperentangled states in both the spatial mode and the polarization DOFs with linear optics. We first introduce our parameter-splitting method to concentrate the systems in the partially hyperentangled states with known parameters, including partially hyperentangled Bell states and cluster states. Subsequently, we present another two nonlocal hyperentanglement concentration protocols (hyper-ECPs) for the systems in partially hyperentangled unknown states, resorting to the Schmidt projection method. It will be shown that our parameter-splitting method is very efficient for the concentration of the quantum systems in partially entangled states with known parameters, resorting to linear-optical elements only. All these four hyper-ECPs are feasible with current technology and they may be useful in long-distance quantum communication based on hyperentanglement as they require only linear optical elements.

We present a method for implementing a weak optical Kerr interaction (single-mode Kerr Hamiltonian) in a measurement-based fashion using the common set of universal elementary interactions for continuous-variable quantum computation. Our scheme is a conceptually distinct alternative to the use of naturally occurring, weak Kerr nonlinearities or specially designed nonlinear media. Instead, we propose to exploit suitable off-line prepared quartic ancilla states together with beam splitters, squeezers, and homodyne detectors. For perfect ancilla states and ideal operations, our decompositions for obtaining the measurement-based Kerr Hamiltonian lead to a realization with near-unit fidelity. Nonetheless, even by using only approximate ancilla states in the form of superposition states of up to four photons, high fidelities are still attainable. Our scheme requires four elementary operations and its deterministic implementation corresponds to about 10 ancilla-based gate teleportations. We test our measurement-based Kerr interaction against an ideal Kerr Hamiltonian by applying them both to weak coherent states and single-photon superposition states.

A scheme of photon echo-based quantum memory in the optimal optical QED cavity with off-resonant Raman atomic transition is proposed. The scheme employs the atomic ensembles characterized by an optically thin resonant transition and natural inhomogeneous broadening of the resonant line composed of the arbitrary narrow homogeneous spectral components. The scheme provides robust and quite simple coherent control of the light atoms dynamics that can be implemented by using an existing optical technique and opens a practical way for realization of the efficient long-lived multimode optical quantum memory.

We consider the problem of characterizing genuine multiparticle entanglement for permutationally invariant states using the approach of positive partial transpose mixtures. We show that the evaluation of this necessary biseparability criterion scales polynomially with the number of particles. In practice, it can be evaluated easily up to ten qubits and improves existing criteria significantly. Finally, we show that our approach solves the problem of characterizing genuine multiparticle entanglement for permutationally invariant three-qubit states.

The OH molecule is currently of great interest from the perspective of ultracold chemistry, quantum fluids, precision measurement, and quantum computation. Crucial to these applications are the slowing, guiding, confinement, and state control of OH, using electric and magnetic fields. In this article, we show that the corresponding eight-dimensional effective ground-state Stark-Zeeman Hamiltonian is exactly solvable and explicitly identify the underlying chiral symmetry. Our analytical solution opens the way to insightful characterization of the magnetoelectrostatic manipulation of ground-state OH. Based on our results, we also discuss a possible application to the quantum simulation of an imbalanced Ising magnet.

The intramolecular energy transfer, dynamical entanglement of vibrations, and decoherence process in triatomic molecular systems are studied. The benchmark molecules of ${\mathrm{H}}_2\mathrm{S}$, ${\mathrm{NO}}_2$, and ${\mathrm{O}}_3$ are sampled to investigate the intramolecular energy transfer and dynamical entanglement of stretching-stretching vibrational modes in triatomic molecular systems by restricting the bending vibration to its ground state. The comparative study is applied to explore the dynamical differences of initial local-mode and normal-mode characteristic states. Also, the decoherence process of the stretching-stretching qubits system caused by the bending vibration is discussed.

We report the results of ab initio calculations for excitation of the $\tilde{a}$ ${}^3{B}_1$, $\tilde{A}$ ${}^1{B}_1$, $\tilde{b}$ ${}^3{A}_1$, $\tilde{B}$ ${}^1{A}_1$, 1 ${}^3{A}_2$, and 1 ${}^1{A}_2$ states of water by low energy electron impact. The calculations are carried out in an eight-channel close-coupling approximation using the complex Kohn variational method. Particular attention is paid to the elimination of pseudoresonances that can occur when correlated target states are employed. Differential and integral cross sections are reported and compared with the most recent experimental and theoretical results.

Modified effective-range theory (MERT) was developed in the 1960s to describe electron and positron scattering from atoms. The theory was frequently used to extrapolate measured total cross sections down to the zero-energy region, which is inaccessible experimentally. However, the applicability of the model was usually limited to the very low energy range where experimental data to be extrapolated are rare. We have proposed [Idziaszek and Karwasz, Phys. Rev. A 73, 064701 (2006)] a different way of employing MERT by exploiting the properties of an analytical solution of the Schrödinger scattering equation with the long-range polarization potential. This alternative approach allows the validity of MERT to be extended to higher energies. At present we are applying this procedure for electron and positron scattering on He, H${}_2$, Ar, and CH${}_4$. The scattering amplitude and the effective-range parameters for $s$ and $p$ partial waves are obtained through a fitting inversion procedure applied to integral cross sections spanning most or all of the elastic region. The derived parameters are then used to obtain differential and momentum transfer cross sections; the agreement with experiments is very good.

Lithographically patterned micrometer-scale electrode arrays are used to examine the effects of controlled surface electric fields on Rydberg-atom–surface interactions. The data show that application of modest electrode biases (∼±1 V) can lead to a transition from ionization of the incident atoms by short-range tunneling to field ionization well above the target surface. The resulting ions can be efficiently detected using ion collection fields whose strengths are substantially smaller than those required for direct field ionization pointing to the application of surface ionization in the detection of low-$n$ Rydberg atoms. The data are analyzed with the aid of a Monte Carlo model and further demonstrate the critical role that local surface fields can play in governing the nature of atom-surface interactions.

This paper derives an analytic expression for the density per unit area of charges confined in a two-dimensional configuration in a Penning trap with an anharmonic applied trap potential that is expressed as a multipole expansion. This expression is used to find the optimum potential, with a given number of multipoles, for trapping a plasma with the most uniform possible density per unit area. Minimum energy states in such an optimized trap potential are evaluated numerically and the resulting crystal structures are shown to be defect-free over the central region of the plasma where the density is most nearly uniform. The paper also briefly considers the possibility of using an $\ell =3$ rotating wall trap potential in order to confine minimum energy states with triangular symmetry and no defects.

We consider the problem of the implementation of stimulated Raman adiabatic passage (STIRAP) processes in degenerate systems, with a view to be able to steer the system wave function from an arbitrary initial superposition to an arbitrary target superposition. We examine the case of an $N$-level atomic system consisting of $N-1$ ground states coupled to a common excited state by laser pulses. We analyze the general case of initial and final superpositions belonging to the same manifold of states, and we cover also the case in which they are nonorthogonal. We demonstrate that for a given initial and target superposition, it is always possible to choose the laser pulses so that in a transformed basis the system is reduced to an effective three-level $\Lambda$ system, and standard STIRAP processes can be implemented. Our treatment leads to a simple strategy, with minimal computational complexity, which allows us to determine the laser-pulse shape required for the wanted adiabatic steering.

Magnetic-dipole, electric-quadrupole, and hyperpolarizability effects on clock energy levels are analyzed in detail for Sr atoms in Stark potentials of red-detuned and blue-detuned magic-wavelength optical lattices. A difference between ac Stark shifts in traveling and standing waves is determined numerically. Differences between magic wavelengths for atoms in traveling and standing waves are presented and strategies for minimizing uncertainties of the clock frequency are indicated explicitly. Significant suppression of hyperpolarizability effect is demonstrated analytically for a blue-detuned in comparison with a red-detuned lattice, thus enabling essential deepening of trap potentials, reducing tunneling between lattice wells and collision effects.

We investigate direct above-threshold ionization in diatomic molecules, with particular emphasis on how quantum interference is altered by a driving field of nonvanishing ellipticity. This interference may be either temporal, i.e., related to ionization events occurring at different times, or spatial, i.e., related to the electron emission at different centers in the molecule. Employing the strong-field approximation and saddle-point methods, we find that, in general, for nonvanishing ellipticity, there will be a blurring of the temporal and spatial interference patterns. The former blurring is caused by the electron velocity component perpendicular to the major polarization axis, while spatial interference is washed out as a consequence either of $s-p$ mixing or of the temporal dependence of the ionization prefactor. Both types of interference are analyzed in detail in terms of electron trajectories, and specific conditions for which sharp fringes occur are provided.

In this paper we analyze the possibility of persistent currents of a two-species bosonic mixture in the one-dimensional ring geometry. We extend the arguments used by  F. Bloch [Phys. Rev. A 7, 2187 (1973)] to obtain a criterion for the stability of persistent currents for the two-species system. If the mass ratio of the two species is a rational number, persistent currents can be stable at multiples of a certain total angular momenta. We show that the Bloch criterion can also be viewed as a Landau criterion involving the elementary excitations of the system. Our analysis reveals that persistent currents at higher angular momenta are more stable for the two-species system than previously thought.

We evaluate the nonlocal spatial interference displayed by Einstein-Podolsky-Rosen entangled particle pairs after they pass through a double-grating arrangement. An entanglement criterion is derived which serves to certify the underlying entanglement only from the observed spatial correlations. We discuss the robustness of the scheme along with a number of possible realizations with matter waves.

In this paper we explore the spin-orbit-induced bound state and molecular signature of the degenerate Fermi gas in a narrow Feshbach resonance based on a generalized two-channel model. Without the atom-atom interactions, only one bound state can be found even if spin-orbit coupling exists. Moreover, the corresponding bound-state energy depends strongly on the strength of spin-orbit coupling, but is influenced slightly by its type. In addition, we find that when increasing the strength of spin-orbit coupling, the critical point at which the molecular fraction vanishes shifts from zero to the negative detuning. In the weak spin-orbit coupling, this shifting is proportional to the square of its strength. Finally, we also show that the molecular fraction can be well controlled by spin-orbit coupling.

We study ultracold bosonic atoms with synthetic three-dimensional spin-orbit (SO) coupling in a cubic optical lattice. In the superfluidity phase, the lowest energy band exhibits one, two, or four pairs of degenerate single-particle ground states depending on the SO-coupling strengths, which can give rise to condensate states with spin stripes for weak atomic interactions. In the deep Mott-insulator regime, the effective spin Hamiltonian of the system combines three-dimensional Heisenberg exchange interactions, anisotropy interactions, and Dzyaloshinskii-Moriya interactions. Based on Monte Carlo simulations, we numerically demonstrate that the resulting Hamiltonian with an additional Zeeman field has a rich phase diagram with spiral, stripe, vortex crystal, and especially Skyrmion crystal spin textures in each $xy$-plane layer. The obtained Skyrmion crystals can be tunable with square and hexagonal symmetries in a columnar manner along the $z$ axis, and moreover are stable against the interlayer spin-spin interactions in a large parameter region.

We report on the analysis of the spatial beam profile of high-order harmonic radiation originating from a semi-infinite gas cell (SIGC). We experimentally assign contributions of electron trajectories to different spatial regions of the harmonic radiation beam. The angular divergence of the harmonic radiation is studied for the first time in a SIGC as a function of different phase-matching parameters. We relate the ratio of the dipole phase coefficients to the coherence time and divergence angle measurements. Simulations, including high-order-harmonic propagation, give further insight into the generation process and the influence of phase matching. The analysis reveals that the SIGC enables tuning of the cutoff frequency by altering the absorption of the generating medium.

We propose a measure of quantumness based on an addition-then-subtraction operation. We demonstrate how this measure can distinguish between classical and bosonic particles by investigating in detail multiparticle bosonic systems. Experimental schemes implementing this measure for bosons in all-optical and atom-cavity systems are provided. We also apply this measure to single-mode fermionic systems.